The importance of proper vehicle selection in the detection of minoxidil sensitivity.

The correct selection of vehicles for patch testing specific substances is essential in the evaluation of suspected allergic contact dermatitis. A reaction to a chemical may be dependent not only on the concentration tested but also on the vehicle in which it is tested. In cases of suspected allergic contact dermatitis to minoxidil, propylene glycol, and alcohol formulations, patch testing is usually done with the formulation of minoxidil in alcohol and/or minoxidil in petrolatum. An acute contact dermatitis of the scalp developed in a 34-year-old white man while he was using minoxidil (Rogaine [2% minoxidil in a solution of alcohol 60% vol/vol, propylene glycol, and water]) solution. Patch testing with 2% minoxidil in alcohol and 2% minoxidil in petrolatum showed no reaction, while both Rogaine solution and 2% minoxidil in propylene glycol produced papulovesicular plaques. Patch testing in cases of suspected minoxidil allergic contact dermatitis should include testing with minoxidil in propylene glycol. Omitting this testing may cause diagnoses and therapeutic formulation alternatives to go unrecognized.