Allergic Contact Dermatitis to Topical Preparations Containing Minoxidil: A Systematic Review and Individual Participant Data Meta-Analysis

Topical minoxidil is generally well tolerated, yet there have been a few reports of allergic contact dermatitis (ACD) confirmed through patch testing. This systematic review and individual participant data meta-analysis sought to elucidate the primary allergens in patients exhibiting ACD in response to topical minoxidil formulations and to ascertain the appropriate testing concentrations and vehicles of minoxidil itself. A comprehensive search was conducted across the PubMed, Scopus, and Embase databases utilizing the keywords "minoxidil" and "contact dermatitis," or "contact allergy," or "contact eczema." Studies documenting ACD in patients using topical minoxidil confirmed by patch testing were deemed eligible. Our analysis included 46 studies encompassing 99 patients with patch-test-confirmed ACD to minoxidil-based topical treatments. The majority of these patients (93.9%) were exposed to minoxidil without additional active components. Minoxidil itself was identified as the primary allergen in 74.7% of the patients, with propylene glycol being the next most common allergen at 17.1%. Other allergens identified included estradiol, butylene glycol, methylchloroisothiazolinone/methylisothiazolinone, canrenone, and latanoprost. The most effective concentration was found to be 2% minoxidil in propylene glycol, which yielded a 100% positivity rate. The findings indicate that minoxidil is the predominant allergen in ACD reactions to its topical formulations, followed by propylene glycol. For the accurate diagnosis of ACD related to minoxidil, patch testing with 2% minoxidil in propylene glycol is recommended, as are separate tests for propylene glycol and other potential allergenic ingredients.