Finasteride: what it is, what it is licensed for, and what comes with it

Finasteride is a prescription tablet that blocks an enzyme called 5-alpha-reductase. In men with male pattern hair loss it slows the loss and, in the trials the licence rests on, partly reversed it. It is also the medicine behind a long and unresolved argument about sexual and psychiatric effects, some of which have been reported as continuing after the tablets stop. Both of those things are true at once, and this page holds them together rather than choosing one.

What finasteride is licensed to do

In the United States the lower-strength tablet has been approved since 1997 for male pattern hair loss in men, and only in men. The FDA-approved labelling puts the restriction in capitals — men only — and adds that effectiveness in the receding corners at the temples has not been established, a detail the marketing rarely carries. The higher-strength tablet is a different licence, for an enlarged prostate.

There is no single answer to what finasteride is, because there are three different regulatory objects wearing the name: the tablet, the topical versions, and its close relative dutasteride. They are not one decision.

That grid matters more than it looks. A licensed medicine has been assessed by a regulator for a stated use; a compounded one has not been assessed at all; an off-label prescription means the regulator never examined that use, so the judgement sits entirely with the prescriber. Japan took the further step of approving dutasteride for male pattern hair loss in 2015, which is why you will find confident claims online that it is "approved" — true in Tokyo, not in Washington, London or Brussels.

The one step it works on

Testosterone becomes dihydrotestosterone — DHT — when 5-alpha-reductase acts on it. There are two types of the enzyme, and the label attributes about two-thirds of circulating DHT to type II, the one this drug is built against. Blocking it lowers DHT in blood and in scalp tissue; the label reports a 65% fall in serum DHT within a day of the first tablet.

Why that helps hair is the second half of the mechanism. In balding scalp, the label says, the follicles are miniaturised and the DHT content is higher than in scalp that still grows hair. Take away much of the DHT and the miniaturising pressure eases. What the label will not claim is which of those two reductions — blood or scalp — is doing the work: it says the relative contributions have not been defined. The mechanism has its own page, because almost every argument about side effects is really an argument about what else that enzyme was doing.

What it does to hair, in the trials' own numbers

The licence rests on three twelve-month randomised trials in 1,879 men aged 18 to 41 with mild to moderate loss, with extensions that ran five years. The most legible result is the photograph panel: standardised photographs, rated blind, at year five.

At five years, 48% of the men on finasteride were rated as having more hair than at the start, 42% as unchanged, and 10% as having lost hair. In the placebo group the same panel rated 6% as having more hair, 19% unchanged and 75% as having lost some. Hold those two columns side by side and the honest summary of this drug appears: its commonest outcome over five years was not regrowth but a stalemate, and the stalemate is the point, because the untreated column kept sliding.

Two warnings about reading that. The men rated at year five are the ones who stayed in the study, which flatters any long trial. And the trial population was young, male and early-stage, so it says nothing about a fiftieth birthday, a fully bare crown, or a woman. Whether this counts as regrowth or as holding is worth its own page.

What it can do to you

In year one of the same hair-loss trials, the investigators recorded sexual adverse experiences they judged possibly, probably or definitely drug-related. The figures are small, and the placebo column is not zero — which is the reason both columns are drawn.

Reduced sex drive was recorded in 1.8% of the men on the drug against 1.3% on placebo; erectile dysfunction in 1.3% against 0.7%; an ejaculation disorder in 1.2% against 0.7%. Counting men who reported at least one of the three, the label gives 3.8% against 2.1%, a difference it reports as statistically significant. It adds that the effects resolved in men who stopped and in most of those who carried on, and that the yearly incidence fell to 0.3% or below by year five.

If that were the whole record this would be a mild drug with a small, reversible side effect. It is not the whole record. Everything that makes finasteride controversial sits in a different section of the same label — the post-approval reports, which have no denominator and no placebo group, and which include sexual dysfunction that continued after treatment stopped, male infertility and poor semen quality, male breast cancer, and depression, suicidal ideation and behaviour. The side-effect record separates those two kinds of evidence properly, because conflating them is how both sides of this argument go wrong.

Who it is not for

This is the part of the label with the least ambiguity in it, and the part most often skipped.

• Women who are or might become pregnant. Use in pregnancy is contraindicated: by blocking the conversion of testosterone to DHT, the drug can affect the development of a male fetus's genitals. The label also tells women who are or may be pregnant not to handle crushed or broken tablets.
• Women generally, for hair loss. The tablet is not indicated for women, and the one trial in the label — 137 postmenopausal women — could not demonstrate any effect on hair counts or on any of the other measures.
• Children. Not indicated, and safety and effectiveness have not been established.
• Anyone whose hair loss has not been diagnosed. Thyroid disease, iron deficiency, alopecia areata, scarring alopecias, telogen effluvium after illness or stress, and reactions to other medicines all look like thinning hair and none of them is treated by this. A diagnosis comes first.

There is also a prostate-testing wrinkle worth carrying to a doctor: the drug lowers PSA, the blood marker used in prostate screening, so a PSA result taken while on it has to be interpreted knowing that. And in a seven-year prevention trial at five times the hair-loss strength, high-grade prostate cancer was found in 1.8% of men on finasteride against 1.1% on placebo — the label says the significance of that for men taking the hair-loss tablet is unknown, which is exactly as far as the evidence goes.

How people actually get a prescription

Four routes exist and they differ in cost, in how much of a clinician you get, and in who carries the risk. The cheapest is not the worst one.

One fact makes the whole table easier to read: finasteride is long off patent and sold generically, so the tablet itself is rarely where the money goes. What differs between routes is the consultation and the convenience.

What a first appointment is supposed to cover

The regulators have written down what they expect a prescriber to do, which makes it unusually easy to tell whether a consultation — in a room or through a screen — is doing its job. The MHRA's instructions to prescribers, published 11 May 2026, are the clearest statement of it.

• Your medical record is reviewed, and you are asked directly whether you have a history of depression or suicidal ideation, before anything is prescribed.
• The risks are explained at the point of prescribing — not buried in a leaflet you find later — including that sexual dysfunction has been reported to persist after treatment has stopped.
• You are pointed at the patient card and the leaflet that come in UK packs, which exist precisely because these warnings were not reaching people.
• Follow-up is arranged: the same advice tells prescribers to review patients regularly for psychiatric and sexual side effects, which means somebody has to be there to review you.

A consultation that asks none of that, and a questionnaire with no question about mood, is not meeting the standard its own regulator has published. That is a fair thing to notice about any route, including the paid one below.

What this site will not tell you

It will not tell you a strength, a schedule, or how to split anything. It will not tell you whether to start, because that answer depends on facts about you that no web page has. It will not show you before-and-after photographs, which are the least reliable evidence in this field and the easiest to stage. And it will not rate this medicine out of ten. A score would imply a recommendation, and a recommendation is not ours to make.