Finasteride Side Effects: Rare but Serious Adverse Events

Does finasteride carry a boxed warning?

No. Neither the 1 mg label (Propecia, male pattern hair loss) nor the 5 mg label (Proscar, benign prostatic hyperplasia) has a boxed warning [1][2]. The events people ask about most — depression, suicidal thoughts, sexual problems that outlast treatment — sit in the Postmarketing Experience section. Reports in that section are voluntary, come from a population of uncertain size, and cannot reliably establish a causal relationship, so they show that an event was reported, not that finasteride caused it [1][2]. The high-grade prostate cancer warning is different: it is a numbered Warnings and Precautions subsection supported by a randomized trial [2].

What the FDA changed, and when

The Propecia label of June 2021 listed postmarketing 'Nervous System/Psychiatric: depression'. On May 31, 2022 the FDA issued a Safety Labeling Change order under section 505(o)(4) of the FD&C Act, citing new safety information on the risk of suicidal ideation and behavior, an additional risk of sexual dysfunction, and psychoneurocognitive adverse events. Supplement S-030, approved August 1, 2022, changed the wording to 'depression, suicidal ideation and behavior' [3]. The current Propecia label (effective October 14, 2024) lists depression, suicidal ideation and behavior, and the current Proscar label (effective March 18, 2026) lists depression and suicidal ideation [1][2].

How common are sexual side effects in the trials?

In three 12-month placebo-controlled Propecia trials (945 men on finasteride, 934 on placebo, ages 18 to 41), drug-related decreased libido was reported by 1.8% versus 1.3%, erectile dysfunction by 1.3% versus 0.7%, and ejaculation disorder by 1.2% versus 0.7%. Combined, 3.8% of men on finasteride reported at least one of these versus 2.1% on placebo (p=0.04), and 1.2% versus 0.9% discontinued because of them [1]. Each individual rate fell to 0.3% or less by the fifth year [1]. A 2019 meta-analysis of 15 randomized placebo-controlled trials covering 4,495 men (finasteride 1 mg or dutasteride 0.5 mg) calculated a relative risk of sexual dysfunction of 1.57 (95% CI 1.19 to 2.08) overall and 1.66 (95% CI 1.20 to 2.30) for finasteride alone [6].

Those figures come with a question about how well the trials captured events. A 2015 meta-analysis of 34 finasteride trials in androgenetic alopecia found that none had adequate safety reporting, 76% evaluated safety for one year or less, and funnel plots suggested sexual adverse effects were systematically under-detected [7].

At 5 mg, the PLESS trial (1,524 men on finasteride, 1,516 on placebo, ages 45 to 78 with BPH) reported year-one impotence in 8.1% versus 3.7%, decreased libido in 6.4% versus 3.4%, and decreased ejaculate volume in 3.7% versus 0.8%. In years 2 to 4 there was no difference between groups for impotence (5.1% versus 5.1%) or decreased libido (2.6% versus 2.6%) [2].

Sexual dysfunction that continued after stopping

The Propecia label lists 'sexual dysfunction that continued after discontinuation of treatment, including erectile dysfunction, libido disorders, ejaculation disorders, and orgasm disorders', and 'male infertility and/or poor seminal quality', noting that normalization or improvement in seminal quality has been reported after stopping [1]. The Proscar label says such persistent events were reported rarely in men taking 5 mg for BPH, that most of those men were older and taking other medicines, and that the independent role of Proscar is unknown [2].

One published description comes from standardized interviews with 71 otherwise healthy men aged 21 to 46 who reported new sexual side effects lasting at least three months after stopping finasteride. Mean duration of use was 28 months and mean persistence 40 months; 94% reported low libido, 92% erectile dysfunction, 92% decreased arousal and 69% orgasm problems [8]. Because the men volunteered with the symptom and were recalling their own history, the study cannot show how often this happens or prove that finasteride caused it [8].

A 2016 study compared finasteride users with persistent symptoms, finasteride users without symptoms, and men who had never used it. The symptomatic group had impaired sexual function, higher depression scores, a more negative affect balance and more cognitive complaints, but normal scores on objective cognitive testing and no evidence of androgen deficiency, decreased peripheral androgen action or persistent peripheral inhibition of the enzyme [9].

Depression and suicidal ideation

The Propecia label lists depression, suicidal ideation and behavior, and the Proscar label lists depression and suicidal ideation [1][2]. The supporting data run in two directions.

In a 2012 cross-sectional study, 61 former finasteride users with persistent sexual side effects were compared with 29 men who had hair loss but had never used the drug. Depressive symptoms, defined as a Beck Depression Inventory-II score of 14 or higher, were present in 75% (46 of 61) versus 10% (3 of 29), P<0.0001 [10]. The men were self-selected and the design cannot establish cause [10].

Two large cohort studies of older men looked at suicide. In Ontario, 93,197 men aged 66 or older who started a 5-alpha reductase inhibitor were compared with matched unexposed men. Suicide was not increased (hazard ratio 0.88, 95% CI 0.53 to 1.45). Self-harm was higher only in the first 18 months (HR 1.88, 95% CI 1.34 to 2.64), and depression was higher in the first 18 months (HR 1.94, 95% CI 1.73 to 2.16) and afterward (HR 1.22, 95% CI 1.08 to 1.37), with absolute increases of 17 self-harm and 237 depression cases per 100,000 patient-years [4].

In Sweden, 2,236,876 men aged 50 to 90 were followed, including 70,645 who started finasteride at a median age of 73. Finasteride was associated with depression (HR 1.61, 95% CI 1.48 to 1.75), a hazard that stayed constant over time, but not with completed suicide (HR 1.22, 95% CI 0.99 to 1.49) [5]. Neither cohort was made up of younger men taking 1 mg for hair loss; both were drawn largely from older men treated for prostate enlargement [4][5].

High-grade prostate cancer: the 5 mg signal

The prostate cancer warning rests on data from the 5 mg dose. In the Prostate Cancer Prevention Trial, 18,882 men aged 55 or older with a normal digital rectal examination and a PSA of 3.0 ng/mL or lower took finasteride 5 mg or placebo for seven years. Among men with data for the final analysis, prostate cancer was detected in 803 of 4,368 men taking finasteride (18.4%) and 1,147 of 4,692 taking placebo (24.4%), a 24.8% relative reduction (95% CI 18.6 to 30.6), while Gleason 7 to 10 tumors were more common on finasteride (6.4% versus 5.1% of men) [11].

The label states the contrast for Gleason 8 to 10 cancer specifically: 1.8% on finasteride versus 1.1% on placebo, with a similar pattern for dutasteride over four years (1% versus 0.5%) [2]. It says 5-alpha reductase inhibitors may increase the risk of high-grade prostate cancer, and that whether the effect of reducing prostate volume, or study-related factors, influenced these results has not been established [2]. It also says the clinical significance of the finding for men taking 1 mg Propecia is unknown, and the Proscar label states it is not approved to prevent prostate cancer [2].

PSA matters here. Proscar lowers serum PSA by approximately 50% within six months, and the Proscar label instructs clinicians to double an isolated PSA value after six months or more of treatment when comparing it with normal ranges for untreated men [2]. Any confirmed rise from the lowest PSA value on treatment should be evaluated even if the number is still in the normal range, and percent free PSA does not need adjustment [2]. With 1 mg, mean PSA in men aged 18 to 41 fell from 0.7 ng/mL to 0.5 ng/mL at month 12 [1].

Male breast cancer and other postmarketing reports

Male breast cancer appears on the postmarketing list of both labels, with the note that the relationship to long-term finasteride use is unknown [1][2]. Trial counts are sparse and inconsistent: in MTOPS (3,047 men) there were 4 cases of breast cancer in men treated with finasteride and none in men not treated with it; in PLESS (3,040 men) there were 2 cases in placebo-treated men and none on finasteride; in PCPT (18,882 men) there was 1 case in each arm [2].

Other postmarketing entries include hypersensitivity reactions (rash, itching, hives, angioedema), testicular pain, hematospermia, breast tenderness and breast enlargement [1]. In the Propecia trials, the incidences of breast tenderness and enlargement, hypersensitivity reactions and testicular pain did not differ from placebo [1].

Pregnancy and handling

Finasteride is contraindicated in pregnancy and not indicated for women; the 1 mg product is for men only [1]. DHT is necessary for normal development of male genitalia, so inhibiting its production may cause abnormalities of the external genitalia in a male fetus [1]. The labels warn that women who are or may become pregnant should not handle crushed or broken tablets, because the coating prevents contact with the active ingredient only while the tablets are intact. They instruct a pregnant woman who contacts them to wash the area immediately with soap and water [1][2].

Animal data cited in the 5 mg label: in rats given oral finasteride during organogenesis at 0.1 to 86 times the maximum recommended human dose, hypospadias occurred in 3.6% to 100% of male offspring [2]. Exposure through semen is small; the highest amount measured in semen of men taking 5 mg was estimated at 50- to 100-fold less than the 5 mcg dose that had no effect on circulating DHT in men [2].

Semen, ejaculate volume and fertility

In healthy volunteers treated for 24 weeks, Proscar had no clinically meaningful effect on sperm concentration, motility, morphology or pH, but median ejaculate volume fell 0.6 mL (22.1%) with a corresponding drop in total sperm per ejaculate. Values stayed within the normal range and reversed after stopping, with an average 84 weeks to return to baseline [2].

With 1 mg, the median ejaculate volume change was -0.3 mL (-11%) versus -0.2 mL (-8%) on placebo over 48 weeks [1]. A double-blind trial that randomized 181 men aged 19 to 41 to finasteride 1 mg or placebo for 48 weeks, with semen analysis in a subset of 79, found no significant effect on sperm concentration, total sperm per ejaculate, motility or morphology [12]. The label also lists male infertility and poor seminal quality as postmarketing reports, with normalization or improvement reported after discontinuation [1].