Finasteride

The standard prescription drug for male pattern hair loss — blocks conversion of testosterone to DHT, the androgen that miniaturises follicles. The most evidence-backed oral option for keeping hair, and the most argued-about because of persistent-side-effect reports.

Finasteride blocks the type 2 form of 5-alpha reductase, the enzyme that converts testosterone into dihydrotestosterone. DHT is the androgen that miniaturises genetically susceptible scalp follicles, shrinking them a little more with each growth cycle until they stop producing visible hair. Blocking its production lowers scalp DHT substantially and halts that miniaturisation, which is why the drug reliably stops loss and only sometimes regrows what is already gone. It does nothing about hair loss that is not androgen-driven, which is why a correct diagnosis matters before starting.

Strong: decades of trials show it halts loss in the large majority and regrows some hair in many. DHT reduction ~70% at standard prescribing. Effect requires continuous use — stopping returns you to baseline trajectory within a year.

• Large randomised trials in male pattern hair loss show most men stop losing hair and a meaningful proportion see visible regrowth, sustained over five-year follow-up. This is among the better-evidenced drugs in cosmetic medicine.
• It also has long-standing evidence at higher doses for benign prostatic hyperplasia, which is what it was originally developed for.
• Sexual side effects — reduced libido, erectile difficulty, ejaculatory changes — appear in trials at low single-digit percentages, typically only a percentage point or two above placebo, and usually resolve on stopping.
• Persistent symptoms after stopping are reported in the literature and in regulatory adverse-event data. How often this happens and why is genuinely contested, and the honest position is that the mechanism is not established and the frequency is not settled — not that the reports do not exist.
• Psychiatric risk is on the label and was strengthened, not softened, on review: in 2025 the European regulator confirmed suicidal ideation as an adverse effect of finasteride tablets, with frequency unknown, alongside existing depression warnings.
• Regulators including the FDA and EMA have required label updates covering depression and suicidal ideation, and several agencies have issued safety reviews on psychiatric and sexual effects.

Plasma half-life is short, around 5-6 hours, but that is misleading because the enzyme inhibition lasts far longer than the drug does — which is why once-daily dosing works and why scalp DHT stays suppressed. Effects on hair take time to show: three to six months before change is visible, and twelve months before a fair judgement. Stopping reverses the benefit within roughly a year. It lowers serum PSA by around half, which must be communicated to any clinician interpreting a prostate screening result.

Sexual side effects (libido, erectile function) occur in a small percentage in trials; the contested part is post-finasteride syndrome — persistent symptoms after stopping, which regulators list while the literature fights over causality and rate. Mood effects reported. Crushed-tablet exposure is dangerous in pregnancy (topical/handling warning). Anyone running exogenous androgens changes this calculus entirely.

• Women who are or may become pregnant — this is an absolute contraindication, including handling broken tablets
• Anyone with a history of significant depression, without discussing it with a prescriber first
• Anyone trying to conceive who is concerned about the reported effects on semen parameters
• Anyone whose hair loss has not been properly diagnosed as androgenetic

• Pregnancy — this is the critical one. Taking finasteride in pregnancy is contraindicated because it can cause genital malformations in a male fetus. Handling intact coated tablets is not the same thing and is not prohibited; the specific warning is against handling crushed or broken tablets.
• PSA testing — finasteride substantially lowers PSA and can mask a clinically important rise. Do not apply a correction factor yourself; the clinician interpreting the result needs to know you are taking it.
• Testosterone therapy — more substrate for the enzyme, though the inhibition still holds.
• Alcohol and drugs affecting mood — worth noting given the psychiatric signal.

• Photographs at a fixed distance and lighting — the only reliable way to judge slow hair change, because memory is not.
• Mood, explicitly. Regulators added depression warnings for a reason, and this is easy to miss in yourself.
• Sexual function, tracked honestly, ideally from a baseline before starting.
• PSA, if you are in the age range for prostate screening — tell whoever interprets it that you take finasteride, because it lowers the reading and how to account for that is their judgement, not a fixed multiplier.

Hair maintained by the drug is lost over roughly six to twelve months after stopping, returning to the trajectory you would have been on. Sexual side effects often resolve, in trials sometimes even during continued treatment, but the timing is variable and persistent post-treatment symptoms are recognised. The contested question is the minority who report persistent symptoms; regulators have taken those reports seriously enough to change labels while the frequency remains unsettled. There is no physical withdrawal syndrome and no taper is needed — stopping is stopping.

US: prescription-only (not scheduled); telehealth prescribing is trivial to obtain.

Papers about this compound, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.

• The Efficacy and Safety of Finasteride Combined with Topical Minoxidil for Androgenetic Alopecia: A Systematic Review and Meta-analysis
  Aesthetic Plast Surg, 2020
• Adverse Sexual Effects of Treatment with Finasteride or Dutasteride for Male Androgenetic Alopecia: A Systematic Review and Meta-analysis
  Acta Derm Venereol, 2019
• The efficacy and safety of dutasteride compared with finasteride in treating men with androgenetic alopecia: a systematic review and meta-analysis
  Clin Interv Aging, 2019

• Comparison of oral minoxidil, finasteride, and dutasteride for treating androgenetic alopeciaJ Dermatolog Treat · 2022
• Finasteride for hair loss: a reviewJ Dermatolog Treat · 2022
• Efficacy and safety of topical finasteride spray solution for male androgenetic alopecia: a phase III, randomized, controlled clinical trialJ Eur Acad Dermatol Venereol · 2022
• Risk of Depression Associated With Finasteride TreatmentJ Clin Psychopharmacol · 2021
• Sexual side effects of 5-α-reductase inhibitors finasteride and dutasteride: A comprehensive reviewDermatol Online J · 2017
• Topical finasteride for male and female pattern hair loss: Is it a safe and effective alternative?J Cosmet Dermatol · 2022
• Post-Finasteride Syndrome. Literature ReviewArch Esp Urol · 2022
• Finasteride in the treatment of men with androgenetic alopecia. Finasteride Male Pattern Hair Loss Study GroupJ Am Acad Dermatol · 1998
• FinasterideExpert Opin Drug Metab Toxicol · 2010
• The effects of finasteride on scalp skin and serum androgen levels in men with androgenetic alopeciaJ Am Acad Dermatol · 1999
• Comparitive effectiveness of finasteride vs Serenoa repens in male androgenetic alopecia: a two-year studyInt J Immunopathol Pharmacol · 2012
• Finasteride and androgenic alopecia; from therapeutic options to medical implicationsJ Dermatolog Treat · 2020