5α-reductase Inhibitors: Finasteride and Dutasteride in Feminising Therapy

The 5α-reductase inhibitors finasteride and dutasteride are widely used in feminising hormone therapy, yet their role is poorly evidenced. This review grades them by mechanism, evidence and safety. Unlike other androgen-directed agents, 5α-reductase inhibitors act only on the conversion of testosterone to dihydrotestosterone (DHT) — neither suppressing testosterone production nor antagonising the androgen receptor — so their effect is partial, and they do not lower, and can raise, serum testosterone. Because DHT is generated from testosterone, reliable testosterone suppression reduces the substrate for DHT formation, making a large additive feminising effect biologically less likely; no controlled trial in transfeminine people has tested this, and the most authoritative appraisal finds additive benefit unclear. The clearest indication-specific rationale is scalp androgenetic alopecia; use for other residual androgen-sensitive features is theoretical and essentially unstudied. WPATH SOC8 recommends against routine use as feminising therapy. The review sets out the safety signals — including the 2024–2026 European and UK regulatory strengthening around psychiatric and sexual effects, teratogenicity, and PSA suppression — with per-claim certainty grading. This is health information, not medical advice.