The quality of evidence is downgraded by suspected publication bias.
A Cochrane review included 8 studies with a total of 2 488 subjects. The studies examined efficacy and tolerability of a single application of high-concentration (8%) topical capsaicin in chronic neuropathic pain in adults. The duration of application of high-concentration topical capsaicin varied between 30 and 90 minutes, with most participants treated for 60 minutes. Two studies used a placebo control and 6 used 0.04% topical capsaicin as an 'active' placebo to help maintain blinding. Participants had pain due to postherpetic neuralgia (PH N, 4 studies), HIV-neuropathy (2 studies), painful diabetic neuropathy (PDN, 1 study), and persistent pain after inguinal herniorrhaphy (1 study).
| Outcome* | Relative effect (95% CI) | Per cent with outcome (control) | Per cent with outcome (8% capsaisin) | NNTB (95% CI) | Participants (studies) |
|---|---|---|---|---|---|
| *PIR=pain intensity reduction; PGIC=Patient Global Impression of Change | |||||
| ≥ 30% PIR 2 to 8 weeks | 1.3 (1.1 to 1.5) | 34% | 43% | 11 (6.8 to 26) | 1272 (4 studies) |
| ≥ 30% PIR 2 to 12 weeks | 1.3 (1.1 to 1.5) | 37% | 46% | 10 (6.3 to 28) | 973 (3 studies) |
| ≥ 50% PIR 2 to 8 weeks | 1.4 (1.1 to 1.9) | 20% | 29% | 12 (7.2 to 41) | 870 (3 studies) |
| ≥ 50% PIR 2 to 12 weeks | 1.3 (1.0 to 1.7) | 24% | 33% | 11 (6.1 to 62) | 571 (2 studies) |
| PGIC much/very much improved at 8 weeks | 1.4 (1.1 to 1.8) | 25% | 36% | 8.8 (5.3 to 26) | 571 (2 studies) |
| PGIC much/very much improved at 12 weeks | 1.6 (1.2 to 2.0) | 25% | 39% | 7.0 (4.6 to 15) | 571 (2 studies) |
In patients with postherpetic neuralgia, a single application of a high-concentration (8%) capsaicin patch provided significant pain relief for up to 12 weeks compared to control (table ). For painful HIV-neuropathy, one study (n=307) reported the proportion of participants who were much or very much improved at 12 weeks (27% with high-concentration capsaicin and 10% with 'active' placebo; RR 2.8, 95% CI 1.4 to 5.6; NNTB=5.8, 95% CI 3.8 to 12). More participants had average 2 to 12-week pain intensity reductions over baseline of at least 30% with capsaicin than control (RR 1.4, 95% CI 1.1 to 1.7; 2 studies, n=801, NNTB=11, 95% CI 6.2 to 47). For peripheral diabetic neuropathy, one study (n=369) reported about 10% more participants who were much or very much improved at 8 and 12 weeks but the results were not statistically significant for 12 weeks. One small study (n=46) with persistent pain following inguinal herniorrhaphy did not show a difference between capsaicin and placebo for pain reduction.
Local adverse events were common, but not consistently reported. Serious adverse events were no more common with active treatment (3.5%) than control (3.2%). Adverse event withdrawals did not differ between groups, but lack of efficacy withdrawals were somewhat more common with control than active treatment, based on small numbers of events. No deaths were judged to be related to study medication.
A Cochrane reviewComment: Use of capsaicin at the high concentration of 8% is associated with increased local skin reactions, primarily burning, stinging, and erythema, that affects many people, whether or not they obtain good pain relief, but these effects can be managed and resolve quickly after the single application.
A systematic review