Topical 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) solution has been widely used for the treatment of androgenetic alopecia for over 15 years now and the substance is currently approved for this indication in 2% and 5% formulation. Typical side effects of this topical treatment include irritative dermatitis going along with pruritus, erythema, scaling and dryness, which occur especially at the onset of the therapy. In some cases, allergic contact dermatitis or exacerbation of seborrheic dermatitis has been reported. While most of the patients with allergic contact dermatitis described in the literature showed a positive sensitization to the vehicle substance propylene glycol evaluated by patch testing, reactions to the active ingredient minoxidil are rare. Here, we report a case of allergic sensitization to minoxidil, which we evaluated and differentiated from an irritative reaction by a combination of patch testing and lymphocyte transformation test. The differentiation of allergic and irritative adverse effects and the identification of the causative allergen are of major relevance for the proceeding and adjustment of the therapy. Patients with sensitizations against propylene glycol are candidates for preparations with alternative solvents but can proceed treatment with minoxidil. In contrast, patients with allergies to the active ingredient itself are no longer candidates for treatment with minoxidil and should undergo alternative therapeutic options.