Finasteride (Proscar)
finasteride 5 mg — for benign prostatic hyperplasia
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Symptomatic BPH — symptom improvement | Adult males with enlarged prostate | Monotherapy | FDA Approved |
| BPH — reduce risk of acute urinary retention | Adult males with enlarged prostate | Monotherapy | FDA Approved |
| BPH — reduce need for surgery (TURP/prostatectomy) | Adult males with enlarged prostate | Monotherapy | FDA Approved |
| BPH symptomatic progression — combined with doxazosin | Adult males with enlarged prostate | Adjunctive (with doxazosin) | FDA Approved |
Finasteride 5 mg is a Type II 5-alpha-reductase inhibitor first approved by the FDA in 1992 that reduces conversion of testosterone to dihydrotestosterone (DHT), the primary androgen driving prostatic enlargement. By lowering intraprostatic DHT levels by approximately 80%, finasteride produces a ~20% reduction in prostate volume over 3–6 months. This translates into improved lower urinary tract symptoms, increased peak urinary flow rate, and a reduced risk of disease progression requiring surgical intervention. The PLESS trial demonstrated a 67% relative risk reduction in acute urinary retention and a 64% reduction in BPH-related surgery over four years. The MTOPS trial established the benefit of combination therapy with doxazosin for preventing symptomatic progression. Finasteride is not approved for the prevention of prostate cancer.
Male pattern hair loss (androgenetic alopecia): Finasteride 1 mg (Propecia) is separately FDA-approved for this indication. The 5 mg BPH formulation should not be used for hair loss. Evidence quality: High (separate approval).
Hirsutism in women (off-label): Used in some settings for excessive hair growth in women of non-childbearing potential, though not FDA-approved. Evidence quality: Low to moderate. Absolutely contraindicated in pregnancy.
Dosing
Adult Dosing — By Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| BPH — monotherapy for symptom control | 5 mg once daily | 5 mg once daily | 5 mg/day | With or without food; fixed dose, no titration needed Symptom benefit may take 6–12 months; prostate volume begins to reduce within 3 months |
| BPH — combination with alpha-blocker (doxazosin) | 5 mg once daily | 5 mg once daily + doxazosin 4–8 mg/day | 5 mg/day | Doxazosin titrated separately (1→2→4→8 mg weekly) MTOPS showed combination reduces symptomatic progression vs either monotherapy |
| BPH — patients with renal impairment | 5 mg once daily | 5 mg once daily | 5 mg/day | No dose adjustment required Metabolite levels increase but are well tolerated (FDA PI) |
| BPH — elderly patients (≥65 years) | 5 mg once daily | 5 mg once daily | 5 mg/day | No dose adjustment; half-life prolonged to ~8 h but no clinical significance AUC ~15% higher in patients ≥70 vs younger men |
| BPH — hepatic impairment | 5 mg once daily | 5 mg once daily | 5 mg/day | Use with caution; extensively hepatically metabolised Formal PK studies in hepatic impairment not performed |
Unlike alpha-blockers which provide symptom relief within days, finasteride requires 6–12 months of continuous therapy for maximum clinical benefit. The pharmacological effect on DHT is rapid (serum DHT drops ~70% within days), but prostatic tissue remodelling is slow. On discontinuation, DHT levels return to baseline within ~2 weeks, and prostate volume regrows to near-baseline within ~3 months. Patients should be counselled to continue therapy even if early improvement is modest.
Pharmacology
Mechanism of Action
Finasteride is a competitive and specific inhibitor of Type II 5-alpha-reductase, an intracellular enzyme concentrated in the prostate, liver, and skin that converts testosterone to the more potent androgen dihydrotestosterone (DHT). Finasteride forms a slowly reversible enzyme complex with Type II 5-alpha-reductase (enzyme complex half-life ~30 days), producing sustained suppression of DHT production even with relatively rapid clearance of the parent drug from plasma. It has no affinity for the androgen receptor. By reducing intraprostatic DHT content by approximately 80%, finasteride induces apoptosis of prostatic epithelial cells and progressive reduction in gland volume. A single 5 mg dose reduces serum DHT by up to 70%, with peak suppression at 8 hours, maintained throughout the 24-hour dosing interval. Serum testosterone levels increase by approximately 10–20% but remain within the physiological range. PSA levels decrease by approximately 50% within six months of treatment.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Bioavailability ~63% (range 34–108%); Tmax 1–2 h; Cmax 37 ng/mL (range 27–49); food does not affect bioavailability | Can be taken with or without meals; rapid absorption supports once-daily dosing |
| Distribution | Vd = 76 L (range 44–96 L); ~90% protein bound; crosses blood-brain barrier; slow accumulation with multiple dosing | Steady-state trough concentrations ~47–54% higher than single dose in elderly; found in semen at very low concentrations (50–100 fold below pharmacological threshold) |
| Metabolism | Extensive hepatic metabolism via CYP3A4 to two metabolites with <20% of parent activity | Caution in hepatic impairment (not studied); does not inhibit or induce CYP450 enzymes, making drug interactions unlikely |
| Elimination | Feces 57% (range 51–64%), urine 39% (range 32–46%) as metabolites; t½ 5–6 h (18–60 yr), ~8 h (≥70 yr); plasma clearance ~165 mL/min | Despite short plasma half-life, pharmacological effect on DHT persists ~4 days after a single dose due to slow enzyme complex dissociation (t½ ~30 days) |
Side Effects
The following adverse reaction data are from the 4-year, placebo-controlled PLESS study (n = 3,040) for finasteride 5 mg monotherapy in BPH, supplemented by the MTOPS study data for combination therapy.
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Impotence (erectile dysfunction) | 18.5% mono; 22.6% combo (vs 12.2% placebo) | Most frequently reported adverse reaction; highest in Year 1 (8.1% mono vs 3.7% placebo), decreases to placebo level in Years 2–4 (5.1% vs 5.1%) |
| Decreased libido | 10.0% mono; 11.6% combo (vs 5.7% placebo) | Dose- and duration-dependent; no significant difference from placebo after Year 1 (2.6% vs 2.6%); incidence decreases with continued therapy |
| Adverse Effect | Incidence (Yr 1) | Clinical Note |
|---|---|---|
| Impotence | 8.1% vs 3.7% placebo | Drug-related; declines to placebo rates after first year of treatment |
| Decreased libido | 6.4% vs 3.4% placebo | Drug-related; declines to placebo rates after first year |
| Decreased volume of ejaculate | 3.7% vs 0.8% placebo | Consistent with pharmacological mechanism; 22.1% median decrease in ejaculate volume observed in healthy volunteers |
| Gynecomastia (breast enlargement) | 0.5% Yr 1; 1.8% Yrs 2–4 (PLESS); 2.2% (MTOPS) | Cumulative incidence increases with duration; monitor for breast tenderness |
| Breast tenderness | 0.4% Yr 1; 0.7% Yrs 2–4 | May precede or accompany gynecomastia |
| Abnormal ejaculation | 7.2% (MTOPS) vs 2.3% placebo | More prominent in combination therapy (14.1% with doxazosin) |
| Rash | 0.5% vs 0.2% placebo | Generally mild and self-limiting |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| High-grade prostate cancer (Gleason 8–10) | 1.8% vs 1.1% placebo (PCPT) | Detected over 7-year follow-up | Ensure prostate cancer is excluded before initiating therapy; ongoing PSA monitoring with doubling adjustment; evaluate any confirmed PSA rise from nadir |
| Male breast cancer | Very rare (4 cases in MTOPS, n=3047) | Years of treatment | Investigate any new breast mass or persistent breast changes; relationship to finasteride currently unknown |
| Persistent sexual dysfunction after discontinuation | Rare (postmarketing reports) | Variable; may persist post-discontinuation | Document and evaluate; endocrine workup if persistent; refer to specialist if needed. Causal role of finasteride not definitively established. |
| Angioedema (face, lips, tongue, throat) | Very rare (postmarketing) | Any time during therapy | Permanent discontinuation; emergency airway management; finasteride contraindicated with known hypersensitivity |
| Depression / suicidal ideation | Rare (postmarketing) | Variable | Screen for mood changes at follow-up; consider discontinuation if mood disorder develops; refer for psychiatric evaluation |
| Reason for Discontinuation | Incidence | Context |
|---|---|---|
| Sexual dysfunction (combined) | 3.7% | Includes impotence, decreased libido, ejaculatory changes; reversible in most upon discontinuation |
| All other causes | <1% | Non-sexual adverse events rarely led to discontinuation in the 4-year trial |
Sexual side effects (impotence 8.1%, decreased libido 6.4%) are most prominent during the first year of therapy and decline to placebo rates thereafter. Clinicians should proactively discuss these at initiation, emphasising the temporal pattern of improvement and that effects are generally reversible upon discontinuation. For patients with combination therapy, the additive sexual effects of alpha-blockers should also be discussed. Ejaculate volume decreases by a median of 22% (0.6 mL), which is expected from the drug’s mechanism (FDA PI).
Drug Interactions
Finasteride has a remarkably clean drug interaction profile. Despite being metabolised by CYP3A4, it does not inhibit or induce the cytochrome P450 system. Formal interaction studies with antipyrine, digoxin, propranolol, theophylline, and warfarin showed no clinically meaningful interactions. In the PLESS and MTOPS studies, finasteride was used concomitantly with alpha-blockers, ACE inhibitors, calcium channel blockers, diuretics, beta-blockers, NSAIDs, HMG-CoA reductase inhibitors, benzodiazepines, and quinolone antibiotics without significant adverse interactions.
Monitoring
-
PSA
Baseline; re-establish at 6 months; then per screening guidelines
Routine PSA decreases ~50% on finasteride. After 6 months, establish a new baseline. Double measured PSA values for comparison with normal untreated ranges. Any confirmed increase from the lowest PSA on treatment warrants cancer evaluation, even if within “normal” range. Percent free PSA is not affected and requires no adjustment. -
Digital Rectal Exam
Baseline; then annually
Routine Rule out prostate carcinoma before starting therapy. BPH and prostate cancer can coexist. Annual DRE complements PSA monitoring. New nodules or induration require urological evaluation regardless of PSA level. -
BPH Symptoms (IPSS/AUA-SI)
Baseline; 3–6 months; then annually
Routine Symptom scores to assess therapeutic response. Maximum benefit may take 6–12 months. Patients with large residual urine volumes or severely diminished flow should be monitored for obstructive uropathy — they may not be candidates for finasteride. -
Sexual Function
At each follow-up visit
Routine Proactively ask about erectile function, libido, and ejaculatory changes. Most sexual side effects occur in Year 1 and decrease with continued therapy. Document any persistent symptoms after discontinuation. -
Breast Changes
At each visit; patient self-report
Trigger-based Gynecomastia incidence increases with treatment duration (0.5% Year 1 to 1.8% Years 2–4). Any new breast mass, pain, or nipple discharge should be investigated to exclude male breast cancer. -
Mood / Depression
At follow-up visits
Trigger-based Postmarketing reports of depression and rarely suicidal ideation. Screen for mood changes, particularly in patients with psychiatric history. Consider discontinuation if mood disorder develops.
Contraindications & Cautions
Absolute Contraindications
- Women who are or may become pregnant (Pregnancy Category X) — Finasteride inhibits DHT production required for normal male fetal genital development. Even skin contact with crushed/broken tablets can pose risk to male fetus.
- Known hypersensitivity to finasteride or any excipient — Postmarketing reports of angioedema including swelling of face, lips, tongue, and throat
- Pediatric patients — Finasteride is not indicated for use in children; safety and effectiveness in pediatric patients have not been established
Relative Contraindications (Specialist Input Recommended)
- Patients with suspected prostate cancer — Finasteride is not a treatment for prostate cancer; PSA reduction may mask cancer detection. Full urological evaluation required before starting therapy.
- Patients with large post-void residual volumes or severely diminished urinary flow — May not be candidates for finasteride therapy; risk of obstructive uropathy requires surgical assessment
Use with Caution
- Hepatic impairment — Finasteride is extensively hepatically metabolised; formal PK studies in liver disease not performed. Use clinical judgement.
- Male partners of women of childbearing potential — Finasteride is present in semen at extremely low concentrations (estimated 50–100-fold below pharmacologically active doses). Rhesus monkey studies at far higher exposures showed no fetal harm, but condom use or drug discontinuation before planned conception is a reasonable precaution.
- Patients concerned about sexual function — Year 1 rates of impotence (8.1%) and decreased libido (6.4%) are significantly above placebo; discuss before prescribing
- History of depression or mood disorders — Postmarketing reports of depression; monitor for changes in mood
In the 7-year PCPT trial (n = 18,882), men taking finasteride 5 mg daily had a higher incidence of Gleason score 8–10 prostate cancer compared with placebo (1.8% vs 1.1%). A similar finding was observed with dutasteride in the REDUCE trial (1.0% vs 0.5%). Whether this represents a true increase in high-grade cancer risk, detection bias from prostate volume reduction, or study-related factors has not been established. Finasteride is not approved for prostate cancer prevention.
Patient Counselling
Purpose of Therapy
Finasteride works by blocking the hormone that causes the prostate to enlarge. It gradually shrinks the prostate gland, which improves urinary symptoms over time and reduces the risk of sudden urinary blockage or the need for prostate surgery. Unlike medications that relax the prostate muscle (alpha-blockers), finasteride addresses the underlying cause of prostate growth, but it takes longer to work — typically 6 to 12 months for full effect.
How to Take
Take one 5 mg tablet once daily, with or without food. Take it at approximately the same time each day. Do not break or crush the tablets. Continue taking finasteride even if symptoms improve; stopping the medication will allow the prostate to regrow within a few months.
Sources
- Proscar (finasteride 5 mg) Tablets. Full Prescribing Information. Organon Global Inc. Revised 2021. FDA Label (2021) Primary source for all FDA-approved dosing, contraindications, adverse reaction rates (PLESS and MTOPS data), and pharmacokinetic parameters.
- Proscar (finasteride) Tablets. Full Prescribing Information. Merck & Co., Inc. 2014. FDA Label (2014) Earlier label version containing additional pharmacokinetic detail on steady-state accumulation and semen concentrations.
- McConnell JD, Bruskewitz R, Walsh P, et al. The effect of finasteride on the risk of acute urinary retention and the need for surgical treatment among men with benign prostatic hyperplasia (PLESS). N Engl J Med. 1998;338(9):557–563. doi:10.1056/NEJM199802263380901 Landmark 4-year RCT (n = 3,040) demonstrating 67% RRR in acute urinary retention and 64% RRR in BPH-related surgery with finasteride 5 mg daily.
- McConnell JD, Roehrborn CG, Bautista OM, et al. The long-term effect of doxazosin, finasteride, and combination therapy on the clinical progression of benign prostatic hyperplasia (MTOPS). N Engl J Med. 2003;349(25):2387–2398. doi:10.1056/NEJMoa030656 MTOPS trial (n = 3,047) establishing superiority of combination finasteride + doxazosin over either monotherapy for preventing symptomatic BPH progression.
- Thompson IM, Goodman PJ, Tangen CM, et al. The influence of finasteride on the development of prostate cancer (PCPT). N Engl J Med. 2003;349(3):215–224. doi:10.1056/NEJMoa030660 7-year prevention trial (n = 18,882) showing reduced overall prostate cancer incidence but increased high-grade (Gleason 8–10) cancer with finasteride.
- Roehrborn CG, Boyle P, Nickel JC, Hoefner K, Andriole G; ARIA3001, ARIA3002, ARIA3003 Study Investigators. Efficacy and safety of a dual inhibitor of 5-alpha-reductase types 1 and 2 (dutasteride) in men with benign prostatic hyperplasia. Urology. 2002;60(3):434–441. doi:10.1016/s0090-4295(02)01905-2 Comparative reference for dutasteride efficacy, providing context for the 5-alpha-reductase inhibitor drug class.
- McVary KT, Roehrborn CG, Avins AL, et al. Update on AUA Guideline on the Management of Benign Prostatic Hyperplasia. J Urol. 2011;185(5):1793–1803. doi:10.1016/j.juro.2011.01.074 AUA guideline recommending 5-alpha-reductase inhibitors for men with demonstrable prostatic enlargement and moderate-to-severe symptoms.
- Gravas S, Cornu JN, Gacci M, et al. EAU Guidelines on Management of Non-Neurogenic Male LUTS, incl. BPO. European Association of Urology. 2024. EAU Guidelines Current European guideline positioning 5ARIs for men with LUTS and enlarged prostate (>40 mL), recommending combination therapy for progressive disease.
- Bull HG, Garcia-Calvo M, Andersson S, et al. Mechanism-based inhibition of human steroid 5 alpha-reductase by finasteride: enzyme-catalyzed formation of NADP-dihydrofinasteride, a potent bisubstrate analog inhibitor. J Am Chem Soc. 1996;118(10):2359–2365. doi:10.1021/ja953069t Foundational paper elucidating the mechanism of finasteride as a bisubstrate analog inhibitor with slow enzyme complex dissociation (t½ ~30 days).
- Rittmaster RS. 5alpha-reductase inhibitors in benign prostatic hyperplasia and prostate cancer risk reduction. Best Pract Res Clin Endocrinol Metab. 2008;22(2):389–402. doi:10.1016/j.beem.2008.01.016 Comprehensive review of 5ARI pharmacology, DHT suppression kinetics, and dual role in BPH treatment and cancer chemoprevention controversy.
- Steiner JF. Clinical pharmacokinetics and pharmacodynamics of finasteride. Clin Pharmacokinet. 1996;30(1):16–27. doi:10.2165/00003088-199630010-00002 Primary PK reference detailing bioavailability (~63%), protein binding (~90%), CYP3A4 metabolism, half-life ranges by age, and renal impairment data.
- Finasteride. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024. NCBI Bookshelf Concise clinical pharmacology overview consolidating metabolism (CYP3A4), elimination half-life (5–6 h, up to 8 h in elderly), and dosing for BPH and alopecia.
- Liu L, Zhao S, Li F, et al. Effect of 5α-reductase inhibitors on sexual function: a meta-analysis and systematic review of randomised controlled trials. J Sex Med. 2016;13(9):1297–1310. doi:10.1016/j.jsxm.2016.07.006 Meta-analysis quantifying the sexual side effect profile of 5ARIs across RCTs, providing pooled estimates of erectile dysfunction and libido changes.