A tier · strong
Finasteride has very large randomised datasets including the 18,882-participant Prostate Cancer Prevention Trial and multi-year phase III hair-count studies, but an unusually contested long-term safety literature holds it below the top grade.
finasteride
Finasteride is a type II 5-alpha-reductase inhibitor approved for benign prostatic hyperplasia and male androgenetic alopecia, with large randomised trials supporting efficacy in both. Its safety literature is contested, with regulators in 2025 confirming suicidal ideation as a labelled side effect and a persistent, unresolved debate over post-finasteride syndrome.
the explanation
Finasteride blocks conversion of testosterone into the more potent androgen DHT, which shrinks the prostate and slows male pattern hair loss. Reported side effects include sexual dysfunction that in some men is described as persisting after stopping, and regulators added warnings about depression and suicidal thoughts in 2025.
regulatory status
Approved prescription medicine
FDA-approved in 1992 at 5 mg for benign prostatic hyperplasia and in 1997 at 1 mg for male androgenetic alopecia. Indicated for men only and contraindicated in pregnancy. The European Commission issued a final decision on 22 August 2025 requiring product information updates warning of mood changes, depression and suicidal thoughts, plus a patient card for the 1 mg product. Not approved for use alongside anabolic steroids.
how it works · proposed mechanism
Finasteride blocks the enzyme that converts testosterone into dihydrotestosterone in androgen-target tissues.
Type II 5-alpha-reductase inhibition
Finasteride binds the type II isoenzyme concentrated in prostate and hair follicle tissue, forming a near-irreversible enzyme-inhibitor complex. Serum DHT falls by roughly 70% while testosterone is preserved or slightly raised.
Tissue androgen withdrawal
DHT drives prostatic stromal growth and follicular miniaturisation in genetically susceptible scalp. Lowering local DHT shrinks prostate volume and slows or partially reverses androgenetic hair loss.
Neurosteroid pathway overlap
The same enzyme family reduces progesterone and deoxycorticosterone en route to allopregnanolone and other GABAergic neuroactive steroids. This overlap is the leading proposed, though unproven, explanation for reported mood and persistent post-treatment symptoms.
what’s reported
evidence shape
34 sourcesThe composition of the cited literature by study type. This describes the shape of the evidence, not its quality — and it does not by itself set the grade.
⚠ the catch
Finasteride's large randomised base establishes efficacy for prostate volume reduction and hair retention in men prescribed and monitored for those specific conditions, where PSA masking, sexual side effects and mood changes are actively watched for. Using it off-label to blunt androgenic effects of anabolic steroids is unstudied, has no efficacy data in that context, and stacks a drug whose own regulators added suicidal-ideation warnings in 2025 on top of exogenous androgen exposure that independently affects mood, lipids and the hypothalamic-pituitary-gonadal axis.
key published findings
- Human trial (Prostate Cancer Prevention Trial, NEJM 2003, n=18,882 men aged 55 and over): prostate cancer period prevalence was 18.4% with finasteride versus 24.4% with placebo, a 24.8% relative reduction, but Gleason 7 or higher tumours were more frequent at 6.4% versus 5.1%.
- Human trial (PCPT adverse effects): reduced ejaculate volume 60.4% vs 47.3%, erectile dysfunction 67.4% vs 61.5%, loss of libido 65.4% vs 59.6%, and gynecomastia 4.5% vs 2.8% for finasteride versus placebo at the 5 mg dose.
- Human trial (Propecia phase III hair-count studies): at 12 months men on finasteride 1 mg had 107 more hairs in a 1-inch diameter circle than placebo, rising to a 277-hair difference by year 5, with roughly 65% showing increased hair growth at one year versus 37% on placebo.
- Regulatory data (Propecia US label): decreased libido 1.8% vs 1.3% placebo, erectile dysfunction 1.3% vs 0.7%, ejaculation disorder 1.2% vs 0.7%, with 3.8% versus 2.1% reporting one or more sexual adverse experiences; the label notes sexual dysfunction continuing after discontinuation and reports of depression and suicidal ideation.
- Regulatory action (EMA Article 31 referral, European Commission decision 22 August 2025): suicidal thoughts were confirmed as a side effect of finasteride 1 mg and 5 mg tablets, requiring product information updates on mood changes, depression and suicidal thoughts and a patient card for the 1 mg product; skin sprays were unaffected and dutasteride received precautionary wording only.
limitations of the evidence
- The post-finasteride syndrome literature is described in systematic review as ill-defined and contested, with a 2019 review identifying three studies showing full reversibility against eleven documenting irreversible adverse events; its status as a distinct pathology remains debated.
- Persistent-symptom reports are dominated by case series, registries and self-reported post-marketing data, which are vulnerable to reporting and selection bias and cannot establish incidence or causality.
- The PCPT high-grade cancer finding is confounded by finasteride's effect on prostate volume and PSA, which improves biopsy detection sensitivity, and long-term follow-up showed no difference in overall survival.
documented safety signals
- Depression and suicidal ideation: the European Commission confirmed suicidal thoughts as a side effect of both 1 mg and 5 mg finasteride on 22 August 2025, mandating product information warnings and a patient card for the hair-loss product; most reported cases involved the 1 mg dose and the precise frequency could not be determined.
- Post-finasteride syndrome: a reported constellation of persistent sexual, neurological and psychological symptoms continuing after discontinuation; a 2019 systematic review found eleven studies documenting irreversible adverse events against three showing full reversibility, and the entity remains contested rather than established.
- Sexual dysfunction: erectile dysfunction, decreased libido and ejaculatory disorder are the most common adverse effects, with a 2010 Cochrane review confirming increased risk; the US label states sexual dysfunction has continued after treatment stopped.
- High-grade prostate cancer: Gleason 7 or higher tumours were more common in PCPT (6.4% vs 5.1%), and the Propecia label cites Gleason 8-10 disease at 1.8% versus 1.1% on placebo.
- PSA suppression, which can mask prostate cancer detection and requires interpretive adjustment of screening values.
- Gynecomastia and breast tenderness; rare male breast cancer reported post-marketing.
- Embryo-fetal toxicity: contraindicated in pregnancy because of the risk of abnormal external genitalia development in a male fetus; women who are or may become pregnant should not handle crushed or broken tablets.
- Hypersensitivity reactions including rash, pruritus, urticaria and angioedema.
- Testicular pain, reduced ejaculate volume and infertility with abnormal semen parameters, reported as generally improving after discontinuation.
- Because finasteride suppresses DHT while leaving or slightly raising testosterone, using it alongside exogenous androgens does not remove androgen exposure and has never been evaluated for efficacy or safety in that combination.
identity
| full name | Finasteride |
| category | Ancillary & Endocrine |
| modality | small molecule |
| formula | C23H36N2O2 |
| molar mass | 372.55 g/mol |
| cas | 98319-26-7 |
| half-life | Approximately 5-6 hours in adults; greater than 8 hours in older men |
laboratory handling
| storage | Film-coated tablets stored at controlled room temperature, approximately 20-25 degrees C, protected from light and moisture in the manufacturer's closed container. |
| solubility | White crystalline powder, freely soluble in chloroform and in lower alcohols including ethanol and methanol, and practically insoluble in water. |
| co-studied with | Sometimes discussed off-label as a way to reduce androgenic hair or prostate effects during androgen use, but there are no trials in that setting, DHT reduction does not offset androgen exposure from non-reducible compounds, and the drug now carries regulator-mandated suicidal-ideation warnings in the EU. |
Handling information describes laboratory practice for a research reagent. It is not a preparation guide for use in humans.
the receipts
6 citedUS prescribing information · 2024 · official
European Medicines Agency · 2025 · official
BfArM (German Federal Institute for Drugs and Medical Devices) · 2025 · official
Wikipedia drug monograph (referenced compilation) · 2026 · review
New England Journal of Medicine · 2003 · randomized
New England Journal of Medicine · 2013 · randomized
others in Ancillary & Endocrine