Q. I have pain from osteoarthritis in both knees. I'm curious about the cream made from a substance in hot peppers. How does it relieve pain?
A. You're referring to capsaicin, the substance in chili peppers that gives them their hot taste. Capsaicin is an ingredient in many over-the-counter topical pain-relief preparations, which include creams, gels, lotions, patches, and sticks. When first applied, topical capsaicin causes a burning sensation. This sensation lessens within a few minutes, and also over time with repeated applications. There are few, if any, systemic side effects.
We don't know exactly how capsaicin works, but it's thought to stimulate the release of substance P, a chemical that helps transmit pain signals from sensory nerve fibers to the brain. After several applications of capsaicin, local stores of substance P (and possibly other chemical pain messengers) become depleted, and the nerve fibers in that area transmit fewer pain signals.
Studies comparing topical capsaicin with a placebo for arthritis pain have yielded inconsistent results. In one randomized trial involving people with either osteoarthritis or rheumatoid arthritis, capsaicin applied to knees four times a day for four weeks was more effective than a placebo in reducing pain. But topical capsaicin showed only a moderate to poor benefit in a pooled analysis of three studies involving patients with arthritis. The authors of that study suggested that capsaicin might be useful when given along with other pain relievers, such as nonsteroidal anti-inflammatory drugs, or as an option for people who can't tolerate other pain-relief treatments.
Capsaicin provides only temporary relief, so it must be applied four times a day. Before you use it on a large area, apply some on a small patch of skin to make sure you're not allergic to any of the ingredients. Apply enough to cover the painful area, and rub it into your skin until it disappears. Then wash your hands immediately to avoid accidentally getting it into your eyes, nose, or mouth. Don't apply capsaicin to broken or irritated skin, and don't use a heating pad on the area you're treating. It may be a week or so before you feel the full effect on your knee pain. If you don't notice any improvement after four weeks, stop using capsaicin.