Eight studies (3,114 patients) were included in the review. Six were RCTs, one was an open-label extension study and one a non-randomised single-arm study. Sample sizes ranged from 150 to 529; the RCT of gabapentin was significantly smaller than the other trials.
All the RCTs were considered to have been fair quality; methods of blinding, allocation concealment and randomisation were unclear and losses to follow-up were high.
Efficacy: Short-term response (reduction of pain by at least 30%) was higher in patients treated with gabapentin (51%) than in those given placebo (RR 1.7, 95% CI 1.1 to 2.5; numbers-need-to-treat of five; one RCT; 150 patients). Response rates to pregabalin ranged from 26% to 50% with a pooled risk ratio of 1.4 (95% CI 1.3 to 1.6; numbers-need-to-treat of eight; four RCTs; 2,757 patients). There was no evidence of a dose-response effect. Data on time to loss of response were also reported.
Safety: The most common adverse events for both pregabalin and gabapentin were dizziness, headache, somnolence and oedema. Dizziness was the most common event with pregabalin (38% of patients) and headache was the most common with gabapentin (27% of patients); it appeared that this was statistically significantly higher than with pregabalin. Other frequent adverse events were weight gain, dry mouth, amblyopia and euphoria with pregabalin (numbers-needed-to-harm ranged from 11 to 21) and sedation and light-headedness with gabapentin (numbers-needed-to-harm were five for sedation and eight for light-headedness).
Discontinuation for adverse events was more common in pregabalin- and gabapentin-treated patients than in placebo groups; this difference was statistically significant in pregabalin trials.