Finasteride Side Effects in Men: Rates, Risk Factors, and What to Expect

This page provides the exact rates of finasteride side effects in men as reported in double-blind, placebo-controlled clinical trials — the gold standard for attributing adverse events to a drug. Understanding the difference between trial-reported rates and anecdotal reports is critical to making an informed decision.

Side Effect Rates: Finasteride 1 mg vs. Placebo (Men)

Side Effect	Finasteride 1 mg	Placebo	Excess vs. placebo
Decreased libido	1.8%	1.3%	+0.5 percentage points
Erectile dysfunction	1.3%	0.7%	+0.6 percentage points
Ejaculation disorder	0.8%	0.4%	+0.4 percentage points
Breast tenderness/enlargement	0.4%	0.2%	+0.2 percentage points
Rash	0.5%	0.3%	+0.2 percentage points

Key takeaway: In the pivotal double-blind trials, the excess rate of any sexual side effect attributable to finasteride (above placebo) was approximately 1.1 percentage points. Roughly 95–98% of men in clinical trials did not report sexual adverse events attributable to finasteride. These numbers are widely quoted; it is equally important to know that Post-Finasteride Syndrome — persistent effects after stopping — is a real but poorly quantified risk that is not fully captured by these trial figures.

The Nocebo Effect: Anxiety and Reported Side Effects

In open-label (non-blinded) studies, reported rates of sexual side effects are substantially higher than in double-blind trials. Mondaini et al. (2007) found that men who were told about finasteride’s potential sexual side effects before starting it reported 3× higher rates of ED (44%) than men who were not informed (15%). This is the nocebo effect — adverse effects caused by expectation of harm rather than the drug’s pharmacological action.

This does not mean that finasteride’s sexual side effects are all nocebo-mediated — the double-blind trial data clearly shows a real pharmacological effect. It means the total reported side effect rate in clinical practice includes both pharmacological and psychological components, and that anxiety about taking finasteride can itself cause or worsen sexual dysfunction.

Risk Factors: Who Is More Likely to Experience Side Effects?

The clinical trial data does not identify specific predictors of finasteride-related sexual side effects. From available evidence, men with the following characteristics may have higher baseline susceptibility:

• Pre-existing sexual dysfunction, anxiety, or depression before starting finasteride
• High anxiety about sexual side effects (nocebo-prone individuals)
• Higher age (overlapping with natural age-related sexual function decline)
• Cardiovascular risk factors (independent predictors of erectile dysfunction)

What to Do If You Experience Side Effects

• Report to your prescribing physician: Do not stop finasteride abruptly without medical guidance, particularly if you have concerns about persistent effects
• Evaluate baseline vs. on-drug: Document your sexual function before starting finasteride so you have a true baseline to compare
• Give it time: Minor initial changes in the first 1–2 months are common and often resolve; persistent effects warrant follow-up
• If effects persist after stopping: Seek evaluation; post-discontinuation persistent sexual symptoms should be investigated for PFS and for other contributing conditions

See the full guides: Complete side effects hub | Sexual side effects | Post-Finasteride Syndrome | Depression and finasteride

Frequently Asked Questions

What percentage of men have no side effects from finasteride?

In double-blind clinical trials, approximately 95–98% of men taking finasteride 1 mg did not report the specific sexual adverse events measured (decreased libido, ED, ejaculation disorder). Combined, these three categories occurred in ~3.8% of finasteride users vs. ~2.1% in placebo (about 1.7% attributable excess). The vast majority of men do not report significant adverse effects in controlled trials. Real-world rates may be higher due to open-label context, nocebo effects, and reporting bias.

Are finasteride side effects permanent?

In most clinical trial participants, sexual side effects resolved after stopping finasteride. However, a subset of men report persistent or worsening sexual and other adverse effects after stopping — a phenomenon called Post-Finasteride Syndrome. The frequency of persistent effects is not established by large epidemiological studies. The 2012 FDA label update acknowledges that effects may persist after discontinuation in some patients. See the full PFS guide for more detail.

Sources

• Merck & Co. Propecia (finasteride 1 mg) US Prescribing Information. Revised 2022.
• Mondaini N, et al. “Finasteride 5 mg and sexual side effects: how many of these are related to a nocebo phenomenon?” J Sex Med. 2007;4(6):1708–1712.