Local therapy with capsaicin or ASS in chronic pain

OBJECTIVE: To provide a brief review of the current state of topical treatment with capsaicin or acetylsalicylic acid (ASA) for therapy of chronic pain syndromes. DATA SOURCES: A MEDLINE search was used to find the pertinent literature on "capsaicin" or "ASA" and "chronic pain"; further publications found in these articles were added. CONCLUSIONS: Capsaicin is a white crystalline parent compound of a group of vanillyl fatty acid amines. Because of its highly specific action in neurons it has become an important tool in neuroscience. Because of its effects, it is obvious to try for the therapy of circumscribed neuropathic pain. Capsaicin acts by depleting stores of substance P and other neurotransmitters, resulting in a blockade of a specific group of sensory afferents. The corresponding clinical findings are initial burning and a desensitization of specific C fiber nociceptors after repeated application. The pain relieving potency was observed in various clinical investigations and even in a few controlled, double-blind studies about neuropathic pain syndromes and (osteo)arthritis. In contrast to these findings, a recent study found no significant benefit of capsaicin, probably because this study was the first to use an active placebo. Therefore, and because clinical efficacy and advantages over other therapies have not been demonstrated up to now, capsaicin cannot be classified as standard therapy. It may be a therapeutic option as an alternative or as an adjuvant treatment. Pain reduction was also observed after topical application of ASA/ether mixture in the one and only controlled double-blind study on this issue. Therefore, topical ASA therapy for (post)herpetic neuralgia is mainly based on a few enthusiastic case reports rather than on well founded investigations. Furthermore, the discrimination of local from systemic effects, the toxicological profile of longterm topical treatment, and the mechanism of action has not been evaluated. In conclusion, topical ASA cannot be recommended for routine clinical use at present.