Short answer: In the label's 4-year PLESS trial of men with BPH, the most common Proscar side effects were sexual. In year 1, impotence was 8.1% vs 3.7% on placebo, decreased libido 6.4% vs 3.4% and decreased volume of ejaculate 3.7% vs 0.8%. In years 2 to 4, impotence and libido rates matched placebo. Breast enlargement, breast tenderness and rash were also listed. There is no boxed warning.
What Proscar is approved for: Proscar is a 5α-reductase inhibitor approved for symptomatic benign prostatic hyperplasia (BPH) in men with an enlarged prostate: to improve symptoms and lower the risk of acute urinary retention and of surgery such as TURP or prostatectomy (section 1.1). With the alpha-blocker doxazosin, it is approved to lower the risk of BPH symptoms getting worse (section 1.2). It is not approved to prevent prostate cancer (section 1.3), and it is not the hair loss product; that is Propecia, finasteride 1 mg.
Source
PROSCAR (finasteride) tablets, prescribing information and Patient Information. Organon. DailyMed, version published March 31, 2026. DailyMed setid 6f988153-fc74-4ca4-b29a-111f750c4a4b
No. The current Proscar label has no boxed warning.
These are the label's own numbers from the Adverse Reactions section. Where the label compares with placebo (a dummy treatment), both columns are shown, so you can see how much more often a reaction happened on Proscar.
| Adverse reaction | Year 1 finasteride (%) | Year 1 placebo (%) | Years 2-4 finasteride (%) | Years 2-4 placebo (%) |
|---|---|---|---|---|
| Impotence | 8.1 | 3.7 | 5.1 | 5.1 |
| Decreased Libido | 6.4 | 3.4 | 2.6 | 2.6 |
| Decreased Volume of Ejaculate | 3.7 | 0.8 | 1.5 | 0.5 |
| Ejaculation Disorder | 0.8 | 0.1 | 0.2 | 0.1 |
| Breast Enlargement | 0.5 | 0.1 | 1.8 | 1.1 |
| Breast Tenderness | 0.4 | 0.1 | 0.7 | 0.3 |
| Rash | 0.5 | 0.2 | 0.5 | 0.1 |
4-year placebo-controlled study (PLESS) of 1524 men on Proscar and 1516 on placebo. Only reactions the investigator judged possibly, probably or definitely drug-related, at 1% or more on Proscar and more than placebo, are listed. In years 2 to 4 there was no significant difference between groups in impotence, decreased libido and ejaculation disorder.
| Adverse experience | Placebo (N=737) (%) | Doxazosin 4 mg or 8 mg (N=756) (%) | Finasteride (N=768) (%) | Combination (N=786) (%) |
|---|---|---|---|---|
| Asthenia | 7.1 | 15.7 | 5.3 | 16.8 |
| Headache | 2.3 | 4.1 | 2.0 | 2.3 |
| Hypotension | 0.7 | 3.4 | 1.2 | 1.5 |
| Postural Hypotension | 8.0 | 16.7 | 9.1 | 17.8 |
| Peripheral Edema | 0.9 | 2.6 | 1.3 | 3.3 |
| Dizziness | 8.1 | 17.7 | 7.4 | 23.2 |
| Libido Decreased | 5.7 | 7.0 | 10.0 | 11.6 |
| Somnolence | 1.5 | 3.7 | 1.7 | 3.1 |
| Dyspnea | 0.7 | 2.1 | 0.7 | 1.9 |
| Rhinitis | 0.5 | 1.3 | 1.0 | 2.4 |
| Abnormal Ejaculation | 2.3 | 4.5 | 7.2 | 14.1 |
| Gynecomastia | 0.7 | 1.1 | 2.2 | 1.5 |
| Impotence | 12.2 | 14.4 | 18.5 | 22.6 |
| Sexual Function Abnormal | 0.9 | 2.0 | 2.5 | 3.1 |
MTOPS: 3047 men with symptomatic BPH on Proscar 5 mg a day, doxazosin 4 or 8 mg a day, both, or placebo for 4 to 6 years. The label's body-system heading rows (Body as a whole, Cardiovascular and so on) carry no numbers and are left out. The study was not designed for statistical comparisons of side effects between groups.
In PLESS, 3.7% (57 men) on Proscar and 2.1% (32 men) on placebo stopped treatment because of sexual side effects, the most frequently reported kind. New reports of drug-related sexual side effects decreased the longer treatment went on.
Prostate Cancer Prevention Trial (PCPT), 18,882 men 55 and older over 7 years: Gleason score 8-10 prostate cancer in 1.8% on finasteride 5 mg vs 1.1% on placebo. A 4-year trial of dutasteride showed similar results (1% vs 0.5%).
Male breast cancer: 4 cases in men on finasteride and none in men not on it in MTOPS; 2 cases on placebo and none on finasteride in PLESS; 1 case in each group in PCPT. The label says the link between long-term finasteride and male breast cancer is unknown.
In healthy men on Proscar for 24 weeks, ejaculate volume fell a median 0.6 mL (22.1%), with less total sperm per ejaculate. Values stayed in the normal range and returned to baseline after stopping, on average after 84 weeks.
Proscar lowered PSA by about 50% within six months of treatment.
Reported after approval: allergic reactions including itching, hives and swelling of the lips, tongue, throat and face; testicular pain; blood in semen; sexual problems that continued after stopping; male infertility or poor semen quality; depression; suicidal thoughts; and male breast cancer. Orgasm disorders that continued after stopping were reported with lower-dose finasteride for hair loss.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Lower PSA readings. Proscar cuts PSA by about half. A new PSA baseline is set at least six months after starting, and any confirmed rise from the lowest value on Proscar may signal prostate cancer and is evaluated, even within the normal range. For a single PSA value after six months or more on Proscar, the value is doubled for comparison with normal ranges. Missed doses can also affect PSA results.
High-grade prostate cancer. In the 7-year PCPT trial, finasteride 5 mg a day was linked to more Gleason 8-10 prostate cancer: 1.8% vs 1.1% on placebo. 5α-reductase inhibitors may raise the risk of high-grade prostate cancer.
Risk to a male fetus. Proscar may cause abnormal genitals in a male fetus. Women who are or may be pregnant should not handle crushed or broken tablets; whole coated tablets prevent contact during normal handling. After contact with crushed or broken tablets, the area is washed right away with soap and water.
Children and women. Proscar is not approved for children or for women.
Other urinary conditions. Other conditions with similar symptoms are considered before starting, and prostate cancer and BPH can occur together. Men with a large amount of urine left in the bladder or a very weak stream are watched closely for blockage and may not be candidates for finasteride.
Who should not use it. Women who are or may become pregnant, and anyone allergic to any component of the tablet.
From the Patient Information that comes with Proscar:
Dose Track has a side-effect log. Each entry records the symptom, a severity (none, mild, moderate, severe or life-threatening), the date and an optional note, and can be tied to a specific dose or logged on its own. The history screen shows what you logged over time, which makes it easier to tell your prescriber when a reaction started and whether it is easing. The app records what you enter; it does not judge whether a symptom is serious. For anything on the label's call-your-doctor list, call your doctor.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.