Capsaicin is a natural product of capsicum peppers, and is an active ingredient in many hot foods. Topical capsaicin cream (0.025% and 0.075%) is a substance P depletor, and has on occasion provided relief for both acute herpetic neuralgia (shingles) and post–herpetic neuralgia. Capsaicin is known for its selectivity for and effect on C–fibre nociceptors and heat receptors. Studies have shown its ability to trigger membrane depolarisation and to open non-selective cation channels, which may be either reversible or lytic. Capsaicin is theorised to cause a neurotoxic cellular degeneration of primary afferent nociceptors. Thus, exposure to capsaicin results in activation, desensitisation and, under certain conditions, the destruction of lightly myelinated or unmyelinated primary afferent fibres.
Recent studies suggest a clinical role for topical capsaicin at doses of 5%–10% in patients with intractable pain. Compliance may be a problem with this medication, since it needs to be applied 4–5 times a day for several weeks before any significant benefit is appreciated and it has intense initial burning effects.