Gabapentin

Agent report for Kya, 5 September 2026. Written because the Southampton neurosurgery registrar recommended gabapentin over a nerve block and her partner and the patient said they need much more information before they trial it. Evidence-based; guideline, licence and trial figures are cited with author, year, journal and URL or PMID. Where something is clinical judgement rather than citation it is labelled as such. No prognosis or survival figures anywhere in this file. Not medical advice: it is a brief so the family can ask the right questions and run the trial with their eyes open. Trial populations are diabetic neuropathy, post-herpetic neuralgia, cancer pain and epilepsy, not skull-base tumour pain; treat every rate as the best available approximation.

Patient facts used: 34-year-old woman, 51 kg after 12 to 15 percent weight loss, probable clival chordoma with cavernous sinus and prepontine extension, right third and fourth nerve palsies (diplopia, ptosis), continuous right trigeminal-distribution pain with flares and neck spasms. Current drugs: Oramorph 10 mg as needed (30 mg taken today), diclofenac 100 mg suppository 16-hourly, omeprazole, paracetamol, a laxative. Kidney function results pending. Not pregnant, not breastfeeding.

1. What gabapentin is and how it works

• Gabapentin was designed in the 1970s as a chemical analogue of GABA, the brain's main inhibitory transmitter, and named accordingly. It turned out not to work through GABA at all: it does not bind GABA-A or GABA-B receptors, is not converted into GABA, and does not block GABA reuptake. Its analgesic and anticonvulsant action comes from binding the alpha-2-delta (α2δ) subunit of voltage-gated calcium channels on nerve terminals in the spinal cord dorsal horn, the trigeminal nucleus and the brain. That binding reduces the trafficking of calcium channels to the nerve terminal and so reduces release of excitatory transmitters (glutamate, substance P, CGRP) from sensitised, over-firing pain neurons. (Bockbrader 2010, Clinical Pharmacokinetics, PMID 20818832, for the mechanism statement; Sills 2006, Current Opinion in Pharmacology, PMID 16376147, and Field 2006, PNAS, PMID 17088553, for the α2δ knock-in evidence; the latter two are cited from general knowledge of the papers and were not re-fetched today.)
• Why that matters for nerve pain: injured or irritated sensory nerves up-regulate α2δ, and gabapentin has most effect on those sensitised pathways and comparatively little on normal pain signalling. This is why it does nothing useful for a healthy volunteer's cold-pressor pain on its own (Eckhardt 2000, Anesthesia and Analgesia, PMID 10866910) but helps a proportion of people with nerve injury pain.
• It is not an opioid, not an anti-inflammatory and not a sedative in the benzodiazepine sense. It is added to those, not swapped for them.
• Pharmacokinetics that shape everything below (Neurontin SmPC section 5.2, https://www.medicines.org.uk/emc/product/3195/smpc; Bockbrader 2010): absorbed slowly by a saturable transporter in the gut, so bioavailability falls from about 60 percent at 900 mg a day to about 33 percent at 3600 mg a day; peak level 3 to 4 hours after a dose; half-life 5 to 7 hours (hence three times a day dosing); not bound to plasma proteins; not metabolised by the liver at all; eliminated unchanged by the kidney. Food has no clinically significant effect.

2. Evidence in neuropathic pain

2a. The Cochrane review (Wiffen 2017)

Wiffen PJ, Derry S, Bell RF et al. Gabapentin for chronic neuropathic pain in adults. Cochrane Database of Systematic Reviews 2017, CD007938. PMID 28597471. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD007938.pub4/full. 37 trials, 5914 participants, mostly at 1200 mg a day or more, trials lasting 4 to 12 weeks, mostly post-herpetic neuralgia and painful diabetic neuropathy.

• Post-herpetic neuralgia, 1200 mg a day or more: substantial benefit (at least 50 percent pain relief or "very much improved") in 32 percent on gabapentin vs 17 percent on placebo, NNT 6.7 (95 percent CI 5.4 to 8.7). Moderate benefit (at least 30 percent relief or "much improved") 46 percent vs 25 percent, NNT 4.8.
• Painful diabetic neuropathy: substantial benefit 38 percent vs 21 percent, NNT 5.9. Moderate benefit 52 percent vs 37 percent, NNT 6.6.
• Adverse-event withdrawals, all conditions: 11 percent vs 8.2 percent, NNH 30 (high-quality evidence).
• Any adverse event: 63 percent vs 49 percent, NNH 7.5.
• Serious adverse events: 3.2 percent vs 2.8 percent, not different from placebo.
• Individual events on gabapentin: dizziness 19 percent, somnolence 14 percent, gait disturbance 14 percent, peripheral oedema 7 percent.
• Authors' plain-language summary: "Around 3 or 4 out of 10 participants achieved this degree of pain relief with gabapentin, compared with 1 or 2 out of 10 for placebo. Over half of those treated with gabapentin will not have worthwhile pain relief but may experience adverse events." And, important for the patient: "Evidence for other types of neuropathic pain is very limited."

The NeuPSIG meta-analysis (Finnerup 2015, Lancet Neurology, PMID 25575710) gives NNTs for 50 percent relief across all neuropathic pain of 7.2 for gabapentin, 7.7 for pregabalin, 6.4 for SNRIs (mainly duloxetine) and lower (better) for tricyclics, about 3.6, with a 10 percent overstatement of effects from publication bias. All four are first-line on GRADE.

2b. NICE CG173

NICE CG173, Neuropathic pain in adults: pharmacological management in non-specialist settings (2013, last updated 2020), https://www.nice.org.uk/guidance/cg173. Recommendation 1.1.8: offer a choice of amitriptyline, duloxetine, gabapentin or pregabalin as initial treatment for neuropathic pain (except trigeminal neuralgia); 1.1.9: if the first is not effective or not tolerated, offer one of the remaining three, and consider switching again if needed; 1.1.13: carbamazepine as initial treatment for trigeminal neuralgia; review early treatment for titration and tolerability, then regularly. NICE does not rank the four. (The NICE site blocked this agent today; the wording is carried from research thread 08, where it was verified.) NG193 (2021) advises against starting gabapentinoids for chronic primary pain; that does not apply to the patient, whose pain has a structural cause.

2c. Trigeminal and facial neuropathic pain

The patient's pain is not classic trigeminal neuralgia (brief electric stabs from vascular compression). It is painful trigeminal neuropathy plus dural and bone pain from a mass involving the cavernous sinus, with third and fourth nerve palsies alongside. The trigeminal neuralgia literature is therefore only partly relevant.

• The European Academy of Neurology guideline (Bendtsen 2019, European Journal of Neurology, PMID 30860637) puts carbamazepine and oxcarbazepine first for trigeminal neuralgia and lists gabapentin as an add-on or alternative on low-quality evidence.
• Lemos 2008 (Clinical Journal of Pain, PMID 18180639): 36 trigeminal neuralgia patients randomised to gabapentin, ropivacaine trigger-point blocks, or both; the combination gave faster and greater relief at a lower gabapentin dose and with fewer side effects than gabapentin alone. Small, single-centre, but it is direct evidence that a block plus a low gabapentin dose can beat a higher gabapentin dose alone.
• Zhao 2023 (Frontiers in Neurology, PMID 37200780): meta-analysis of 18 randomised trials, 1604 patients, gabapentin vs carbamazepine in primary trigeminal neuralgia; gabapentin had a higher response rate (OR 2.02) and far fewer adverse events (OR 0.28). Judgement: nearly all the included trials are small Chinese trials of uncertain quality, so treat the direction (gabapentin is better tolerated than carbamazepine) as reliable and the size of the efficacy difference as unproven.
• There is no randomised evidence in skull-base tumour facial pain. Anyone who says gabapentin is "proven" for this exact situation is extrapolating; so is anyone who says it is useless.

2d. Cancer-related neuropathic pain

• Caraceni 2004 (Journal of Clinical Oncology, PMID 15254060): the one good placebo-controlled trial in this setting. 121 patients with neuropathic cancer pain partially controlled on opioids; gabapentin titrated from 600 to 1800 mg a day over 10 days on top of a stable opioid dose. Average pain 4.6 vs 5.4 out of 10 (p = 0.025); dysaesthesia improved significantly; adverse-event withdrawals 7.6 percent vs 7.3 percent. A modest but real difference within 10 days at a modest dose in patients already on opioids, which is the patient's situation.
• Mishra 2012 (American Journal of Hospice and Palliative Care, PMID 21745832): 120 patients, amitriptyline vs gabapentin vs pregabalin vs placebo with rescue morphine; all three active drugs helped, pregabalin most, and pregabalin spared the most morphine.
• Kane 2018 (Palliative Medicine, PMID 28604172): systematic review of opioid plus antidepressant or antiepileptic vs opioid alone in cancer pain; pooling four gabapentinoid trials, no significant added benefit (SMD 0.16, CI -0.19 to 0.51) and more adverse events, but the pooled patients had mixed bone, visceral and neuropathic pain and the authors say benefit in definite neuropathic cancer pain "cannot be excluded". Honest reading: in mixed tumour pain gabapentinoids add side effects without much relief; in clearly neuropathic pain (Caraceni's population) they help some people.

2e. Combination with morphine

• Gilron 2005 (New England Journal of Medicine, PMID 15800228): 57 patients, crossover, gabapentin vs sustained-release morphine vs both. Mean pain 4.15 on gabapentin alone, 3.70 on morphine alone, 3.06 on the combination; the maximum tolerated dose of each drug was lower in combination; constipation, sedation and dry mouth were the commonest adverse effects. This is the evidence for the strategy "a little of each rather than a lot of one".
• Gilron 2009 (Lancet, PMID 19796802): gabapentin plus nortriptyline better than either alone (2.3 vs 3.2 vs 2.9 out of 10). Relevant to the "what if it only half works" question in section 8.
• Chaparro 2012 Cochrane (PMID 22786518) confirms that opioid plus gabapentinoid combinations do better than single agents on pain in the small number of trials, at the cost of more sedation.

2f. What "several nerves involved" implies

This is judgement, but it follows from the pharmacology. A nerve block is anatomical: it numbs the territory of the one nerve or ganglion injected. The patient has irritation of the trigeminal system (the pain), the third and fourth nerves (the eye signs), the dura of the cavernous sinus and prepontine cistern, and clival bone, with neck spasm on top. No single injectable target covers that. Gabapentin acts on the sensitised second-order neurons in the trigeminal nucleus and dorsal horn where all those inputs converge, so it is indifferent to which nerve is firing. That is the registrar's reasoning and it is sound. The two are not exclusive: Lemos 2008 suggests a block for the worst-defined territory (cheek and temple via sphenopalatine ganglion, neck via occipital nerve) can lower the gabapentin dose needed. Ask for both, not either.

2g. How it compares with pregabalin, duloxetine and the tricyclics

• Direct head-to-head trials (Sadegh 2024, Pain Reports, PMID 38932764; 30 trials, 4087 patients): tricyclics and gabapentin/pregabalin are equivalent for pain (mean difference 0.10 on a 10-point scale, moderate-quality evidence); no proven difference between SNRIs and gabapentinoids or between SNRIs and tricyclics; more dropouts for adverse events with SNRIs and opioids than with tricyclics.
• OPTION-DM (Tesfaye 2022, Lancet, PMID 36007534): the largest UK crossover trial of first-line pathways in diabetic neuropathic pain; amitriptyline-first, pregabalin-first and duloxetine-first pathways gave the same pain relief; combinations helped the partial responders; the side-effect signatures differed (dizziness with pregabalin, nausea with duloxetine, dry mouth with amitriptyline).
• Indirect comparison in diabetic neuropathy (Quilici 2009, BMC Neurology, PMID 19208243): duloxetine, pregabalin and gabapentin comparable for pain; duloxetine more nausea, gabapentinoids more dizziness and somnolence.
• Cochrane side-effect rates for the alternatives (verified in thread 08): pregabalin somnolence 11 to 15 percent and dizziness 13 to 22 percent at 300 to 600 mg (Derry 2019, PMID 30673120); amitriptyline any adverse event 55 vs 36 percent (Moore 2015, PMID 26146793); duloxetine nausea 29 vs 10 percent, 16 percent stopping for side effects (Lunn 2014, PMID 24385423).
• Bottom line on comparison: on the evidence there is no efficacy reason to choose gabapentin over the other three, and no efficacy reason to reject it. The choice is about side-effect profile, interactions and speed. Gabapentin's advantages for the patient: no liver metabolism, no serotonergic interaction with anything, no anticholinergic load, no sodium problem, the only one of the four with a placebo-controlled trial in opioid-treated neuropathic cancer pain (Caraceni), and the Gilron evidence that it lets the morphine dose stay lower. Its disadvantages: the most dizziness and drowsiness of the four at working doses, slow titration, additive sedation and the respiratory warning with morphine, and blurred or double vision listed as a common effect in a woman who already has diplopia.

3. Dosing and titration

Licence (Neurontin SmPC section 4.2, https://www.medicines.org.uk/emc/product/3195/smpc):

• Standard start for neuropathic pain: 300 mg once on day 1, 300 mg twice on day 2, 300 mg three times on day 3; or 300 mg three times a day from day 1. Then increase by 300 mg a day every 2 to 3 days to a maximum of 3600 mg a day in three divided doses. Minimum one week to reach 1800 mg, two weeks to 2400 mg, three weeks to 3600 mg.
• Renal adjustment (creatinine clearance in mL/min, total daily dose): 80 or above, 900 to 3600 mg; 50 to 79, 600 to 1800 mg; 30 to 49, 300 to 900 mg; 15 to 29, 150 to 600 mg; below 15, 150 to 300 mg. Because gabapentin is cleared only by the kidney, the pending creatinine and eGFR must be seen before the dose climbs, and repeated while she is on a regular NSAID at low body weight. Note that eGFR formulas over-estimate clearance in a small, underweight patient; the pharmacist should use a weight-based Cockcroft-Gault estimate. (Judgement on the last point, standard pharmacy practice.)
• Stopping: taper over a minimum of one week regardless of indication (SmPC 4.2 and 4.4). Longer at higher doses is sensible; see section 6.

The slow schedule for the patient (judgement, built from the SmPC, the MHRA opioid warning and the Caraceni dosing; this is what to ask the prescriber to write up rather than the label schedule):

• Start 100 to 300 mg at night only for 3 nights. Night-first because the drowsiness then coincides with sleep, which she needs, and because the first dose in a 51 kg opioid-treated patient shows you her sensitivity before she has three doses a day on board.
• If clear-headed by day: 300 mg at night plus 100 to 300 mg in the morning for 3 to 4 days, then 300 mg three times a day (900 mg total). That is the dose at which Caraceni's cancer patients were already showing benefit and the SmPC's first working dose.
• Hold at 900 mg for at least a week and judge. If partial benefit and no fog: steps of 300 mg a day every 3 to 7 days towards 1800 mg. Cochrane's working range is 1800 to 3600 mg, but at her weight and on morphine 1800 mg is a fair ceiling for the first trial; going above it is a specialist pain-team decision.
• Do not step up on a day she is drowsier, dizzier or foggier than the day before. Step up on a plateau, not on a slope.
• Fair trial length: Caraceni saw separation within 10 days at 600 to 1800 mg; the Cochrane trials ran 4 to 12 weeks; NICE says review early for titration and tolerability. Judgement: 2 weeks at 900 to 1800 mg tells you whether there is any signal; 4 to 8 weeks at the best tolerated dose is a full trial. If there is nothing at all after 2 weeks at 1800 mg, more time is unlikely to change the answer.
• How to take it: with or without food (SmPC 5.2; NHS, https://www.nhs.uk/medicines/gabapentin/how-and-when-to-take-gabapentin/); swallow whole; space doses evenly, for example morning, afternoon and bedtime. Not within 2 hours after an aluminium or magnesium antacid (Gaviscon, Maalox and similar), which cut absorption by up to 24 percent (SmPC 4.5). Omeprazole is not an antacid and does not interact. Missed dose: take it unless within 2 hours of the next; never double up (NHS).

4. Side effects: rates, time course, and the ones that matter for her

Rates are from the Cochrane review (1200 mg a day or more) and the Neurontin SmPC section 4.8 frequency categories (very common is more than 1 in 10; common is 1 in 100 to 1 in 10).

• Dizziness: 19 percent vs 6.6 percent on placebo (Cochrane). Very common (SmPC).
• Somnolence, drowsiness, sedation: 14 percent vs 5.2 percent (Cochrane). Very common (SmPC).
• Fatigue: very common (SmPC).
• Ataxia and gait disturbance: 14 percent (Cochrane); ataxia very common (SmPC). At 51 kg with poor sleep this is a falls risk; she should stand up slowly and not walk the corridor alone at night during titration.
• Peripheral oedema (ankle swelling): 7 percent (Cochrane); common (SmPC). Cosmetic and reversible; it is not heart or kidney failure, but a new swollen ankle should be reported so nobody mistakes it for something else.
• Weight gain and increased appetite: both common (SmPC). In 44 epilepsy patients on high doses for a year or more, 10 gained over 10 percent of body weight and 15 gained 5 to 10 percent; the gain started in months 2 to 3 and levelled off by 6 to 9 months (DeToledo 1997, Therapeutic Drug Monitoring, PMID 9263379). Anorexia is also listed as common, so appetite can go either way, but the net population effect is weight gain. For a woman who has lost 12 to 15 percent of her weight this is one of the few side effects that would be welcome.
• Blurred vision (SmPC term "amblyopia"), diplopia and nystagmus: all common (SmPC). This matters for the patient specifically. She already has diplopia from third and fourth nerve palsies. Gabapentin can worsen double vision and cause jerky eye movements, and the family and the ward need a written baseline description of her eyes before the first dose so that any change can be attributed. Worsening diplopia during titration is a reason to hold the dose and tell the team, not to assume the tumour has grown; and equally, a change in eye signs must not be dismissed as "just the gabapentin" without a doctor looking. Both errors are possible; the baseline prevents both.
• Tremor: common (SmPC).
• Nausea and vomiting: common (SmPC; NHS lists feeling or being sick and diarrhoea).
• Dry mouth, headache, infections: common (NHS, SmPC).
• Cognitive effects: the SmPC lists amnesia and "thinking abnormal" as common; the NHS page lists memory problems. The best controlled data: 40 healthy volunteers randomised to 12 weeks of gabapentin (titrated to 3600 mg a day or the highest tolerated), topiramate or placebo, with cognitive testing after at least 2 weeks at a plateau dose; gabapentin affected 1 of 24 cognitive measures, topiramate 12 of 24 (Salinsky 2005, Neurology, PMID 15753411). In older healthy adults, gabapentin and carbamazepine both produced mild effects compared with no drug, gabapentin slightly less (Meador 2001, Neurology, PMID 11422333). Patient reports of word-finding trouble and fogginess are commonest during titration and at each step up. Do they settle? The NHS page says sleepiness, tiredness and dizziness "should wear off" as the body adjusts and headaches usually go within the first week; the Salinsky data show that at a steady dose after 2 weeks the measurable cognitive cost is small. There is no trial that times the settling precisely; "worst in the first 1 to 2 weeks and after each increase, mostly settled by 2 to 4 weeks at a stable dose" is a synthesis of product information and clinical experience, not a single citation. Judgement: the honest way to describe it to the patient is "you will probably feel it for the first week or two and after each step up; if it has not lifted after two weeks at a steady dose, that dose is too high for you".
• Mood: the NHS page lists mood changes as common. The SmPC 4.4 warning: a meta-analysis of placebo-controlled trials of antiepileptic drugs showed a small increased risk of suicidal thoughts and behaviour; the mechanism is unknown; patients should be monitored and told to seek advice if such thoughts emerge. This is a class warning applied to every antiepileptic after the FDA's 2008 pooled analysis of 199 trials (about 0.43 percent vs 0.24 percent, roughly one extra person in 500; figures from general knowledge of the FDA alert, not re-fetched today). A later claims study found no increase in suicide attempts on antiepileptics in bipolar disorder (Gibbons 2009, Archives of General Psychiatry, PMID 19996040). The signal is small and contested but the rule is simple: any new low mood, agitation, hopelessness or thoughts of self-harm is reported the same day.
• Rare serious reactions (SmPC 4.4; NHS): DRESS (drug reaction with eosinophilia and systemic symptoms: fever, rash, swollen glands, liver or kidney involvement, usually 2 to 8 weeks in); anaphylaxis (lip, tongue or throat swelling, difficulty breathing); acute pancreatitis (persistent stomach pain); hallucinations; severe respiratory depression (section 5). All are rare, all are stop-and-call-a-doctor events.
• Proportion who stop because of side effects in trials: 11 percent vs 8.2 percent on placebo across all conditions (Cochrane); 7.6 percent vs 7.3 percent in the opioid-treated cancer trial (Caraceni 2004). So roughly one person in ten stops, and only about one in thirty stops because of the drug rather than because of being in a trial.

5. Interactions and cautions for her

5a. The MHRA warning on gabapentin with opioids

MHRA Drug Safety Update, 26 October 2017, "Gabapentin (Neurontin): risk of severe respiratory depression", https://www.gov.uk/drug-safety-update/gabapentin-neurontin-risk-of-severe-respiratory-depression. Advice: be aware of the risk of CNS depression including severe respiratory depression with gabapentin; consider dose adjustment in patients at higher risk; risk factors are compromised respiratory function or respiratory disease, neurological disease, renal impairment, concomitant CNS depressants, and older age. Patients should seek help for any trouble breathing or shallow breaths. The MHRA update of 16 April 2019 (https://www.gov.uk/drug-safety-update/pregabalin-lyrica-gabapentin-neurontin-and-risk-of-abuse-and-dependence-new-scheduling-requirements-from-1-april) adds: gabapentin and pregabalin "can cause depression of the central nervous system, resulting in drowsiness, sedation, and potentially fatal respiratory depression, particularly if used concomitantly with opioid medicines and alcohol".

The patient has two of the listed risk factors: neurological disease and an opioid. Renal function is pending and could add a third.

• The mechanism (Tambon 2023, Expert Opinion on Drug Safety, PMID 36932941, systematic review of 25 studies): the two drugs depress breathing by different routes and the effects add; and experimental work shows a single gabapentinoid dose can reverse the tolerance to respiratory depression that opioid users build up. In observational data the combination roughly increases the odds of respiratory depression or opioid-related death by about 1.3-fold in perioperative and chronic pain settings.
• The population evidence (Gomes 2017, PLoS Medicine, PMID 28972983): among Ontario opioid users, 1256 opioid-related deaths matched to 4619 controls; gabapentin co-prescription in the preceding 120 days was associated with a 49 percent increase in the odds of opioid-related death (adjusted OR 1.49, CI 1.18 to 1.88), and moderate or high gabapentin doses with about a 60 percent increase. NSAID co-prescription showed no such association. The same group found a similar signal for pregabalin (Gomes 2018, Annals of Internal Medicine, PMID 30140853). These are relative risks in a population with much misuse and high opioid doses; the absolute risk in a supervised 34-year-old on 30 mg of oral morphine a day is small, but it is not zero and it is the reason the morphine dose stays modest while the gabapentin climbs.
• What to watch for and when: the additive sedation is largest in the first days after starting and after each step up, and in the 1 to 4 hours after a gabapentin dose coincides with a morphine dose. Signs that matter: hard to rouse, breathing under 10 to 12 a minute or shallow, long pauses in breathing during sleep, snoring that is new, blue or grey lips, confusion. These are the same signs as opioid toxicity and are handled the same way: wake her, sit her up, call for help, do not give the next dose of either drug. Judgement: on a ward, ask for respiratory rate and sedation score to be recorded at every set of obs for the first 72 hours after starting and after each increase; at home, someone should be in the house for the first three nights.

5b. Morphine raises gabapentin levels

SmPC section 4.5: in 12 healthy volunteers, 600 mg gabapentin taken 2 hours after 60 mg controlled-release morphine had a 44 percent larger area under the curve than gabapentin alone (Eckhardt 2000, Anesthesia and Analgesia, PMID 10866910), because morphine slows gut transit and lets more of the slowly absorbed gabapentin be taken up. The same study showed the combination gave more analgesia than either alone. Practical meaning: a given gabapentin dose is worth more, in effect and in side effects, in a person on morphine. Another reason to start low.

5c. Alcohol and other sedatives

Additive CNS depression (SmPC 4.4; MHRA 2019). The NHS page says alcohol is permitted but will add drowsiness. Judgement for the patient: none during titration and none within a few hours of a morphine dose. No benzodiazepines, zopiclone, sedating antihistamines (chlorphenamine, promethazine, cyclizine is sedating too) or cannabis products without the team knowing, because each one stacks on the same respiratory and sedative pathway. If an anti-sickness drug is added alongside the morphine, ondansetron or metoclopramide is less sedating than cyclizine.

5d. Antacids

Aluminium or magnesium antacids reduce gabapentin bioavailability by up to 24 percent; take gabapentin at the earliest 2 hours after an antacid (SmPC 4.5). Omeprazole is unaffected.

5e. No liver or CYP interactions

Gabapentin is not metabolised, not protein-bound, does not induce or inhibit liver enzymes and has no interaction with oral contraceptives (SmPC 4.5; Bockbrader 2010). It will not alter the levels of dexamethasone, chemotherapy, antiemetics or anything else she may be given later. Cimetidine slightly reduces its renal excretion, not clinically important. This is a genuine advantage over carbamazepine.

5f. Driving

Under the drug-driving law in England, Scotland and Wales (https://www.gov.uk/drug-driving-law) there are two offences: driving while impaired by any drug, legal or illegal; and driving with more than a specified blood level of certain listed drugs. Gabapentin and pregabalin are not on the specified list (the listed prescription medicines are amphetamine, clonazepam, diazepam, flunitrazepam, lorazepam, methadone, morphine and other opioids, and oxazepam). Morphine is on the list, with a medical defence for people taking it as prescribed and not impaired. So for the patient the position is: the impairment offence applies to gabapentin, the specified-limit offence applies to her morphine, and the SmPC and NHS advice is not to drive, cycle or operate machinery while affected by dizziness or drowsiness. Realistically, with diplopia from two cranial nerve palsies she should not be driving at all until an ophthalmologist or the DVLA says otherwise, and the diplopia itself is notifiable to the DVLA. Judgement on the last sentence, from the DVLA's general fitness-to-drive rules on double vision, not re-fetched today.

5g. Pregnancy

Not relevant; recorded only because it is a standard caution.

5h. Appetite and weight in a patient losing weight

Increased appetite and weight gain are common (SmPC; DeToledo 1997). For most patients this is a drawback; for the patient it is, if anything, a small benefit, and it is another reason not to fear the drug's metabolic profile. Peripheral oedema can add a kilogram or two that is fluid rather than nutrition; weigh her weekly and look at the ankles so the two are not confused.

6. Dependence, misuse and withdrawal

• Why it became a Class C controlled drug: the Advisory Council on the Misuse of Drugs advised reclassification in 2016 because of rising deaths involving pregabalin and gabapentin, diversion and misuse, mostly in combination with opioids and other sedatives; the government announced the change in October 2018 and it took effect on 1 April 2019 (gov.uk announcement, https://www.gov.uk/government/news/pregabalin-and-gabapentin-to-be-controlled-as-class-c-drugs; MHRA Drug Safety Update 16 April 2019). Both drugs are Class C under the Misuse of Drugs Act and Schedule 3 under the 2001 Regulations: prescriptions must be signed and are valid for 28 days, pharmacists may ask for identification on collection, and good practice is a supply of no more than 30 days. Possession without a prescription and supply to others are offences. This changes the paperwork, not the medicine.
• The real risk for someone without a history of misuse: low. In the systematic review of gabapentin addiction and dependence (Mersfelder 2016, Annals of Pharmacotherapy, PMID 26721643), every published case of addiction was in a patient with a previous history of alcohol, cocaine or opioid misuse, at an average dose above 3000 mg a day (range 600 to 8000); surveys put misuse at 1.1 percent in the general population and 22 percent in people attending drug treatment services. The 2021 update (Evoy 2021, Drugs, PMID 33215352; 55 further studies) confirms that misuse concentrates in people with opioid use disorder, who take it to boost the opioid effect. The MHRA's advice to prescribers is to evaluate for a history of substance misuse before prescribing and to watch for dose escalation and drug-seeking. The patient has no such history. Judgement: the addiction risk to her is not a reason to avoid the drug; the physiological dependence that develops in anyone after weeks on it is a reason to plan the taper before the first dose.
• Withdrawal if stopped abruptly: anxiety, insomnia, nausea, pain, sweating, tremor, headache, depression (SmPC 4.4); the NHS list adds agitation, panic, palpitations, shaking, body aches and restlessness. In case reports it began 12 hours to 7 days after stopping (Mersfelder 2016), typically within 1 to 2 days. In an epilepsy patient sudden withdrawal can provoke seizures; the patient does not have epilepsy, but a mass touching the brain is a reason not to test that.
• The taper: the SmPC minimum is one week for any dose and any indication. Judgement, matching the SmPC titration times in reverse: from 900 mg, drop by 300 mg every 2 to 3 days over about a week; from 1800 mg, over 2 weeks; from higher doses, over 3 weeks or more. If she is being switched to pregabalin the two can be cross-tapered overnight by most pain teams because the mechanism is the same; that is a prescriber decision.

7. Monitoring and stop rules the family can use

The two-line daily diary (extend the one already proposed in thread 08):

• Line 1, morning: hours slept; worst pain overnight (0 to 10); doses taken since yesterday's entry (gabapentin, Oramorph with times, diclofenac, others).
• Line 2, evening: average pain today (0 to 10); clarity today (0 to 10, where 10 is "entirely herself", scored by the patient and by her partner separately if they differ); any of: drowsy, dizzy, unsteady, double vision worse, ankle swelling, nausea, mood change, word-finding trouble; breathing normal at night, yes or no.

Rules (judgement, built from the SmPC, MHRA advice and the Cochrane rates; agree them with the prescriber and have them written in the notes or on the discharge letter so nobody has to improvise):

• Hold a step: if on the day of a planned increase she is drowsier, dizzier or foggier than 2 days earlier, or has had a fall or near-fall, or her diplopia is worse, or she has needed more Oramorph than usual. Stay at the current dose another 3 to 7 days and re-assess. Holding is free; stepping up too early is what generates the fog.
• Drop back one step: if drowsiness, dizziness or fog persist for more than 7 days at a steady dose without improving; if clarity scores stay 2 or more points below her pre-gabapentin baseline for a week; if new ankle swelling is troublesome; if she is sleeping in the daytime and awake at night.
• Stop (taper) and swap: no pain benefit at all after 2 weeks at 1800 mg a day (or the highest dose she tolerates); or fog or drowsiness that does not lift after a step down; or any mood change that the patient or her partner recognises as not her; or worsening diplopia that the doctors attribute to the drug. Swap to duloxetine (least sedating) or nortriptyline (helps sleep), or to pregabalin if the problem was unpredictable absorption rather than side effects; section 8.
• Stop immediately and call: slow, shallow or paused breathing; cannot be roused normally; rash with fever or swollen glands; lip or throat swelling; persistent severe stomach pain; hallucinations; thoughts of self-harm.
• The honest signal that it is working: not "pain gone". It is a fall in the average daily pain score of 2 points or more from baseline, or a 30 percent fall, sustained for a week; fewer Oramorph doses needed; sleeping through more of the night; and, often before the score moves, a change in the quality of the pain (less burning, less electric, less "alive"). Caraceni's trial found the dysaesthesia score improved before anything else. If the score does not move but she is using half the morphine, that is also a response.
• What dose most responders need: in the placebo-controlled trials the working range was 1800 to 3600 mg a day (Cochrane), and in Gilron 2005 the average maximum tolerated dose fell when morphine was on board. In opioid-treated cancer patients the trial dose was 600 to 1800 mg (Caraceni). Judgement: expect a first signal at 900 mg and a fair verdict at 1800 mg; if 1800 mg is clearly helping but not enough, the pain team can decide whether to go higher or add a second agent, and the Gilron evidence favours adding rather than pushing.

8. The alternatives if it fails or fogs her

• Pregabalin. Same mechanism, same class, same MHRA warnings, same renal clearance. The differences are kinetic: absorbed fast and completely at all doses (bioavailability 90 percent or more vs gabapentin's 60 falling to 33 percent), peak at 1 hour, linear dose response, twice-daily dosing, useful dose (150 mg a day) reached in about a week (Bockbrader 2010, PMID 20818832). Cochrane (Derry 2019, PMID 30673120): somnolence 11 to 15 percent, dizziness 13 to 22 percent, about 1 in 10 stopping for side effects; efficacy of the same order as gabapentin. It is the right swap if gabapentin's problem was inconsistent effect or too slow a climb; it is not a swap for sedation, because the sedation is the same. Weight gain and oedema are somewhat more prominent.
• Duloxetine. The least sedating first-line option (Lunn 2014, PMID 24385423; Finnerup 2015, NNT 6.4). Nausea in about 3 in 10 for the first weeks, reduced appetite in some, 16 percent stop. Watch appetite in a woman losing weight, and sodium. Bleeding risk with the NSAID rises a little (a PPI is already in place). No serotonergic drugs alongside (no tramadol). The right swap when the problem was fog.
• Nortriptyline or amitriptyline. Tricyclics have the best NNT (Finnerup 2015) and were equivalent to gabapentinoids in head-to-head trials with fewer dropouts than SNRIs (Sadegh 2024). Nortriptyline 10 mg at night is the least sedating version and the one to try if sleep is the goal and daytime clarity the constraint; amitriptyline if more sedation is wanted at night. Dry mouth, constipation (stacking with morphine), postural dizziness; ECG at baseline. Gilron 2009 shows nortriptyline plus a low dose of gabapentin beats either alone, which is the natural next move if gabapentin half-works.
• Oxcarbazepine (or carbamazepine). Only if a clearly paroxysmal, trigger-evoked, electric component emerges, because sodium-channel blockers are the drugs for that and gabapentin is not (Bendtsen 2019). Oxcarbazepine over carbamazepine because of enzyme induction, with sodium checks; details in thread 08.
• Non-drug and procedural adjuncts, all with zero cognitive cost and all compatible with gabapentin: lidocaine 5 percent plaster to cheek and temple; intranasal lidocaine for flares; sphenopalatine ganglion block and greater occipital nerve block (Lemos 2008 is the evidence that a block lowers the gabapentin dose needed); heat and positioning for the neck spasm; physiotherapy; an eye patch or Fresnel prism for the diplopia; protected sleep. Detail and sources in thread 08, sections 1j and 3.

9. Questions for the prescriber, and the bottom line

Questions to ask (one action per message in the room; here as a list):

1. What starting dose and what schedule, in writing? We would like 100 to 300 mg at night for the first three nights, then a step every 3 to 7 days, not the day 1, 2, 3 label schedule, because she is 51 kg and on morphine.
2. What is her eGFR, and has the dose been checked against the SmPC renal table using her actual weight?
3. What is the target dose for a fair trial, and at what point do we call it a failure? (Our suggestion: verdict at 1800 mg after 2 weeks.)
4. Will respiratory rate and a sedation score be recorded at every set of obs for the first 72 hours and after each increase? At home, what breathing signs should make us stop both drugs and call?
5. Can a baseline description of her eye movements and double vision be written down before the first dose, so we can tell drug effect from tumour effect?
6. Does the morphine plan change? Morphine raises gabapentin levels by nearly half, and the two together allow lower doses of each. Should the Oramorph become a low regular dose rather than as needed, and should the total stay where it is while gabapentin climbs?
7. What is the stop rule for fog or drowsiness, and what is the swap: duloxetine or nortriptyline? Will that be written in the discharge letter so the GP can act without another referral?
8. Can a sphenopalatine ganglion or occipital nerve block be done alongside, so she may need less gabapentin?
9. Who prescribes the ongoing supply, given the 28-day controlled-drug rules, and who reviews her at 2 weeks?
10. How will we taper if it does not suit her?

The bottom line for her partner and the patient, as honestly as it can be put:

• A trial is reasonable. Gabapentin is one of four NICE first-line drugs for nerve pain, none of which is proven better than the others; it is the only one with a placebo-controlled trial in people already on opioids for neuropathic cancer pain, where it helped within 10 days at modest doses; it acts where several irritated nerves converge rather than on any one of them, which fits her anatomy; it has no liver or drug-level interactions with anything she may need later; and the weight gain most people dislike would suit her.
• The costs are real and specific. At working doses about 1 in 5 people are dizzy and about 1 in 7 are drowsy, against roughly 1 in 15 and 1 in 20 on placebo; about 1 in 10 stop because of side effects; over half get no worthwhile relief. Blurred or double vision is a common effect in a woman who already has diplopia. With morphine the sedation adds up and the MHRA has warned about breathing; the population risk is measurable, the individual risk at her doses under supervision is small but is the reason the morphine stays modest and someone watches her breathing in the first nights.
• The conditions that make it safe enough: kidney function known before the climb; a slow, night-first schedule; one step at a time on a plateau; a written eye baseline; breathing watched for 72 hours after each change; the diary kept; and the stop rule and the swap agreed in advance and written down.
• What would make us stop: fog or drowsiness that does not lift after a step down and a week's wait; no pain signal at all after two weeks at 1800 mg; any change in mood that is not her; worsening double vision the doctors attribute to the drug; or any breathing, rash or allergic event, which stops it the same hour.
• What gabapentin will not do: it will not treat the tumour, it will not work in a day, and it will not remove the need for the anti-inflammatory and a steady, low morphine dose while it builds. It is one layer, added slowly, and the whole point of the diary is that if it costs the patient more than it gives, that becomes obvious within two weeks and it comes off again over one more.

10. Sources

Licence, regulator and guideline documents - Neurontin 300 mg hard capsules, Summary of Product Characteristics (Viatris), electronic medicines compendium. Sections 4.2, 4.4, 4.5, 4.8, 5.2. https://www.medicines.org.uk/emc/product/3195/smpc (fetched 5 Sep 2026) - MHRA Drug Safety Update, 26 October 2017. Gabapentin (Neurontin): risk of severe respiratory depression. https://www.gov.uk/drug-safety-update/gabapentin-neurontin-risk-of-severe-respiratory-depression (fetched) - MHRA Drug Safety Update, 16 April 2019. Pregabalin (Lyrica), gabapentin (Neurontin) and risk of abuse and dependence: new scheduling requirements from 1 April. https://www.gov.uk/drug-safety-update/pregabalin-lyrica-gabapentin-neurontin-and-risk-of-abuse-and-dependence-new-scheduling-requirements-from-1-april (fetched) - Home Office and Department of Health and Social Care, 15 October 2018. Pregabalin and gabapentin to be controlled as Class C drugs. https://www.gov.uk/government/news/pregabalin-and-gabapentin-to-be-controlled-as-class-c-drugs (fetched) - GOV.UK. Drugs and driving: the law. https://www.gov.uk/drug-driving-law (fetched) - NHS. Gabapentin: how and when to take it; side effects; common questions. https://www.nhs.uk/medicines/gabapentin/ (fetched) - NICE CG173. Neuropathic pain in adults: pharmacological management in non-specialist settings. 2013, updated 2020. https://www.nice.org.uk/guidance/cg173 (NICE site returned 403 today; recommendations carried from thread 08 where they were verified) - NICE NG193. Chronic pain (primary and secondary) in over 16s. 2021. https://www.nice.org.uk/guidance/ng193 - Bendtsen L et al. European Academy of Neurology guideline on trigeminal neuralgia. European Journal of Neurology 2019. PMID 30860637.

Systematic reviews and meta-analyses - Wiffen PJ, Derry S, Bell RF et al. Gabapentin for chronic neuropathic pain in adults. Cochrane Database of Systematic Reviews 2017, CD007938. PMID 28597471. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD007938.pub4/full - Finnerup NB et al. Pharmacotherapy for neuropathic pain in adults: a systematic review and meta-analysis. Lancet Neurology 2015. PMID 25575710. - Derry S et al. Pregabalin for neuropathic pain in adults. Cochrane 2019, CD007076. PMID 30673120. - Moore RA et al. Amitriptyline for neuropathic pain in adults. Cochrane 2015, CD008242. PMID 26146793. - Lunn MPT et al. Duloxetine for treating painful neuropathy, chronic pain or fibromyalgia. Cochrane 2014, CD007115. PMID 24385423. - Chaparro LE et al. Combination pharmacotherapy for the treatment of neuropathic pain in adults. Cochrane 2012, CD008943. PMID 22786518. - Sadegh AA et al. A systematic review and meta-analysis of randomized controlled head-to-head trials of recommended drugs for neuropathic pain. Pain Reports 2024. PMID 38932764. - Quilici S et al. Meta-analysis of duloxetine vs pregabalin and gabapentin in the treatment of diabetic peripheral neuropathic pain. BMC Neurology 2009. PMID 19208243. - Kane CM et al. Opioids combined with antidepressants or antiepileptic drugs for cancer pain: systematic review and meta-analysis. Palliative Medicine 2018. PMID 28604172. - Zhao X et al. The efficacy and safety of gabapentin vs carbamazepine in patients with primary trigeminal neuralgia: a systematic review and meta-analysis. Frontiers in Neurology 2023. PMID 37200780. - Tambon M et al. Non-opioid antinociceptive drugs: risk of respiratory depression and death related to concomitant use of gabapentinoids in addition to opioids. Expert Opinion on Drug Safety 2023. PMID 36932941. - Mersfelder TL, Nichols WH. Gabapentin: abuse, dependence, and withdrawal. Annals of Pharmacotherapy 2016. PMID 26721643. - Evoy KE et al. Abuse and misuse of pregabalin and gabapentin: a systematic review update. Drugs 2021. PMID 33215352. - Bockbrader HN et al. A comparison of the pharmacokinetics and pharmacodynamics of pregabalin and gabapentin. Clinical Pharmacokinetics 2010. PMID 20818832.

Trials and cohort studies - Caraceni A et al. Gabapentin for neuropathic cancer pain: a randomized controlled trial from the Gabapentin Cancer Pain Study Group. Journal of Clinical Oncology 2004. PMID 15254060. - Mishra S et al. A comparative efficacy of amitriptyline, gabapentin, and pregabalin in neuropathic cancer pain: a prospective randomized double-blind placebo-controlled study. American Journal of Hospice and Palliative Care 2012. PMID 21745832. - Gilron I et al. Morphine, gabapentin, or their combination for neuropathic pain. New England Journal of Medicine 2005. PMID 15800228. - Gilron I et al. Nortriptyline and gabapentin, alone and in combination for neuropathic pain: a double-blind, randomised controlled crossover trial. Lancet 2009. PMID 19796802. - Tesfaye S et al. OPTION-DM: amitriptyline supplemented with pregabalin, pregabalin supplemented with amitriptyline, and duloxetine supplemented with pregabalin for diabetic peripheral neuropathic pain. Lancet 2022. PMID 36007534. - Lemos L et al. Gabapentin supplemented with ropivacaine block of trigger points improves pain control and quality of life in trigeminal neuralgia patients when compared with gabapentin alone. Clinical Journal of Pain 2008. PMID 18180639. - Eckhardt K et al. Gabapentin enhances the analgesic effect of morphine in healthy volunteers. Anesthesia and Analgesia 2000. PMID 10866910. - Gomes T et al. Gabapentin, opioids, and the risk of opioid-related death: a population-based nested case-control study. PLoS Medicine 2017. PMID 28972983. - Gomes T et al. Pregabalin and the risk for opioid-related death: a nested case-control study. Annals of Internal Medicine 2018. PMID 30140853. - Salinsky MC et al. Effects of topiramate and gabapentin on cognitive abilities in healthy volunteers. Neurology 2005. PMID 15753411. - Meador KJ et al. Comparative cognitive effects of carbamazepine and gabapentin in healthy senior adults. Neurology 2001. PMID 11422333. - DeToledo JC et al. Changes in body weight with chronic, high-dose gabapentin therapy. Therapeutic Drug Monitoring 1997. PMID 9263379. - Gibbons RD et al. Relationship between antiepileptic drugs and suicide attempts in patients with bipolar disorder. Archives of General Psychiatry 2009. PMID 19996040.

Mechanism (cited from general knowledge of the papers, not re-fetched today) - Sills GJ. The mechanisms of action of gabapentin and pregabalin. Current Opinion in Pharmacology 2006. PMID 16376147. - Field MJ et al. Identification of the α2δ-1 subunit of voltage-dependent calcium channels as a molecular target for pain mediating the analgesic actions of pregabalin. PNAS 2006. PMID 17088553. - US FDA, 2008. Alert on suicidality and antiepileptic drugs (pooled analysis of 199 placebo-controlled trials).

Caveats - No trial exists in skull-base tumour facial pain. Rates are from diabetic neuropathy, post-herpetic neuralgia, trigeminal neuralgia, neuropathic cancer pain and epilepsy. - The slow titration schedule, the 1800 mg verdict point, the "2 to 4 weeks to settle" time course, the diary and the stop rules are clinical judgement assembled from the licence, the MHRA advice and the trial dosing, not single citations. They are proposals for the prescriber to accept, change or reject. - NICE pages were not reachable today; CG173 wording is from thread 08. - The DVLA position on diplopia is stated from general knowledge of the fitness-to-drive standards and was not re-fetched.