Six RCTs were included in the review (total number of participants 8,733). Studies ranged between a Tuldor score of 6 and 9. Five RCTs were included in meta-analyses: four evaluated pregabalin and one evaluated gabapentin. The authors stated that none of the studies controlled for the amount of co-medications or changes in other therapies and that external validity was limited due to exclusion of several disease states that were relevant to fibromyalgia syndrome and a limited demographic of patients.
The percentage of completers ranged from 38.4% to 78.6% for treatment groups and from 19.2% to 82.7% for placebo groups. Numbers reported below were numbers of patients included in the analysis and included double counting of some control groups.
Pain (five studies, n=4,250, 13 comparisons): Compared with placebo, gabapentin and pregabalin were associated with a significant but small reduction in pain in fibromyalgia syndrome patients (SMD -0.28, 95% CI -0.36 to -0.20, I2=41.7%). NNT for a 30% or greater reduction in pain was 8.5 (95% CI 6.4 to 12.6).
Compared with placebo, gabapentin and pregabalin significantly improved sleep function (SMD -0.39, 95% CI -0.48 to -0.29, I2=42.7%; four studies, 10 comparisons), anxiety (SMD -0.18, 95% CI -0.27 to -0.10; two studies, n=2,237, six comparisons), health-related quality of life (SMD -0.30, 95% CI -0.46 to -0.15, I2=43.4%; two studies, four comparisons) and fatigue (SMD -0.16, 95% CI -0.23 to -0.09; three studies, nine comparisons).
Gabapentin and pregabalin did not significantly affect levels of depression in fibromyalgia syndrome patients.
Based on Cohen's categories for size effect, gabapentin and pregabalin had a negligible effect on fatigue and anxiety and a small effect on pain, sleep and health-related quality of life; the reductions were statistically significant.
Adverse events: Adverse events were inconsistently reported. Compared to placebo, gabapentin and pregabalin were associated with a significant greater risk of withdrawal due to adverse events (NNH 9.5, 95% CI 7.6 to 12.8). Pregabalin was associated with significantly greater risks of dizziness, somnolence, weight gain, peripheral oedema and negative neurocognitive effects than placebo. Gabapentin was associated with significantly greater risks of dizziness and weight gain than placebo.