Allergic contact dermatitis from minoxidil

In a study from 32 years ago, 8 patients (1 male and 7 females, aged 24 to 41 years) who had been using minoxidil solution for androgenetic alopecia developed allergic contact dermatitis. Patch testing revealed that 7 of the patients were allergic to minoxidil alone, while one had a simultaneous allergy to both minoxidil and propylene glycol. The study emphasized the importance of the vehicle in patch testing for minoxidil, as all patients reacted to the topical minoxidil solution used and to minoxidil 2% in propylene glycol and water. However, tests with minoxidil in petroleum or ethanol were negative in some patients. The study concluded that propylene glycol, which enhances skin penetration of substances including minoxidil, is necessary for both the hair growth effects of minoxidil and to elicit contact sensitivity.