Recent postmarketing reports and a US Food and Drug Administration analysis have documented uncommon persistent sexual and nonsexual side-effects in a subset of younger men who have taken finasteride 1 mg for androgenic alopecia. While the mechanisms of the sexual side-effects in humans is incompletely understood, one study found lower cerebrospinal fluid concentrations of dihydrotestosterone, progesterone, dihydroprogesterone and allopregnanolone, and higher levels of testosterone, 5α-androstane-3α,17β-diol, and pregnenolone. Another study found up-regulation of the androgen receptor in the human foreskin with a mean of 5 years after finasteride discontinuation.
Irwig MS. Safety concerns regarding 5α reductase inhibitors for the treatment of androgenetic alopecia. Curr Opin Endocrinol Diabetes Obes. 2015 Jun;22(3):248-53. doi: 10.1097/MED.0000000000000158. [PubMed]