Sixteen studies (n=1,556) were included in the review.
Four studies scored 4 out of 5 on the quality assessment, ten scored 3, and two scored 2. On the 16-point validity assessment, the scores ranged from 9 to 14.
Efficacy.
Based on 313 patients in 4 studies, capsaicin resulted in a statistically significant improvement in neuropathic pain at 4 weeks (RR 1.4, 95% CI: 1.1, 1.7); the corresponding NNT was 6.4 (95% CI: 3.8, 21). No results for the assessment of heterogeneity were presented. Capsaicin also resulted in a statistically significant improvement in neuropathic pain at 8 weeks (RR 1.4, 95% CI: 1.2, 1.7) and a corresponding NNT of 5.7 (95% CI: 4.0, 10), based on 656 patients in 6 studies. There was evidence of statistical heterogeneity for the comparison at 8 weeks (P=0.02; I2=62.5%).
Based on 368 patients in 3 studies, capsaicin resulted in a statistically significant improvement in musculoskeletal pain at 4 weeks (RR 1.5, 95% CI: 1.1, 2.07) and a corresponding NNT of 8.1 (95% CI: 4.6, 34, P=0.01). There was no evidence of statistical heterogeneity (P=0.36; I2=2.5%). Subgroup analyses showed no significant effect of trial size or outcome measured.
Adverse events.
Based on 300 patients in 4 studies, capsaicin caused a statistically significant higher rate of local adverse events at 8 weeks than placebo when used for neuropathic pain (RR 3.2, 95% CI: 2.2, 4.6; NNH 2.5, 95% CI: 2.0, 3.3).
Based on 190 patients in 3 studies, capsaicin caused a statistically significant higher rate of local adverse events at 4 weeks than placebo when used for musculoskeletal pain (RR 5.0, 95% CI: 2.6, 9.6; NNH 2.6, 95% CI: 2.06, 3.6).
Withdrawals due to adverse events were significantly higher with capsaicin than placebo when used for neuropathic pain (5 studies with 503 patients; RR 5.5, 95% CI: 2.6, 12; NNH 7.5, 95% CI: 5.5, 12) and for musculoskeletal pain (4 studies with 398 patients; RR 2.5, 95% CI: 1.1, 5.6; NNH 16, 95% CI: 9.1, 63).