Gabapentin is a structural analogue of the neurotransmitter gamma-aminobutyric acid (GABA) approved for use in adults with postherpetic neuralgia. Gabapentin does not bind to GABA(A) or GABA(B) receptors. Its mechanism of action in humans is unclear, but may involve binding to alpha2delta calcium channel subunits in animal models. Reductions in the mean daily pain score from baseline to week 7 or 8 of treatment (primary endpoint) were significantly greater with gabapentin 1800-3600 mg/day than placebo therapy in two well designed trials in patients with postherpetic neuralgia. The proportion of responders (patients showing a > or =50% reduction in mean daily pain score at endpoint versus baseline) was significantly greater with gabapentin than placebo. Daily sleep rating scores, the Short Form McGill Pain Questionnaire (total pain scores), Patient and Clinician Global Impression of Change and measures on the Short Form-36 Health Survey (including physical functioning, role-physical, bodily pain, vitality or mental health) improved to a significantly greater extent with gabapentin than placebo. Adverse events associated with gabapentin in patients with postherpetic neuralgia were usually mild to moderate in intensity, with dizziness, somnolence and peripheral oedema being commonly reported.