Finasteride Side Effects: Complete Guide
Medically reviewed by [Reviewer Name, MD] + [PharmD Name, PharmD] | Last reviewed: August 2026 | Editorial policy | Sources
Finasteride is generally well tolerated — the majority of men who take it experience no significant side effects. However, a minority do report adverse effects, predominantly sexual in nature, and a smaller subset report persistent symptoms after stopping (Post-Finasteride Syndrome). This page presents the complete side effect profile drawn from FDA prescribing information and peer-reviewed clinical data, without minimizing or overstating the risks.
Summary: Side Effect Frequencies from Clinical Trials
| Side Effect | Finasteride 1 mg (AGA) | Placebo | Finasteride 5 mg (BPH) | Placebo |
|---|---|---|---|---|
| Decreased libido | 1.8% | 1.3% | 3.7% | 2.1% |
| Erectile dysfunction | 1.3% | 0.7% | 8.1% | 3.7% |
| Ejaculation disorder (decreased volume) | 0.8% | 0.4% | 7.7% | 2.1% |
| Gynecomastia (breast enlargement or tenderness) | 0.4% | 0.2% | Not separately reported | — |
| Testicular pain | Rare (<1%) | — | Not separately reported | — |
In most men who experienced sexual side effects in clinical trials, the effects resolved after discontinuation. The exception — persistent effects after stopping — is addressed in the Post-Finasteride Syndrome section below.
Sexual Side Effects
Sexual dysfunction is the most commonly reported category of finasteride side effects. Three main types are reported:
Decreased Libido (Reduced Sex Drive)
Decreased libido occurred in 1.8% of finasteride 1 mg users vs. 1.3% on placebo — an excess of 0.5 percentage points. The absolute risk is low, and the mechanism is not fully understood; it may relate to reduced central or peripheral neurosteroid levels (as DHT is a precursor to neuroactive steroids such as allopregnanolone) or to anxiety about side effects (the nocebo effect contributes meaningfully in open-label studies). Most cases in trials resolved after stopping finasteride.
Erectile Dysfunction (ED)
Erectile dysfunction occurred in 1.3% of finasteride 1 mg users vs. 0.7% on placebo in the pivotal AGA trials. The rate is higher at the 5 mg BPH dose (8.1% vs. 3.7% placebo) — largely because the BPH trial population was older and had higher baseline rates of ED. In the majority of trial participants, ED resolved after stopping finasteride.
Ejaculation Disorders
Decreased ejaculate volume is the most commonly reported ejaculatory adverse effect, occurring in 0.8% of finasteride 1 mg users vs. 0.4% on placebo. This is attributable to finasteride’s suppression of DHT in the seminal vesicles and prostate, which reduces their secretory contribution to ejaculate volume. Most cases resolve on discontinuation.
Detailed guide: Finasteride and sexual side effects
Post-Finasteride Syndrome (PFS)
2012 FDA Label Update: The FDA updated finasteride prescribing information to add that sexual adverse effects — including decreased libido, erectile dysfunction, and ejaculatory disorders — may persist after discontinuation in some patients.
Post-Finasteride Syndrome (PFS) refers to a constellation of reported persistent adverse effects — primarily sexual (ED, loss of libido, ejaculatory dysfunction), but also neuropsychological (depression, cognitive impairment, emotional blunting, fatigue) and physical (genital numbness, penile atrophy, gynecomastia) — that continue or begin after stopping finasteride.
The honest evidence summary:
- PFS is clinically recognized and acknowledged in FDA labeling
- Case series, registry data (Khera et al., 2021), and patient-reported outcome studies document the phenomenon in a subset of former finasteride users
- The true frequency of persistent PFS-type symptoms is not yet established by a large, methodologically sound epidemiological study
- Proposed biological mechanisms include persistent epigenetic dysregulation of androgen receptor signaling, neurosteroid pathway disruption, and altered GABAergic function — none conclusively proven in humans as of 2026
- Men experiencing persistent symptoms after stopping finasteride should be evaluated by a physician; the PFS Foundation (pfsfoundation.org) supports ongoing research
Complete Post-Finasteride Syndrome evidence review
Gynecomastia
Gynecomastia (breast tissue enlargement or tenderness in men) was reported in 0.4% of finasteride 1 mg users vs. 0.2% on placebo. This is likely related to the minor compensatory increase in testosterone (and subsequent aromatization to estradiol) that can accompany DHT suppression. Cases of gynecomastia should be evaluated by a physician, as any breast lump in a man warrants clinical assessment regardless of medication use.
Finasteride and gynecomastia: causes, frequency, and what to do
PSA Effects — Important for Cancer Screening
Finasteride reduces serum PSA (prostate-specific antigen) levels by approximately 50% after 6 months of treatment, regardless of dose. This applies to both the 1 mg (hair loss) and 5 mg (BPH) doses.
Clinical implication: Men on finasteride who undergo PSA screening for prostate cancer should have their PSA value doubled by their physician when interpreting results, to estimate the PSA level they would have without finasteride. Any confirmed increase in PSA while on finasteride — even a value within the normal range — should be evaluated as a potential cancer signal. PSA velocity (rate of change) is not affected by finasteride and remains a useful monitoring parameter.
Depression and Neuropsychological Effects
Post-marketing surveillance and a 2019 EMA safety review led to an update of European finasteride labels to include depression, suicidal ideation, and emotional disorders as potential adverse effects. The US label does not currently include a depression warning, though the FDA has received post-marketing reports.
The proposed mechanism involves DHT’s role as a precursor to neuroactive steroids (neurosteroids), particularly allopregnanolone, a positive allosteric modulator of GABA-A receptors. Reduced neurosteroid levels may affect mood regulation. Epidemiological studies on this association have yielded mixed results.
Men with a personal or family history of depression should discuss this risk with their physician before starting finasteride. Any new or worsening depressive symptoms during finasteride treatment should be reported to a physician promptly.
Finasteride and depression: what the evidence shows
Teratogenicity — Critical Warning for Women
PREGNANCY CATEGORY X: Finasteride is absolutely contraindicated in pregnancy. Women who are or may become pregnant must not take finasteride and must not handle crushed or broken finasteride tablets. Intact tablets are coated; this coating must not be broken.
Finasteride causes fetal abnormalities of the external genitalia in male pregnancies. Type II 5-alpha-reductase activity is required for normal male genital differentiation in utero; blocking it with finasteride during pregnancy causes hypospadias and ambiguous genitalia in male fetuses. This has been demonstrated in animal models and is supported by the natural experiment of congenital 5-alpha-reductase Type II deficiency in humans.
Side effects of finasteride relevant to women
Prostate Cancer — The PCPT Controversy
The Prostate Cancer Prevention Trial (PCPT; Thompson et al., 2003) found that finasteride 5 mg reduced overall prostate cancer incidence by 24.8% over 7 years. However, the trial also found a higher prevalence of high-grade (Gleason 7–10) cancer in the finasteride arm. Subsequent analyses (Lucia et al., 2007; Thompson et al., 2013) concluded this likely reflected a detection bias — finasteride shrinks prostate volume, improving biopsy sampling accuracy and making high-grade cancer more detectable, not more frequent.
The FDA added a label warning in 2011 noting the observed high-grade cancer association. Major urology guidelines (AUA, EAU) do not conclude that finasteride causes high-grade prostate cancer.
Frequently Asked Questions
How common are finasteride side effects really?
In the controlled pivotal trials, sexual side effects occurred at rates 0.5–0.6 percentage points above placebo at the 1 mg dose. The majority of men taking finasteride 1 mg (roughly 95%+) do not report significant adverse effects in clinical trials. However, observational and post-marketing data — which are less controlled — suggest higher rates in some populations. The placebo-controlled trial data represents the most rigorous available evidence for causation at the 1 mg dose.
Do finasteride side effects go away?
In most men who experience sexual side effects in clinical trials, the effects resolved after stopping finasteride. However, in a subset of men (frequency uncertain), sexual and other adverse effects have been reported to persist or worsen after discontinuation — a pattern called Post-Finasteride Syndrome. If you experience side effects on finasteride, discuss them with your physician rather than stopping abruptly; and if symptoms persist after stopping, seek medical evaluation.
Are finasteride side effects worse at 5 mg than 1 mg?
Yes, the 5 mg (BPH) dose produces higher rates of sexual side effects than the 1 mg (hair loss) dose, as shown in the prescribing information. Erectile dysfunction, for example, was reported in 8.1% of 5 mg users vs. 1.3% of 1 mg users (compared to placebo rates of 3.7% and 0.7% respectively). The BPH trial population was also older and had more baseline comorbidities affecting sexual function, which contributes to this difference.
About this page: Reviewed by [Reviewer Name, MD] and [PharmD Name, PharmD]. Data sourced from FDA prescribing information and peer-reviewed clinical literature. Last reviewed August 2026. Editorial policy.
Sources
- Merck & Co. Propecia (finasteride 1 mg) US Prescribing Information. Revised 2022.
- Merck & Co. Proscar (finasteride 5 mg) US Prescribing Information. Revised 2022.
- U.S. FDA. Drug Safety Communication: Update — 5-alpha reductase inhibitors: FDA adds labeling changes regarding risk of persistent sexual side effects. April 11, 2012.
- Khera M, et al. “A new era of testosterone and prostate cancer: from physiology to clinical implications.” Eur Urol. 2021.
- Thompson IM, et al. “The influence of finasteride on the development of prostate cancer.” N Engl J Med. 2003;349(3):215–224.