Finasteride

What it is

Finasteride blocks type II 5-alpha reductase, the enzyme that converts testosterone into dihydrotestosterone. DHT is what miniaturises hair follicles in genetically susceptible men, so lowering scalp DHT stops the process that causes male pattern baldness.

It is on this site for the same reason tadalafil is: it is approved, cheap, well-evidenced, and it sits in the middle of the same set of goals people are chasing with far less certain compounds.

What the evidence shows

At 1 mg daily, finasteride reduces serum DHT by roughly 70%. In the registration trials, most men stopped losing hair and a meaningful proportion regrew some. It outperforms every other approved option for androgenetic alopecia.

Two things about how it works are worth internalising:

It is preventative more than restorative. It is far better at keeping the hair you have than at bringing back hair you lost years ago. Starting earlier gets better outcomes, which is an uncomfortable thing to learn late.

The benefit stops when you stop. Within about twelve months of discontinuing, you return to the trajectory you would have been on. This is not a course of treatment; it is an ongoing one.

The side effect argument, honestly

This is the part worth reading carefully, because both camps overstate.

Trial data puts sexual side effects at roughly 2 to 4% against about 2% on placebo. That is a real but small absolute difference, and in most men who experience it the effect resolves on stopping.

Post-finasteride syndrome describes persistent sexual, cognitive and mood symptoms continuing after discontinuation. It is reported consistently enough that regulators in several countries have updated labelling, and it is disputed enough that its existence as a distinct entity is not settled. The honest position is that the mechanism is unclear, the incidence is unknown, and the reports are not fabricated.

What follows from that is not "avoid it" or "ignore the noise". It is that this is a real decision with a small but non-zero tail risk attached, made for a cosmetic indication, and it deserves an actual conversation with a doctor rather than a telehealth checkbox.

Practical notes

• Topical finasteride achieves meaningful scalp DHT reduction with lower systemic exposure. The evidence base is thinner than for oral but growing, and it is a reasonable option if systemic effects are the concern.
• It halves your PSA. Tell any doctor ordering a PSA test that you take it, or the result will be misread.
• Minoxidil works by a different mechanism and the two are routinely combined. Finasteride stops the loss, minoxidil stimulates growth.
• Give it six months before judging. Three months is too early, and early shedding is common and not a sign of failure.

Side effects and warning signs

Commonly reported

• Reduced libido, erectile difficulty or reduced ejaculate volume, reported in roughly 2 to 4% of trial participants against about 2% on placebo
• Breast tenderness or enlargement, uncommon but documented
• Mood changes, including depressive symptoms in some reports
• Lowers PSA by roughly half, which matters for prostate cancer screening

Stop and get medical help

• Persistent sexual dysfunction that does not resolve after stopping, the contested post-finasteride syndrome
• New or worsening depressive symptoms or suicidal thinking
• Any breast lump, tenderness or discharge
• Pregnancy in a partner handling the drug, since it can cause genital abnormalities in a male fetus

Sources

1. FDA prescribing information, Propecia (finasteride)regulatory
2. Finasteride for male pattern hair loss, Cochrane-level reviewreview

From the community

Discussion for this compound concentrates in r/Biohackers.

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