Topical capsaicin. A review of its pharmacological properties and therapeutic potential in post-herpetic neuralgia, diabetic neuropathy and osteoarthritis.

Capsaicin, the active principle of hot chili pepper, is thought to selectively stimulate unmyelinated C fibre afferent neurons and cause the release of substance P. Prolonged application of capsaicin reversibly depletes stores of substance P, and possibly other neurotransmitters, from sensory nerve endings. This reduces or abolishes the transmission of painful stimuli from the peripheral nerve fibres to the higher centres. In clinical studies of patients with post-hepatic neuralgia, diabetic neuropathy or osteoarthritis, adjunctive therapy with topical capsaicin achieved better relief than its vehicle in most studies. In a single trial, topical capsaicin in demonstrated similar efficacy to oral amitriptyline in patients with diabetic neuropathy. Topical capsaicin is not associated with any severe systemic adverse effects. However, stinging and burning, particularly during the first week of therapy, is reported by many patients. Topical capsaicin merits consideration as adjuvant therapy in conditions such as post-herpetic neuralgia, diabetic neuropathy and osteoarthritis, where the pain can be chronic and difficult to treat.