**7.11 Contact sensitizers**

Contact sensitizers are a mode of inducing a type IV hypersensitivity reaction, thus making them a form of topical immunotherapy [28]. Diphencyprone (DCP) is the preferred compound. DCP 2% solution is applied after every 10–14 days, on the medial side of upper arm—till there is appearance of local erythema and vesiculation—and this may be repeated up to three times. Warts were then first pared followed by application with stepwise concentration of DCP: 0.01, 0.05, 0.10, 0.25, 0.50, 1.0, 1.5, 2.0, 3.0, 4.0 and 6.0%. Treatments are applied every 1–4 weeks. Resistant palmoplantar warts treated with DCP over 8 years [192] exhibited 88% clearance rate. However, a large percentage of patients developed adverse effects (56%), including painful blistering at the site of sensitization and near warts, pompholyx-like or generalised eczematous eruption, influenza-like symptoms, vesiculation elsewhere due to passive transfer of DCP and inguinal lymphadenopathy. They concluded that patients with recalcitrant palmar, plantar, periungual and digital warts are good candidates for DCP therapy [193].
