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ObjectiveTo assess the effect of different doses of gabapentin (GBP) on cognitive function in treated epileptic patients.MethodsTwenty seven patients with refractory partial seizures commenced a double blind, dose ranging, placebo controlled, crossover study of adjuvant GBP. Each treatment phase lasted three months, during which the dose of GBP or matched placebo was increased stepwise at intervals of four weeks (1200 mg/day, 1800 mg/day, and 2400 mg/day in three daily doses). Psychomotor and memory testing was carried out at the end of each four week period, at which time the patient also completed subjective measures of cognition, fatigue, worry, temper, and dysphoria. A visual analogue scale was used to assess drowsiness and a questionnaire was employed to gauge the severity of side effects.ResultsIn the 21 patients completing the study, GBP produced a significant reduction in median monthly seizure frequency from 7 to 4·3 (P = 0·02), the decrease being most pronounced for secondarily generalised seizures (from 1·0 to 0·3, P = 0·01). Forty three per cent of patients reported a reduction in seizure frequency of at least 50% throughout all GBP doses. Mean (SD) plasma concentrations of GBP at 1200, 1800, and 2400 mg/day were 4·7 (2·6), 6·8 (3·8), and 8·6 (3·3) mg/l respectively. The drug had no effect on composite psychomotor and memory scores; nor was there alteration in any self assessment subscore. The mean drowsiness (P = 0·03) score was higher during treatment with 2400 mg GBP daily compared with matched placebo. Composite psychomotor (r = -0·47, P < 0·01), tiredness (r = 0·42, P < 0·01), and side effect (r = 0·61, P < 0·001) scores correlated significantly with seizure frequency but not with GBP dose.ConclusionGBP is a well tolerated and effective antiepileptic drug which had no measurable effect on cognition but did produce sedation at the highest dose. This study also supports the suggestion that seizures can cause cognitive impairment.