Capsaicin. Neuropathic pain: playing with fire

5% lidocaine medicated plasters are available for local treatment of neuropathic pain. Treatment is generally poorly effective but has few adverse effects, other than local erythema. Capsaicin is a natural chilli pepper extract that depletes sensory nerve endings of substance P, a pain neurotransmitter. It is authorised in the European Union for the treatment of nondiabetic neuropathic pain, in the form of cutaneous patches containing 8% capsaicin. Clinical evaluation of capsaicin patches does not include any trials versus lidocaine plasters. Eight double-blind trials have compared 8% capsaicin patches versus 0.04% capsaicin patches, 5 in postherpetic neuralgia, and 3 in HIV-related neuropathic pain. These trials are only vaguely described in the European Medicines Agency report. Taken separately, they yielded divergent results. It was only when some of the trials were pooled for analysis that any differences emerged between the two doses of capsaicin. The clinical implications are unclear, but efficacy is at best modest. Capsaicin is an irritant that frequently provokes pain and erythema at the site of patch application, and 3% of patients using the patches experienced transient arterial hypertension that the investigators attributed to this pain. Some pharmacological data suggest that repeated application of 8% capsaicin patches might provoke painful nerve damage in the long-term. Patch application and removal by a third party is delicate, due to the strong irritant potential of capsaicin. In practice, when a patient with neuropathic pain requires local treatment, in the absence of a better alternative, it is better to use lidocaine plasters, which are better tolerated and with which we have more experience.