Whole lotta research on this recently:
Topical Formulations Containing Finasteride. Part I: In Vitro Permeation/Penetration Study and In Vivo Pharmacokinetics in Hairless Rat.Monti D1, Tampucci S, Burgalassi S, Chetoni P, Lenzi C, Pirone A, Mailland F.
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AbstractIn hair...