Capsaicin, which is derived from chili peppers, causes vasodilation, itching, and burning when applied to the skin. These actions are attributed to binding with nociceptors, which causes a period of enhanced sensitivity followed by a refractory period of reduced sensitivity. Repeated application leads to desensitization and, thus, relief of some forms of chronic pain. Although systemic adverse effects are rare, local irritation, burning, and erythema are common. Mason and colleagues studied the efficacy of topical capsaicin in relieving chronic neuropathic and musculoskeletal pain.
They searched electronic databases of publications and clinical trials to identify randomized studies of adults treated with capsaicin three to four times daily for a minimum of three weeks for chronic musculoskeletal pain and a minimum of six weeks for neuropathic pain. Each trial was assessed independently for quality and validity by two reviewers, and disputes were settled by consensus. Clinical success was defined as a 50 percent decrease in pain. The numbers of patients who improved, reported adverse events, and withdrew because of adverse events also were counted.
From 38 papers identified, 16 met criteria for inclusion in the meta-analysis. The 1,556 patients had moderate to severe pain (11 trials) or were unresponsive to or intolerant of conventional analgesia (five trials). Three trials prohibited concomitant therapy. Based on three trials involving 368 patients, capsaicin was significantly better than placebo in reducing musculoskeletal pain. The relative benefit was 1.5, and the number needed to treat was eight. Topical capsaicin also significantly improved neuropathic pain at four and eight weeks, with relative benefits of 1.4 compared with placebo and a number needed to treat of 5.5 to 6.5. Overall, about one third of patients reported local adverse reactions. Thirteen percent of capsaicin patients and 3 percent of those treated with placebo withdrew because of adverse events.
The authors conclude that topical capsaicin is superior to placebo in relieving chronic neuropathic and musculoskeletal pain. Local adverse reactions were common but seldom serious. However, local irritation could have led some patients to recognize active treatment and may have caused biased results. Although topical capsaicin has moderate to poor efficacy, it may be particularly useful (alone or in conjunction with other modalities) in patients whose pain has not been controlled successfully with conventional therapy.