Most likely caused by propylene glycol. Treatment may be replaced with 5% minoxidil foam, which is without propylene glycol.[33]Olsen EA, Whiting D, Bergfeld W, et al. A multicenter, randomized, placebo-controlled, double-blind clinical trial of a novel formulation of 5% minoxidil topical foam versus placebo in the treatment of androgenetic alopecia in men. J Am Acad Dermatol. 2007 Nov;57(5):767-74.
http://www.ncbi.nlm.nih.gov/pubmed/17761356?tool=bestpractice.com
Occurs in 1.3% to 1.8% of men treated with finasteride. May resolve during long-term treatment or after discontinuation of drug.[40]Kaufman KD, Olsen EA, Whiting D, et al. Finasteride in the treatment of men with androgenetic alopecia. Finasteride Male Pattern Hair Loss Study Group. J Am Acad Dermatol. 1998 Oct;39(4 Pt 1):578-89.
http://www.ncbi.nlm.nih.gov/pubmed/9777765?tool=bestpractice.com
[60]Drake L, Hordinsky M, Fiedler V, et al. The effects of finasteride on scalp skin and serum androgen levels in men with androgenetic alopecia. J Am Acad Dermatol. 1999 Oct;41(4):550-4.
http://www.ncbi.nlm.nih.gov/pubmed/10495374?tool=bestpractice.com