Finasteride Side Effects: What the 1mg and 5mg Trial Data Actually Show

Almost every finasteride article quotes the same two figures: roughly 1.8 percent report decreased libido and roughly 1.3 percent report erectile dysfunction. Both numbers are real and both come straight from the prescribing information. They also belong to one dose, one trial population and one year of treatment. Put them beside the numbers from the 5 mg prostate trials and the picture shifts enough to change how you read them. This guide sets both dose levels next to each other in their labelled form and works through what each column can and cannot tell you.

The two dose levels get quoted interchangeably and they should not be

Finasteride is sold at 1 mg for male pattern hair loss and at 5 mg for benign prostatic hyperplasia. Same molecule, same mechanism, five times the dose, and two completely separate adverse reaction tables in the FDA label. The 1 mg table comes from three controlled trials of 12 month duration in men with androgenetic alopecia. The 5 mg table comes from PLESS, a four year study in 3,040 men aged 45 to 78 with symptomatic prostate enlargement.

Those two populations are not comparable. The hair loss trials enrolled younger men. The prostate trials enrolled men in their fifties, sixties and seventies, a group with a far higher baseline rate of erectile difficulty regardless of what tablet they are taking. When a forum post claims finasteride causes impotence in 8 percent of users, it is quoting the 5 mg prostate figure and attaching it to a 1 mg hair loss product. That is the single most common error in this topic.

Year one figures for finasteride 1 mg

In the three 12 month trials, 945 men took finasteride 1 mg and 934 took placebo. Adverse experiences judged possibly, probably or definitely drug related and reported in at least 1 percent of either group were:

• Decreased libido: 1.8 percent on finasteride, 1.3 percent on placebo
• Erectile dysfunction: 1.3 percent on finasteride, 0.7 percent on placebo
• Ejaculation disorder: 1.2 percent on finasteride, 0.7 percent on placebo, with decreased volume of ejaculate specifically at 0.8 percent versus 0.4 percent
• Discontinuation because of a drug related sexual adverse experience: 1.2 percent versus 0.9 percent

The placebo column is the part people skip. Roughly two thirds of the men who reported reduced libido on finasteride would have reported it on a sugar tablet. The drug attributable increase for libido is about half a percentage point. For erectile dysfunction it is about six tenths of a percentage point.

The integrated analysis is the more honest single number

Looking at any one symptom understates the total because a man can report more than one. The label also gives a combined figure: 36 of 945 men on finasteride, or 3.8 percent, reported one or more of these experiences, compared with 20 of 934, or 2.1 percent, on placebo. That difference reached statistical significance at p equals 0.04.

So the defensible way to describe finasteride 1 mg is this: about 4 men in 100 will report a sexual side effect in the first year, and about 2 of those 4 would have reported one anyway. The net attributable rate is close to 1.7 percentage points. That is a small number, and it is not zero.

Year one figures for finasteride 5 mg

The PLESS data, covering 1,524 men on 5 mg daily and 1,516 on placebo, look different:

• Impotence: 8.1 percent on finasteride, 3.7 percent on placebo
• Decreased libido: 6.4 percent versus 3.4 percent
• Decreased volume of ejaculate: 3.7 percent versus 0.8 percent
• Ejaculation disorder: 0.8 percent versus 0.1 percent
• Breast enlargement: 0.5 percent versus 0.1 percent
• Breast tenderness: 0.4 percent versus 0.1 percent
• Rash: 0.5 percent versus 0.2 percent

Impotence at 5 mg runs roughly six times the 1 mg rate. Some of that is dose. A meaningful part of it is age, because these men were decades older. The label does not separate the two contributions, and no trial has ever directly compared 1 mg and 5 mg in the same population for sexual outcomes, which is why nobody can tell you exactly how much of the gap is the extra 4 mg.

What happens to those rates after year one

This is the most useful part of the label and the least quoted. In years two to four of PLESS there was no significant difference between the finasteride and placebo groups in impotence, decreased libido or ejaculation disorder. Impotence sat at 5.1 percent in both arms. Decreased libido sat at 2.6 percent in both arms.

At 1 mg the pattern is the same and more pronounced. The label states that the incidence of each of these adverse experiences fell to 0.3 percent or below by the fifth year of treatment. New reports of drug related sexual adverse experiences decreased as therapy continued rather than accumulating.

The practical reading: if finasteride is going to cause a sexual side effect, it usually shows up early. Men who get through the first year without a problem are unlikely to develop one in year three because of the drug.

Ejaculate volume is measured rather than self reported

Self reported symptoms are open to expectation effects. Ejaculate volume was measured. In healthy men taking 1 mg daily, median volume fell by 0.3 mL, about 11 percent, after 48 weeks. Placebo fell by 0.2 mL, about 8 percent. That is a real but small drug effect.

At 5 mg, two studies found a median decrease of roughly 0.5 mL, about 25 percent, compared with placebo, and the label states this was reversible after treatment stopped. The dose relationship here is clear and it is one of the few places in the label where you can see the 5 mg effect cleanly separated from the 1 mg effect.

Discontinuation rates say more than incidence rates

A side effect only matters if it is bad enough to stop treatment. In the 1 mg trials, 1.4 percent of men on finasteride discontinued because of a drug related adverse experience, against 1.6 percent on placebo. More men quit the placebo arm than the drug arm.

At 5 mg, 57 men out of 1,524, or 3.7 percent, stopped because of sexual adverse reactions, against 32 out of 1,516, or 2.1 percent, on placebo. Again the 5 mg figure is higher, and again it is roughly double placebo rather than many times it.

Reversibility: what the label says and what it leaves open

The label states that resolution occurred in men who discontinued finasteride because of these side effects, and in most of those who continued therapy. Both halves of that sentence matter. Symptoms resolving while treatment continues is common, and it is one reason clinicians often suggest giving an early side effect a few weeks before abandoning the drug.

The label is also explicit that stopping reverses the hair benefit. Withdrawal of treatment leads to reversal of effect within 12 months. Anything gained will be lost on roughly the same timeline it was gained.

Persistent sexual dysfunction and post-finasteride syndrome

The postmarketing section of the label lists sexual dysfunction that continued after discontinuation of treatment, including erectile dysfunction, libido disorders, ejaculation disorders and orgasm disorders. This is the cluster now widely called post-finasteride syndrome.

Two things are true at once and both should be said plainly. The reports exist, they are in the FDA label, and the agency thought them significant enough to include. At the same time, postmarketing reports are voluntary, come from a population of uncertain size, and cannot establish frequency or causation. A 2021 review of finasteride for hair loss traced how growing patient complaints and analysis of the FDA adverse event reporting database drove label changes, and noted that men with persistent sexual side effects were found to be at risk of suicide.

Nobody can currently tell you the incidence of persistent symptoms because no dataset supports that calculation. Anyone quoting a precise percentage in either direction is going beyond the evidence.

Depression and the 2011 label change

Depression appears in the postmarketing nervous system and psychiatric section. It was added to the FDA label in 2011 following analysis of adverse event reports. It is not in the clinical trial tables because the controlled trials did not detect a signal at that dose and duration.

If you have a history of depression, this belongs in the conversation with your prescriber before you start rather than after. It does not rule finasteride out, but it changes what you and your doctor should be watching for in the first few months.

Breast tenderness, enlargement and the male breast cancer question

In the 1 mg hair loss trials, breast tenderness and enlargement occurred at rates no different from placebo. At 5 mg over four years, breast enlargement reached 1.8 percent versus 1.1 percent and breast tenderness 0.7 percent versus 0.3 percent.

Male breast cancer appears in postmarketing reports. The trial data are genuinely mixed. In the MTOPS study of 3,047 men there were 4 cases among men on 5 mg and none among untreated men. In the PLESS study of 3,040 men there were 2 cases in the placebo arm and none on finasteride. In the Prostate Cancer Prevention Trial of 18,882 men there was 1 case in each arm. The label concludes that the relationship between long term finasteride use and male breast neoplasia is currently unknown. Any new breast lump, tenderness or nipple discharge should be examined regardless.

The PSA problem for men over 40

Finasteride lowers prostate specific antigen. In men aged 18 to 41 taking 1 mg, mean PSA fell from 0.7 ng/mL at baseline to 0.5 ng/mL at month 12. In older men taking 5 mg for prostate enlargement, PSA falls by roughly 50 percent.

This is not a side effect in itself. It is a test interference problem. If you are old enough to be screened for prostate cancer, your doctor needs to know you take finasteride so the result can be interpreted correctly. The label also warns that any confirmed rise from your lowest PSA value while on finasteride may signal prostate cancer and should be investigated even if the number still sits inside the normal range.

High grade prostate cancer and the PCPT finding

In the seven year Prostate Cancer Prevention Trial, men aged 55 and over taking finasteride 5 mg daily had a higher incidence of Gleason score 8 to 10 prostate cancer than placebo, 1.8 percent versus 1.1 percent. A four year trial of dutasteride found a similar pattern, 1 percent versus 0.5 percent.

The label states directly that the clinical significance of these findings for men taking 1 mg is unknown, and that whether the effect reflects the drug shrinking the prostate, study design factors or something else has not been established. This finding is worth knowing about and it is not a reason on its own to avoid the 1 mg hair loss dose.

Fertility and semen quality

Postmarketing reports include male infertility and poor seminal quality. The label adds that normalisation or improvement of seminal quality has been reported after finasteride was discontinued. If you are actively trying to conceive, this is worth raising with your doctor, and a semen analysis before starting gives you a baseline to compare against.

Hypersensitivity reactions

Postmarketing hypersensitivity reactions include rash, itching, hives and angioedema with swelling of the lips, tongue, throat and face. Angioedema involving the airway is a medical emergency. Stop the tablet and get urgent care rather than waiting to see whether it settles.

Who should not take finasteride at all

Finasteride is indicated for male pattern hair loss in men only. It is not indicated for women or for children. Women who are pregnant or might become pregnant should not handle crushed or broken tablets because of the risk to a male fetus. Intact coated tablets prevent contact with the active ingredient during normal handling, so the caution is specifically about broken or split tablets. This is one of several reasons splitting a 5 mg tablet at home is a poor idea in a household with a woman of childbearing age.

What to do if you develop a side effect

Tell your prescriber rather than stopping silently, because the label pattern of resolution on continued therapy is real and worth knowing about before you decide. Note when it started, since almost all drug related sexual effects appear in the first months. If you are on 5 mg for hair loss, which is off label, moving to the licensed 1 mg dose is the obvious first conversation. Do not stack a second 5-alpha-reductase inhibitor on top. Our dutasteride versus finasteride comparison covers how the two differ on this point.

Where the alternatives sit on side effect risk

Topical minoxidil works through a different mechanism entirely and carries no hormonal effect, which makes it the usual first switch for men who cannot tolerate an oral 5-alpha-reductase inhibitor. Our finasteride versus minoxidil comparison covers the efficacy trade, and our minoxidil strength guide covers which concentration is worth using. Topical finasteride is the other option, and the reasoning behind it is set out in our topical finasteride guide. A 2021 review found no significant efficacy difference between finasteride 5 mg daily, 1 mg daily and topical 1 percent solution, which is the argument for using the lowest exposure that works.

Finasteride products available at SafeRxPills

All of our finasteride tablets are the licensed 1 mg strength except where noted:

• Finalo 1mg by Intas Pharmaceuticals, 16 USD
• Finax 1mg by Dr Reddy's Laboratories, 24 USD
• F-Pecia 1mg by Cipla, 26 USD
• Finpecia 1mg by Cipla, 28 USD
• Healpecia 5mg by Healing Pharma, 30 USD, which is the prostate strength and is not the licensed dose for hair loss

Buying guidance including prescription rules by country is in our finasteride buying guide.

Safety information

Finasteride is a prescription only medicine in the United States, United Kingdom, Australia and Canada. It is indicated for male pattern hair loss in men only and is not for use in women or children. Speak to a licensed clinician before starting, changing or stopping it. Tell your doctor if you have a history of depression, are trying to conceive, or are due for prostate screening. Stop the tablet and seek emergency care for swelling of the lips, tongue, throat or face, or for difficulty breathing. Report any new breast lump, breast tenderness or nipple discharge to your doctor. Do not take finasteride together with dutasteride or any other 5-alpha-reductase inhibitor.

References

• FDA prescribing information for finasteride tablets USP, section 6.1 Clinical Trials Experience, Table 1 for the 1 mg male pattern hair loss trials and Table 2 for the 5 mg PLESS benign prostatic hyperplasia study
• FDA prescribing information for finasteride tablets USP, section 6.2 Postmarketing Experience, covering persistent sexual dysfunction, depression, male breast neoplasia and hypersensitivity reactions
• FDA prescribing information for finasteride tablets USP, sections 5.1 to 5.4, covering exposure of women, effects on prostate specific antigen and the Prostate Cancer Prevention Trial finding
• FDA prescribing information for finasteride tablets USP, section 2 Dosage and Administration, on the 1 mg daily dose and reversal of effect within 12 months of withdrawal
• PubMed PMID 34291720, finasteride for hair loss review, on hair count outcomes, the 2011 addition of depression to the label and the comparison of 1 mg, 5 mg and topical 1 percent
• PubMed PMID 29447628, review of sexual side effects of the 5-alpha-reductase inhibitors finasteride and dutasteride across prescribed dosages