Capsaicin (8-methyl-N-vanillyl-6-nonenamide) is an active component of chilli peppers which are plants belonging to the genus Capsicum. It produces a sensation of burning in any tissue with which it comes into contact.
Capsaicin (and other capsaicinoids) have been used in health applications for centuries and have suggested benefits ranging from cardiovascular health to skin irritations to digestive medicines. In this brief article we are looking at its topical analgesic affects. Capsaicin has played an important role in folk medicine, often based on using like to treat like, for example, treating burning pain with a substance which causes burning pain. The first formal report of the pain-reducing properties of topical capsaicin in the West appeared in 1850 as a recommendation to use an alcoholic hot pepper extract on burning or itching extremities. There are now numerous studies showing beneficial effects of topical capsaicin which has encouraged many companies to produce commercial medication using capsaicin.
Recent Topical capsaicin clinical studies involved three to five topical skin applications per day for periods of 2–6 weeks. Results generally suggested modest beneficial effects against various pain syndromes, including post-herpetic neuralgia (PHN), diabetic neuropathy, and chronic musculoskeletal pain. (Derry S, Lloyd R, Moore RA, McQuay HJ. Topical capsaicin for chronic neuropathic pain in adults. Cochrane Database Syst Rev. 2009:CD007393 Hempenstall K, Nurmikko TJ, Johnson RW, A’Hern RP, Rice AS. Analgesic therapy in postherpetic neuralgia: a quantitative systematic review. PLoS Med. 2005;2:e164).
In another study to see if one strong dose of capsaicin could have enhanced effects, a single 60-min application in patients with neuropathic pain produced effective pain relief for up to 12 weeks (McCormack PL. Capsaicin dermal patch: in non-diabetic peripheral neuropathic pain. Drugs. 2010;70:1831–42). The studies were powerful enough for a recent approval in the EU and USA for a high-concentration capsaicin 8% patch (Qutenza™). Hence the use of capsaicin as a topical analgesic should be actively considered for all pain sufferers from neuropathic pain to arthritis within a clinical setting.
The mechanism of action of topical capsaicin has been ascribed to depletion of substance P. However, experimental and clinical studies show that depletion of substance P from nociceptors is only a correlate of capsaicin treatment and has little, if any, causative role in pain relief. Rather, topical capsaicin acts in the skin to attenuate cutaneous hypersensitivity and reduce pain by a process best described as ‘defunctionalisation’ of nociceptor fibres. Defunctionalization is due to a number of effects that include temporary loss of membrane potential, inability to transport neurotrophic factors leading to altered phenotype, and reversible retraction of epidermal and dermal nerve fibre terminals.