Finasteride side effects in trials, labels and regulators' warnings

Finasteride for hair loss is a prescription tablet, 1 mg once a day on the US and UK labels, and in some European countries a spray for the scalp. Its unwanted effects are the part of the decision that deserves the most time. ^{[1, 2, 3, 4]}

This page organises what each kind of source says about them: the trials, the product labels and the medicines regulators, each with its country and its date. It is written for adult men with male pattern hair loss who are deciding whether to start, or who have noticed something and want to know what is documented. What finasteride does for hair, the dose and the cost are in the full finasteride profile.

A 5 mg tablet of the same medicine is used for an enlarged prostate. This page is about the 1 mg dose, and figures from the 5 mg product do not carry over.

The side effects at a glance

Three kinds of information are involved. They should not be mixed.

The list below sorts each group of effects by the kind of information behind it. The sections that follow give the numbers, the sources and the limits.

Sexual side effects in the trials

The first year, on the US label

The main trial figures come from the US label for the 1 mg tablet. It summarises three trials in men with male pattern hair loss that ran for a year: 945 men took finasteride and 934 took placebo. At least one sexual side effect that the investigators put down to the treatment was recorded in 36 men on finasteride (3.8%) and 20 on placebo (2.1%). ^[1]

The placebo bars matter. Some men taking a dummy tablet also had sexual problems that were put down to the treatment, so the number to read is the gap between the groups: 1.7 percentage points, or about 17 more men in every 1,000 over that year. In the other 909 of the 945 men on finasteride, no sexual side effect of these kinds was recorded as related to the tablet in that year. What the tablet trials found for hair is in the profile, under what the trials measured. ^[1]

A 24-week trial that counted more

A later trial compared dutasteride, a medicine of the same group, with finasteride and with placebo over 24 weeks. Of the 917 men randomly assigned to its five groups, 179 took finasteride 1 mg and 181 took placebo. It tracked sexual and breast-related side effects as a category of special interest. ^[7]

Both columns are higher than on the label, the placebo column included. Studies define these events differently and collect them differently, which is why percentages from separate studies should not be lined up as if they came from one. What the two sources share is the direction: more events with finasteride than with placebo. ^{[1, 7]}

Fifteen trials pooled

A 2019 analysis of sexual side effects in the treatment of male pattern hair loss pooled 15 randomised trials, with 4,495 participants, that compared finasteride or dutasteride with placebo. Sexual side effects were about 1.7 times as likely with finasteride 1 mg as with placebo. For dutasteride 0.5 mg the estimate was about 1.4 times, with a range that includes no difference at all. ^[10]

The analysis has limits of its own. The trials it pooled did not all last the same time. How side effects were recorded, and whether they cleared after stopping, was incompletely reported. The dutasteride group includes a study in healthy volunteers. Its authors declared no conflicts of interest, and we found no statement of specific funding. The two medicines are set against each other in finasteride vs dutasteride. ^[10]

Symptoms that continue after stopping

Some men look for this under the name "post-finasteride syndrome". Our research does not use or define that name. What two of our sources state is narrower: sexual problems that carried on after finasteride was stopped. The US label lists them among the reports received once the tablets were on sale. On 11 May 2026 the UK medicines regulator confirmed that sexual dysfunction linked to finasteride tablets can persist after treatment is stopped. ^{[1, 5]}

What neither gives is a frequency. Reports made after a medicine goes on sale come without a count of how many men took it, so they cannot be turned into a percentage. The trial figures above do not fill the gap, because they were collected during treatment. ^{[1, 5]}

We did not review studies of post-finasteride syndrome as a separate condition. Our research records what the US label and the UK regulator state, does not dismiss the symptoms men report, and does not attach a universal percentage to a syndrome.

The medicine itself leaves the blood quickly. Half of a dose is gone in about five to six hours in men aged 18 to 60. That describes the medicine, not a symptom: how fast it clears neither proves nor rules out an effect that lasts longer. For dutasteride, the US label also says sexual side effects may continue after stopping and adds that the role of the medicine in this is not known, as set out in the dutasteride profile. ^{[1, 11]}

Mood: depression and suicidal thoughts

Depression and suicidal thoughts are recognised side effects of finasteride tablets. The trial figures above do not cover them. What we have comes from the US label's list of reports made after the medicine went on sale and from regulators' safety documents, which are not trials. Each statement has a date and a scope.

The rows are one label, one European safety review and one UK safety update, not three studies. ^{[1, 4, 5]}

Three points from these documents are easy to lose. A frequency that is "not known" means neither rare nor common. Changes in mood can appear with or without sexual side effects. And the European review's conclusion that benefits outweigh risks applies to the uses these medicines are approved for. ^{[4, 5]} The comparison of finasteride and dutasteride shows how the mood warnings differ between the two medicines.

No source we reviewed gives a figure for how often either happens, and nothing on this page estimates one person's risk.

Semen, fertility and pregnancy

The trial in young men

One trial looked at semen directly. It randomly assigned 181 healthy men aged 19 to 41 to finasteride 1 mg or placebo and measured semen in 79 of them, through 48 weeks of treatment and a further 60 weeks without it. It found no significant difference between the groups in the main sperm measures. ^[9]

The limits matter as much as the result. These were healthy volunteers, so the trial does not answer the question for men who already have a fertility problem. It measured semen, not pregnancies or births. And "no significant difference in this group" is a narrower statement than a guarantee for each man. ^[9]

For dutasteride, a 52-week study on its US label, with 27 healthy men on the medicine and 23 on placebo, recorded falls in semen measures. That finding belongs to dutasteride and does not transfer to finasteride in either direction. ^[11]

Pregnancy and handling

For a partner who is pregnant, or for plans to conceive, the leaflet of the exact product in your country is the reference, and it is a question for the prescriber. We do not work out a timetable for conception from these sources.

Breast changes and prostate blood tests

Two further points come up in our sources, and a third card says where our reading of the labels stops.

The breast figure comes from the trial that compared dutasteride with finasteride, and the PSA figure from the US label. Figures from trials of the 5 mg prostate dose are not rates for men taking 1 mg for hair loss. ^{[7, 1]}

The scalp spray: less in the blood, not shown to be safer

A spray puts finasteride on the scalp instead of sending a tablet through the whole body, and it is tempting to assume that must be safer. The one placebo-controlled spray trial we reviewed does not show it.

That trial tested one product, 2.275 mg per mL, used once a day for 24 weeks by men aged 18 to 40. For side effects it counted 181 men on the spray, 181 on the same spray without finasteride and 84 on the 1 mg tablet. ^{[8, 12]}

Three things stand between "less in the blood" and "safer". First, some finasteride does reach the blood, and DHT, the hormone finasteride lowers, still fell there: by about 35% with the spray against about 56% with the tablet. Second, a blood level measures exposure. It is not a rate of side effects, and no study we reviewed shows that one falls in step with the other. Third, the groups were small and the trial lasted 24 weeks, which is too little to say anything about rare events or about symptoms that continue after stopping. ^[8]

Regulators treat the two forms separately. The European review that confirmed suicidal thoughts as a side effect of the tablets found no evidence linking them to the sprays. That is an absence of evidence in that review, not a guarantee. What the spray does for hair is covered on the page about topical finasteride. ^[4]

Compounded sprays in the United States

In the United States no finasteride product for the scalp is approved, alone or mixed with anything else. What is sold there is made to order by compounding pharmacies, and the US regulator has issued a notice about those products. ^[6]

A made-to-order spray is also not the studied spray. Strength, volume and applicator decide how much medicine is delivered, and the trial product gave 0.05 to 0.2 mL a day. A product whose instructions say 1 mL is not that product. ^[8]

We reviewed one large report on products of this kind. A US telehealth company funded an analysis of 638,629 men who were prescribed its scalp products, all containing finasteride 0.3% and minoxidil. Of the 151,352 who answered a follow-up question, 4,034 reported side effects or other negative reactions. The answers are the patients' own and there was no comparison group, so the report cannot show that a scalp product is safer than the tablet. ^[14] The trials of both medicines in one scalp product are on the page about minoxidil and finasteride together.

What happens when treatment is stopped

Stopping raises two separate questions, and an answer to one says nothing about the other: what happens to the hair, and what happens to a side effect.

The first three rows describe the medicine and the hair, the last two describe side effects. ^{[1, 3, 5]}

The reversal on the label describes what was observed. It is not advice to stop, to pause or to change the schedule. A dated note of what changed and when gives a prescriber something to work with. That is an organising suggestion of ours, not something a study tested. The profile has more on what happens to the hair if you stop.

Who the labels say it is not for

Each label defines who its product is for, and the three we checked do not say the same thing. ^{[1, 2, 3]}

The tablet labels are for male pattern hair loss. Hair that falls out suddenly or in patches, or a scalp that is sore or inflamed, is a different situation, covered in types of hair loss and when to get assessed first. The UK regulator also asks prescribers to ask about any history of depression or suicidal thoughts. ^[5]

What is not known

These gaps rest on the same sources as the sections above. ^{[1, 5, 4, 13, 8, 3, 9, 6]}

Where to go next

Side effects are one of eight things we weigh for every ingredient. Finasteride sits in the top tier of our ranking together with minoxidil, and the risks described here are the trade-off recorded there. How we handle evidence, and why we stop where the sources stop, is on the methodology page.