Finasteride for hair loss: how it works, results and side effects

If you have started to notice that the hair at your crown is thinning or that your hairline is slowly creeping back, you have probably heard of finasteride. It is one of the few treatments for hormonal hair loss in men with years of evidence behind it, and also one of the most hotly debated: you will read everything from claims that it solves the problem for good to warnings that it wrecks your sex life. Here we try to separate what the data actually say from the background noise, without recommending anything for you specifically, because only a doctor who assesses your case can do that.

What finasteride is and why it is used for hair

Finasteride is an inhibitor of the type II 5-alpha-reductase enzyme, the one that converts testosterone into dihydrotestosterone (DHT). The block is fairly selective: it has roughly 100 times greater affinity for the type II isoenzyme than for type I. This matters because DHT is the androgen that, in genetically predisposed people, gradually miniaturises the hair follicle: each new hair comes through finer, shorter and less pigmented, until it stops growing altogether. By reducing DHT, finasteride slows that miniaturisation process.

That process has a name: androgenetic alopecia, the most common form of hair loss. It is very common and becomes more frequent with age. In white populations it is estimated to affect around 50% of men by the age of 50 and up to nearly 80% by 70. These are indicative figures: they vary depending on the study, ethnicity and the scale used to measure it (the classic one is the Norwood-Hamilton scale), so they are best read as a trend rather than an exact number.

How it works: less DHT, less pressure on the follicle

The effect on DHT is measurable and marked. In a dose-response study, scalp DHT fell by a median of 64% with 1 mg a day and 69% with 5 mg a day after 42 days, while DHT in the blood drops by around 70%. In other words, the 1 mg dose used for hair already achieves a good part of the local fall in DHT. The aim of treatment is not to make hair grow out of nothing, but to take away the hormonal stimulus that was shrinking the follicle.

What the evidence says about results

The trials underpinning its use are solid. In the one-year pivotal studies, involving 1,553 men aged between 18 and 41, finasteride 1 mg a day improved scalp hair across all assessment techniques compared with placebo, with statistically highly significant differences (P<0.001) at one year and at two years.

Over the longer term, in men with vertex alopecia (the crown), after five years of treatment with 1 mg a day: 90% showed no further visible hair loss on photographic assessment, and 65% showed an increase in hair count. By contrast, in the placebo group 100% of the men continued to lose hair progressively. Read properly, this tells us two things: the drug's main benefit is preventing further loss, and in some cases some density is also regained.

One important caveat, so as not to raise false expectations: efficacy has not been established for receding temples (the bitemporal area) or for hair loss at advanced stages. Put another way, finasteride is not designed to fill in an area that is already bald, but to stabilise things and, in early stages or at the vertex, partially reverse them. The sooner you act, the more there is to preserve.

An ongoing treatment, not a permanent one

The effect depends on continuing to take it. It usually takes 3 to 6 months to see things stabilise, so patience is part of the treatment. And the reverse is also true: if it is stopped, the benefit starts to wear off at 6 months, between 9 and 12 months you are back where you started, and the miniaturisation process picks up where it left off. That is why it is not accurate to present it as a permanent solution; it works while you take it, not afterwards.

Safety: which side effects to watch for

This is the part worth understanding calmly. The Spanish summary of product characteristics lists, among other adverse reactions: mood changes (depressed mood, depression and, less frequently, suicidal ideation), erectile dysfunction and ejaculation disorders. It also includes post-marketing reports of sexual dysfunction that persists after stopping the drug, and reports of breast cancer in men.

In May 2025 the Pharmacovigilance Risk Assessment Committee (PRAC) of the European Medicines Agency concluded a review and recognised suicidal ideation as an adverse reaction to finasteride. It assessed 325 cases in the European database (313 for finasteride and 13 for dutasteride), most of them involving 1 mg finasteride for hair loss. As a result, the product information for finasteride 1 mg now includes a warning and a patient alert card, and notes that sexual dysfunction may contribute to mood changes and suicidal ideation.

Now the level-headed reading, neither playing it down nor blowing it up. The fact that the EMA recognises suicidal ideation does not tell us how many people experience it: the agency classifies it as of unknown frequency, because the available data do not allow it to be estimated. Nor should it be swept under the carpet, precisely because a patient alert card has been added. On persistent sexual dysfunction, honesty means saying two things at once: there are reports of cases that continue after stopping the drug, so it is not true that it always resolves; but so-called post-finasteride syndrome is a reported pattern, not a proven causal link. These are signals under monitoring, not settled certainties.

Common myths, dismantled

• "It grows hair where I am already bald." No: its efficacy has not been established in advanced loss or at the temples. Its role is mainly to slow things down and, in early stages, recover some density.
• "It works for women too." Not in this context: in Spain it is contraindicated in women. What is more, women who are pregnant or may become pregnant should not even handle broken or crushed tablets, because of the risk of genital abnormalities in male fetuses.
• "It is a permanent solution." No: the effect only lasts while you take it; once stopped, the benefit starts to wear off at 6 months and by around 9 to 12 months you are back where you started.
• "The oral version and the topical spray are the same thing." They do not have the same status. The EMA (2025) found no evidence linking the cutaneous spray to suicidal ideation and did not change its information, whereas the FDA (April 2025) issued a warning about compounded topical finasteride. Its regulatory and safety situation differs from that of the tablet.
• "Dutasteride is approved for hair." In Spain, no: it is authorised only for benign prostatic hyperplasia. Its use for hair is off-label or via a pharmacy-compounded preparation, always on prescription.

The situation in Spain

Oral finasteride 1 mg is indeed authorised in Spain for androgenetic alopecia in men aged 18 to 41, in early stages and on prescription. Both the original brand (Propecia) and numerous EFG generics registered with the AEMPS are available (Normon, Cinfa, Stada, Sandoz and Viatris, among others). It is contraindicated in women. The other standard treatment for androgenetic alopecia is topical minoxidil, which works through a different, non-hormonal mechanism and is sometimes considered alongside finasteride. Dutasteride, by contrast, is not authorised in Spain for hair, and compounded topical finasteride does not share the same regulatory status as the oral form.

In short, and what to do with this

Finasteride is a treatment with proven efficacy for slowing and, in part, reversing hormonal hair loss in men, provided it is taken continuously and at stages where there is still follicle left to preserve. It has real benefits and a safety profile updated in 2025 that deserves an honest conversation before starting: deciding whether it is right for you is an individual clinical assessment.

That step is easy to take today. In Spain that route is medical treatment for hair loss with a prior assessment, a prescription where appropriate from a registered doctor and ongoing follow-up by the medical team. You can check in a few minutes whether you are eligible.