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Allergic contact dermatitis caused by acyclovir is rare. We report the 5th case of systemic acyclovir reaction subsequent to acyclovir contact dermatitis, with investigations made to determine an alternative antiviral treatment. A 23-year-old woman, after dermatitis while using Zovirax® cream, went on to develop urticaria after oral acyclovir. Patch tests were performed with the components of Zovirax® cream (acyclovir, propylene glycol and sodium lauryl sulfate) and with other antiviral drugs. Patch tests were positive to Zovirax® cream, acyclovir, valacyclovir and propylene glycol. Patch and prick tests with famciclovir were negative, but its oral administration caused an itchy erythematous dermatitis on the trunk and extremities. Our patient developed a systemic acyclovir reaction subsequent to acyclovir allergic contact dermatitis, with cross-reactions to valacyclovir and famciclovir. Their common chemical structure is the 2-aminopurine nucleus. It is probably this part of the molecule that provokes both contact allergy and systemic reactions. The only antiviral drugs not having this core are foscarnet and cidofovir, and these could therefore be alternatives. [ABSTRACT FROM AUTHOR]