Gabapentin

Observational studies

The efficacy of gabapentin in dosages up to 3600 mg/day as adjunctive therapy has been studied in 2016 patients with partial seizures [ ]. The four most commonly reported adverse events were somnolence (15%), dizziness (10%), weakness (5.8%), and headache (4.5%).

In an uncontrolled trial using dosages up to 6000 mg/day in 50 patients with refractory partial epilepsy, tiredness, dizziness, headache, and diplopia were the most common adverse reactions [ ]. At dosages above 3600 mg/day three patients developed flatulence and diarrhea and two had myoclonic jerks. At least in some patients, gabapentin gastrointestinal absorption did not become saturated within the explored dosage range.

The effectiveness of gabapentin has been studied in 22 patients with bipolar disorder who had an incomplete response to other mood stabilizers [ ]. Somnolence was common (six patients); adverse events that occurred in two patients each included irritability, memory impairment, headache, and tremor. One patient dropped out because of a mild rash.

In 237 children aged 3–12 years with refractory partial seizures gabapentin 24–70 mg/kg/day was used as add-on therapy over 6 months [ ]. There was a more than 50% reduction in partial seizures in 34% of the children. Somnolence was the most common adverse event related to gabapentin. Emotional lability and hostility were related to gabapentin in 3.4% and 3.0% of the patients respectively.

In a non-interventional observational cohort study, using the technique of prescription-event monitoring, patients taking gabapentin were identified from dispensed National Health Service prescriptions [ ]. Outcome data were obtained from questionnaires sent to the doctor 6 months after the initial gabapentin prescription. The cohort included 3100 patients, of whom 136 (4%) were children. The median duration of treatment was 8.1 months. The most frequently reported adverse events during the first month of treatment were neurological: drowsiness/sedation (6.7% of patients), headache/migraine (3.6%), malaise/lassitude (3.5%), dizziness (2.4%), and nausea/vomiting (2.6%). These events were also the commonest reasons for withdrawal of gabapentin and were reported as suspected adverse drug reactions. There were no congenital anomalies in the 11 babies born to women who used gabapentin during the first trimester of pregnancy. The crude mortality rate was five times that in the general population but similar to that in other published studies in patients with epilepsy. Thus, this report does not disclose new unrecognized adverse reactions to gabapentin.

The efficacy of gabapentin 600–900 mg/day for 31 days has been evaluated in 16 patients with neurogenic overactive bladder; there were no significant adverse reactions [ ].

A single preoperative dose of gabapentin 800 mg 2 hours before surgery did not augment postoperative analgesia in 60 patients given interscalene brachial plexus blocks for arthroscopic shoulder surgery in a placebo-controlled study, although headaches were less frequent in those who were given gabapentin (n = 27) [ ]. The incidence of adverse reactions was comparable in the two groups.

In 41 patients with postamputation pain, gabapentin up to 2400 mg/day during the first 30 postoperative days was ineffective [ ]. Adverse events (nausea, stomach ache, fatigue, confusion, nightmares, itching, ataxia) were transient and occurred in nine patients who received gabapentin, and in eight who received placebo. Four patients were withdrawn (two in each group). Medication was reduced or temporarily stopped in seven (gabapentin n = 5; placebo n = 2) because of adverse reactions.

The use of gabapentin 900 mg/day in cardiothoracic surgery has been assessed in 60 consecutive out-patients with refractory pain persisting at 4 weeks or more after thoracic surgery or trauma [ ]. Follow-up data (median 21, range 12–28 months), were available for 45 patients. The mean duration of gabapentin use was 22 (range 1–68) weeks. There were no deaths or major complications. There were minor adverse reactions, mostly somnolence and dizziness, in 18 patients, and had to discontinue gabapentin. Satisfaction with gabapentin was expressed by 40 patients. Adverse reactions were not a source of dissatisfaction in any patient.

Snellen visual acuity, subjective visual function, and eye movement recordings have been retrospectively analysed in 23 patients with nystagmus (13 secondary to multiple sclerosis, three associated with other neurological diseases, two idiopathic infantile, and five with associated ocular diseases) treated with gabapentin (600–2400 mg/day); three had additional memantine 10–20 mg/day [ ]. Adverse reactions to gabapentin were mild, and mostly consisted of transient fatigue. Three patients discontinued gabapentin because of adverse events such as fatigue, memory loss, tingling, and numbness.

When gabapentin was used to treat chronic irritability and recurrent pain in nine neurologically impaired children, care-givers reported marked improvement; one child developed nystagmus and withdrawal was required [ ].

Of eight patients who were refractory to first-line treatment in whom gabapentin was used as second-line treatment for chronic cluster headache in a prospective study, six responded; one had transient drowsiness at high doses and one had sexual dysfunction [ ].

Gabapentin (initially 900 mg/day, increasing up to a maximum of 2400 mg/day) has been evaluated in 55 patients with lumbar spinal stenosis [ ]. Gabapentin increased walking distance and improved pain scores. The most frequent adverse reactions were drowsiness and dizziness, and two patients had transient ataxia. There were no withdrawals.

Comparative studies

Gabapentin and lamotrigine have been compared in an open, parallel-group, add-on, randomized study in 109 patients with uncontrolled partial epilepsy and learning disabilities [ ]. The two drugs were similarly efficacious, with similar incidences of adverse events and serious adverse events. Neither lamotrigine nor gabapentin exacerbated any of the challenging behaviors observed in these patients. The most common adverse reaction to gabapentin was somnolence, which was mostly reported during the initial titration phase.

In a double-blind comparison of gabapentin and lamotrigine in 309 patients with new-onset partial or generalized seizures, the target doses were gabapentin 1800 mg/day and lamotrigine 150 mg/day [ ]. Severe adverse events were reported in 11% of patients taking gabapentin and 9.3% of patients taking lamotrigine. Two patients had serious adverse events thought to be related to the study drug; one took an overdose of gabapentin and the other had convulsions with lamotrigine. The most frequent treatment-related adverse events in both treatment groups were dizziness, weakness, and headache; 11% of patients taking gabapentin and 15% of those taking lamotrigine withdrew because of adverse events. There was an increase of over 7% in body weight from baseline in 14% of the patients taking gabapentin and 6.6% of those taking lamotrigine. There were benign rashes in 4.4% of those taking gabapentin and 11% of those taking lamotrigine.

The hypothesis that both amitriptyline and gabapentin are more effective in relieving neuropathic pain than diphenhydramine has been tested in a randomized, double-blind, triple crossover, 8-week trial in 38 adults with spinal cord injuries [ ]. Maximum daily doses were 2600 mg for gabapentin and 150 mg for amitriptyline. Amitriptyline was more efficacious in relieving neuropathic pain than diphenhydramine. Withdrawal because of possible adverse reactions occurred five times during gabapentin treatment: (1) shortness of breath; (2) dizziness, fatigue, and nausea; (3) increased spasticity and pain; (4) fatigue, drowsiness, constipation, and dry mouth; and (5) severe itching. The four most frequent adverse events were dry mouth, drowsiness, fatigue, and constipation, which were all more common with amitriptyline.

Placebo-controlled studies

In 99 patients with cocaine dependence randomized to either gabapentin 3200 mg/day or placebo for 12 weeks, gabapentin was very well tolerated [ ]. The most frequently reported adverse reactions were dizziness (10%) and tiredness/sedation (8%). There were five serious adverse events, four with gabapentin; three were transient (chest pain, bloody stools, calf pain) and one patient dropped out because of depression with suicidal tendencies; none of these adverse events was judged to be related to gabapentin.

The effect of gabapentin on the perception of pruritus has been tested in a double-blind, randomized, placebo-controlled trial in 16 women [ ]. The starting dose of gabapentin was 300 mg/day in divided doses and it was increased to a maximum of 2400 mg/day or until the pruritus was relieved. Gabapentin was associated with an increased perception of pruritus.

Gabapentin has been used as adjunctive treatment in postoperative pain to reduce the occurrence of postoperative delirium in a double-blind, randomized, placebo-controlled trial in 21 patients [ ]. There was postoperative delirium in 5/12 patients who received placebo versus 0/9 patients who received gabapentin. Two patients (one in each group) had postoperative sedation. None had dizziness, nystagmus, or ataxia.

In a randomized, double-blind, placebo-controlled study, 49 patients with tonsillectomy received gabapentin 1200 mg preoperatively, followed by gabapentin 600 mg bd on the day of operation and gabapentin 600 mg tds for the next 5 days [ ]. Both groups were given rofecoxib 50 mg/day. Postoperatively intravenous morphine was administered in doses of 2.5 mg on request and from 4 hours to 5 days postoperatively, ketobemidone was offered as escape therapy. As a result of the global withdrawal of rofecoxib, the study had to be terminated prematurely when 22 patients had been randomized to gabapentin and 27 to placebo. Gabapentin caused more dizziness, gait disturbance, and vomiting during days 0–5 than placebo.

The effects of different doses of gabapentin on muscle and cutaneous pain have been studied in 16 healthy volunteers in a double-blind, three-session, placebo-controlled crossover comparison [ ]. Muscle pain was induced by infusing 0.5 ml of hypertonic saline into the anterior tibial muscle. Gabapentin pretreatment reduced the sensitivity to electrical induction of skin pain by 14%. Secondary hyperalgesia was also abated. Mechanical pain thresholds were unaffected. Pain induced by intramuscular infusion of hypertonic saline was not affected by gabapentin. Adverse reactions (dizziness, tiredness, weakness, and fatigue) were mild and transient, but more frequent with higher doses of gabapentin: placebo—dizziness (n = 1), tiredness (n = 1); 1200 mg gabapentin single dose—dizziness (n = 5), tiredness (n = 1), fatigue (n = 1); 2600 mg gabapentin four-dose regimen—dizziness (n = 9), tiredness (n = 7), weakness (n = 2), fatigue (n = 1).

Gabapentin (titrated to 2400 mg/day) and conjugated estrogens (0.625 mg/day) have been compared in the treatment of hot flushes in a randomized, double-blind, placebo-controlled trial for 12 weeks in 60 postmenopausal women [ ]. Headache, dizziness, and disorientation were more common with gabapentin; the NNT _H was 4. Other reported adverse reactions in the gabapentin group were gastrointestinal symptoms (n = 4); pain (n = 1); cold, flu, allergy (n = 7); sleep disturbances (n = 4); weight gain and/or edema (n = 2); hormonal problems (n = 2); other (n = 2).

In a double-blind, crossover, placebo-controlled study 6 patients with primary orthostatic tremor were studied while taking gabapentin 600–2700 mg/day, 7 days after drug withdrawal, and again after two 2-week periods of treatment with either gabapentin or placebo, using force platform posturography to quantify postural sway and tremor [ ]. Other tremor medications were continued unchanged. Quality of life improved in all patients. There were no adverse drug-related events and symptomatic benefit was maintained at follow-up (mean 19 months).

In a prospective study 40 patients undergoing lower extremity surgery procedures were randomly assigned to either placebo or oral gabapentin 1200 mg/day before and for 2 days as an adjunct to epidural analgesia after surgery [ ]. Gabapentin reduced pain and analgesic consumption and increased patient satisfaction. The incidence of dizziness was higher with gabapentin (n = 7 versus 1).

The effect of gabapentin in perioperative pain management has been studied in 100 patients undergoing abdominal hysterectomy, who were randomly assigned to one of four treatment groups: placebo before and for 2 days after surgery; rofecoxib 50 mg/day before and after surgery; gabapentin 1200 mg/day before and after surgery; rofecoxib 50 mg/day + gabapentin 1200 mg/day before and after surgery [ ]. The following adverse reactions were attributed to gabapentin (n = 25): dizziness (n = 4), nausea (n = 12), vomiting (n = 9), somnolence (n = 5), diarrhea (n = 2), pruritus (n = 1), urinary retention (n = 1), dry mouth (n = 4). Adverse events observed in the combination group were: dizziness (n = 6), nausea (n = 10), vomiting (n = 7), somnolence (n = 5), diarrhea (n = 2), pruritus (n = 1), urinary retention (n = 1), dry mouth (n = 3).

The efficacy and safety of gabapentin in doses up to 2400 mg/day for neuropathic pain caused by traumatic or postsurgical peripheral nerve injury have been investigated in a double-blind, crossover, randomized, placebo-controlled study in 120 patients [ ]. There was no statistically significant difference in the primary outcome of efficacy, although gabapentin provided significantly better pain relief than placebo. In 11 patients treatment was withdrawn because of adverse events. During gabapentin run-in, one patient had diplopia, ataxia, amnesia, and dysarthria, one had nausea and vomiting, one had dizziness and vertigo, and one had erythema. During gabapentin maintenance treatment, one patient was withdrawn because of cough and another because of vertigo. The most commonly reported adverse events during the gabapentin period were dizziness and vertigo, reported by 33% of the patients, and malaise and tiredness (26%), compared with 7.5% and 14% respectively during placebo treatment.

Gabapentin (1200–2400 mg/day) and placebo have been compared in a 12-week, double-blind, randomized study in 150 patients with pain associated with fibromyalgia [ ]. Gabapentin produced greater improvement in average pain severity scores. Adverse events necessitated withdrawal in 12 patients taking gabapentin and 7 taking placebo. Gabapentin was associated with significantly more dizziness (19 versus 7 patients), sedation (18 versus 3 patients), light-headedness (11 versus 1), and weight gain (6 versus 0).

Gabapentin (300 mg/day initially, increasing to a maximum of 4200 mg/day) and placebo have been compared in a 12-week randomized, placebo-controlled trial in 50 patients with chronic masticatory myalgia [ ]. Gabapentin was better than placebo in reducing pain and masticatory muscle hyperalgesia. The most frequent adverse reactions were dizziness (in seven patients taking gabapentin and two patients taking placebo), drowsiness (n = 7 and 5 respectively), impairment of memory and cognition (4 and 1), dry mouth (3 and 1), and fatigue (3 and 2).

The effects of gabapentin 800 mg and dexamethasone 20 mg on postoperative pain, given together or separately 1 hour before the start of varicocele surgery, have been investigated in a randomized, double-blind, placebo-controlled trial in 60 patients [ ]. Gabapentin + dexamethasone improved postoperative analgesia and prevented postoperative nausea and vomiting better than either drug alone. The most frequent adverse reactions were vomiting, nausea, and dry mouth, which were significantly more common with placebo. Headache and pruritus occurred with similar frequencies in all four groups.

The effects of gabapentin 1200 mg alone and gabapentin 1200 mg + paracetamol 20 mg/kg, 1 hour before surgery, on postoperative pain and morphine consumption in patients undergoing abdominal hysterectomy have been compared in a three-arm, double-blind, placebo-controlled study in 75 patients [ ]. Gabapentin alone and in combination with paracetamol reduced opioid requirements and increased patients’ satisfaction postoperatively. There were no significant differences between the groups in terms of adverse reactions.

The analgesic effect of gabapentin 1.2 g before the operation has been studied in patients undergoing elective hand surgery with intravenous regional anesthesia in a double-blind, placebo-controlled study [ ]. Gabapentin was significantly superior to placebo in several measures of efficacy. Two of those who were given gabapentin and three of those who were given placebo had nausea that required antiemetic drugs and two of those who were given gabapentin complained of dizziness. Another postoperative complaint was dry mouth in four patients given gabapentin and two given placebo. None reported being excessively drowsy after surgery.

Gabapentin 600 mg has been assessed in the prevention of high-altitude headache in a placebo-controlled randomized study an altitude of 3500 m in 202 unacclimatized hotel guests aged 15–65 years [ ]. The total incidence of headaches was not significantly affected by gabapentin, but there was a reduction in the incidence of moderate/severe high-altitude headache. Somnolence was the most common adverse reaction and was more common among gabapentin users. Dizziness, fatigue, and gastrointestinal adverse reactions were not significantly different between the groups.

Gabapentin 2400 mg/day and tiagabine 24 mg/day in reducing cocaine use have been evaluated in a three-arm, 10-week, double-blind, placebo-controlled study in 76 cocaine-dependent methadone-treated patients [ ]. Treatment retention was significantly reduced by gabapentin, but it was ineffective in reducing cocaine use. Adverse reactions were not reported.

Gabapentin up to 1200 mg/day for 8 days has been evaluated as an alternative to benzodiazepines in the treatment of alcohol withdrawal in 35 non-treatment-seeking alcoholic subjects in a double-blind placebo-controlled study [ ]. Gabapentin had no overall effect on drinking or craving. However, no adverse reactions from the combination of gabapentin with alcohol were reported.

The effects of memantine 10–40 mg/day and gabapentin up to 2400 mg/day on congenital nystagmus have been studied in 48 patients in a randomized, three-arm, double-masked, placebo-controlled study over 56 days [ ]. Both drugs improved visual acuity and reduced the intensity of nystagmus. In 16 patients given gabapentin nine had adverse reactions: each felt dizzy, or tired, or sleepless, or light-headed, nauseated, forgetful, and shaky, had headaches, or was depressed; two had to reduce the dosage. Of 15 subjects who took placebo, five had adverse reactions (dizziness, tiredness, light-headedness, nausea, and headaches).

Systematic reviews

The effects of perioperative gabapentin for the control of acute postoperative pain in six studies have been reviewed [ ]. Single preoperative doses of gabapentin 1200 mg were associated with lower risks of vomiting and urinary retention. There was no difference between gabapentin and controls for other adverse events. Five studies used a single preoperative dose of gabapentin under 1200 mg (range 300–900 mg). The risk of sedation was higher than in controls; for other adverse reactions there was no difference. Gabapentin given as multiple doses perioperatively was investigated in five studies and was associated with lower incidences of pruritus and nausea. In one study there was a lower incidence of constipation with gabapentin and no difference for other adverse events. All 16 studies reported nausea as an adverse outcome, and one reported nausea and vomiting as a single adverse event. Gabapentin caused less vomiting than control treatments. The incidence of sedation was higher with gabapentin. Six studies reported the incidence of pruritus: gabapentin was associated with less pruritus. Four trials reported urinary retention, three trials reported constipation, and two reported respiratory depression as adverse events; there were no significant differences between the groups.

In a meta-analysis of 12 randomized controlled trials in 896 patients of the analgesic effects of gabapentin in perioperative and postoperative pain management, preoperative doses were 300–1200 mg and postoperative doses 1200–1800 mg/day for 2–10 days [ ]. Gabapentin was associated with sedation and anxiolysis (OR = 3.28; CI = 1.21, 8.87) but no difference in lightheadedness, dizziness, nausea, or vomiting.

Ear, nose, throat

Rhinitis is uncommon with gabapentin [ ].

Nervous system

Drowsiness, dizziness, fatigue, or ataxia are seen in 10–20% of patients who use gabapentin [ , ]. Of 240 adults followed for about 1 year, only 4% discontinued treatment because of adverse events [ ]. Adverse reactions reported by more than 10% of patients were nystagmus, somnolence, diplopia, tremor, ataxia, and dizziness.

• A 57-year-old woman with diabetes mellitus and uremia on regular hemodialysis, who had bilateral leg dysesthesia, developed severe dizziness and lethargy after a single dose of gabapentin 75 mg; she recovered rapidly after one session of hemodialysis [ ].

Gabapentin occasionally exacerbates absence and myoclonic seizures and causes de novo myoclonus [ ]. Myoclonus has been studied in 104 patients taking gabapentin for epilepsy [ ]. There were 13 cases of mild myoclonus, which did not significantly interfere with daily activities. All the patients had refractory epilepsy and were taking other antiepileptic drugs. Six had a severe chronic static encephalopathy; five had no medical diagnosis other than seizures. Ten developed multifocal myoclonus and three developed focal myoclonus, contralateral to their epileptic focus. Two had an exacerbation of pre-existing myoclonus. Withdrawal led to rapid resolution.

Choreoathetosis, dyskinesias, dystonic or myoclonic movement disorders (including oculogyric crises), and stuttering have been reported rarely [ , ].

• Sensory neuropathy occurred in a 58-year-old man who had been given up to 2400 mg/day for over 5 months for the treatment of head pain [ ]. A mild pruritic rash had been present since starting treatment. Neuropathic symptoms included a burning sensation in the legs and hips. After withdrawal, reduced perception of tactile and noxious stimuli and neurogenic bladder dysfunction (with an associated syncopal episode) were recorded. The neuropathy improved over several months.

Gabapentin is used often to treat neuropathic pain, and its role in causing the sensory neuropathy in this patient was uncertain.

Gabapentin-induced myoclonus may be more common than initially thought. In a retrospective survey, 13 of 104 consecutive patients developed myoclonus at gabapentin dosages of 800–3200 mg/day [ ]. Six of the patients had severe chronic static encephalopathy. Three had focal myoclonus, contralateral to the epileptic focus, and two had exacerbation of pre-existent myoclonus. In all cases the myoclonus was mild and did not interfere with daily activities. The fact that in three patients the electroencephalogram did not show epileptiform discharges suggests that, at least in some cases, the myoclonus is non-epileptic in nature.

Life-threatening myoclonic status has been attributed to gabapentin [ ].

• A 57-year-old man who had hand tremors and jerks involving the upper limbs was given gabapentin 900 mg/day and after 3 days developed continuous, fast-frequency, high-amplitude jerking of the trunk and extremities, which severely impaired his normal activities. Progressive confusion, agitation, and disorientation developed soon after. Five days later he was admitted to the hospital. He was afebrile and had high-frequency, continuous, erratic, multifocal myoclonus, which was asynchronous in different parts of the body. Electroencephalography showed diffuse theta slowing and multifocal spikes. The diagnosis was cortical tremor. He was given thiopental after intubation and ventilation. Gabapentin was withdrawn. Genetic studies showed that he had a haplotype that co-segregates with benign adult familial myoclonic epilepsy. He was treated with levetiracetam 1000 mg/day, which controlled his symptoms.

There have been several reports of gabapentin-induced chorea. A patient with Parkinson’s disease developed new-onset dyskinesia (choreoathetosis involving both upper and lower limbs) when gabapentin was introduced, with full resolution when it was withdrawn [ ].

Three further cases of chorea have been reported in patients without extrapyramidal disorders taking gabapentin.

• A 46-year-old woman developed chorea in her neck, trunk, and limbs soon after taking gabapentin for complex regional pain syndrome in her left arm [ ]. The chorea lasted for 1 year and resolved completely within 2 weeks of withdrawal of gabapentin.

• A 68-year-old hypertensive woman, who reported lumbar pain radiated to the legs associated with paresthesia, developed chorea 30 days after starting to take gabapentin; it resolved 1 week after drug withdrawal [ ].

• A 41-year-old man with a thoracic cord lesion and subsequent severe neuropathic pain had right-sided hemichorea after taking gabapentin; it abated gradually after drug withdrawal [ ]. An MRI scan was normal but functional imaging of regional cerebral blood flow showed hypoperfusion of the left basal ganglia, especially the left caudate.

In a comparison of twice- and thrice-daily gabapentin, 29 stable responders were selected and followed for 3 months [ ]. The mean number of seizures per month was 4.2 at baseline, 1.0 during the thrice-daily and 0.9 during the twice-daily period. Adverse reactions were reported by 11 patients during the thrice-daily period and by five patients during the twice-daily period; sedation and vertigo were the most frequent.

The efficacy and safety of gabapentin in relieving the symptoms of panic disorder have been studied in 103 patients in a randomized, placebo-controlled, double-blind study for 8 weeks [ ]. Adverse events included somnolence, headache, and dizziness. One patient had a serious adverse event, a car accident, while taking gabapentin.

Of 12 patients with moderate to severe dementia and severe behavioral disorders given gabapentin (200–1200 mg/day) for 8 weeks, five had adverse events such as gait instability, emotional instability, and sedation; two patients discontinued treatment prematurely because of severe adverse reactions [ ].

The role of gabapentin in patients with neuropathic pain has been evaluated in a systematic review [ ]. The most common adverse events were dizziness and somnolence, which occurred in about 25% of patients; ataxia occurred in about 8%. Adverse reactions were dose-related.

• A 68-year-old man with essential tremor who was taking propranolol 80 mg/day had several daily episodes of paroxysmal dystonic movements in both hands 2 days after the addition of gabapentin 900 mg/day [ ]. The dose of propranolol was reduced to 40 mg/day and the dystonic movements resolved. The authors suggested that there had been a synergistic effect between propranolol and gabapentin.

• A 60-year-old woman with post-herpetic neuralgia developed asterixis after having taken gabapentin for 4 days [ ]. The authors proposed that the mechanism was GABAergic.

Two cases of gabapentin-related dyskinesia have been reported [ ]. The patients were 60 and 41 years old and took gabapentin 900–1200 mg/day for generalized anxiety. Generalized dyskinetic movements and facial tics appeared after 3 days and disappeared after 2 days of withdrawal.

Myoclonus has been attributed to gabapentin [ ].

• A 66-year old man with impaired renal function had vasculitis with macroglobulinemia, causing a painful sensorimotor neuropathy. He was given gabapentin 900 mg/day and a fentanyl patch every 3 days for 2 years and developed generalized myoclonus. He was unable to stand and walk, his speech was dysarthric, and he had mental slowing. There were signs and symptoms of a polyneuropathy. The gabapentin serum concentration 4 hour after the last gabapentin intake was very high. After withdrawal of gabapentin for 24 hours the myoclonus, dysarthria, and mental slowing disappeared. The serum gabapentin concentration returned to normal.