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5-alpha reductase inhibitor

Finasteride

Finasteride is a selective 5-alpha reductase inhibitor that blocks conversion of testosterone to dihydrotestosterone (DHT), the primary androgen driving prostate growth and hair loss. It is used for benign prostatic hyperplasia (BPH) at higher doses and androgenetic alopecia at lower doses. The most testable fact: finasteride reduces DHT by ~70% but does not lower total testosterone.

Practice questionsHigh-yield facts
Drug class5-alpha reductase inhibitor
MechanismBlocks conversion of testosterone to DHT
Main usesBPH, male pattern baldness
Key adverse effectSexual dysfunction, gynecomastia
4 min readFor medical studentsUpdated 2026-10-15

Mechanism of action

Finasteride competitively and selectively inhibits type II 5-alpha reductase, the enzyme responsible for converting testosterone to dihydrotestosterone (DHT) in tissues including the prostate, skin, and hair follicles.

DHT is the active androgen in these tissues and drives benign prostatic hyperplasia and androgenetic alopecia. By reducing DHT concentration by approximately 70%, finasteride shrinks the prostate and can halt or slow hair loss.

Important distinction: Finasteride does not significantly reduce total serum testosterone or inhibit type I 5-alpha reductase (which predominates in skin and sebaceous glands). This is why it has minimal systemic androgenic effects but targeted tissue effects.

High-yield for exams
  • Finasteride blocks type II 5-alpha reductase, reducing DHT ~70% without lowering total testosterone.
  • Used for BPH (5 mg daily) and male pattern baldness (1 mg daily); effects take 3–12 months.
  • Sexual dysfunction (ED, decreased libido) occurs in 3–5%; usually reverses after stopping.
  • Finasteride reduces PSA by ~50%; double reported PSA when interpreting cancer screening results.
  • Absolutely contraindicated in pregnancy and women of childbearing age; teratogenic (FDA Pregnancy Category X).
  • Combined with alpha-blockers for synergistic BPH symptom relief; combination superior to monotherapy.
  • No major drug interactions; safe with warfarin, NSAIDs, and phosphodiesterase inhibitors.
  • PCPT trial showed slight increase in high-grade prostate cancer risk (mechanism unclear, clinical significance debated).

Indications

Benign Prostatic Hyperplasia (BPH)

Used at 5 mg daily to reduce prostate volume, improve urinary flow rate, and decrease symptoms of lower urinary tract obstruction (hesitancy, nocturia, weak stream). Often combined with alpha-blockers for additive symptom relief.

Androgenetic Alopecia (Male Pattern Baldness)

Used at 1 mg daily to slow or halt progression of hair loss in men. Most effective if started early; regrowth is modest and requires ongoing therapy. Not indicated for women.

Limitation

Effectiveness depends on presence of type II 5-alpha reductase activity; finasteride is ineffective in conditions with isolated type I reductase involvement.

Pharmacokinetics

  • Absorption: Rapidly absorbed from the GI tract; peak plasma levels in 1–2 hours; unaffected by food.
  • Distribution: Crosses blood–brain barrier and accumulates in prostate; highly protein-bound (~90%).
  • Metabolism: Hepatic via CYP3A4; forms active and inactive metabolites.
  • Elimination: Renal (39%) and fecal (57%); terminal half-life approximately 5–7 hours in plasma.
  • Steady state: Requires 6–8 weeks to reach steady state; DHT levels plateau after ~2 weeks.
  • Onset of clinical effect: 3–6 months for BPH symptom improvement; 6–12 months for observable hair regrowth; effect is reversible upon discontinuation.

Adverse effects

Sexual Dysfunction (Most Common)

  • Erectile dysfunction (3–5% of patients).
  • Decreased libido (3%).
  • Ejaculation disorders (2–3%).
  • Effects typically reverse within months of stopping; rarely persistent (post-finasteride syndrome is controversial and rare).

Endocrine

  • Gynecomastia (breast tissue enlargement; 0.5–2%); may require discontinuation.
  • Testicular pain or tenderness (rare).

Other

  • Allergic reactions (rash, pruritus; rare).
  • Headache, dizziness (uncommon).
  • Prostate-specific antigen (PSA) reduction (~50%) — important for cancer screening interpretation.

Note on PSA and Cancer Screening

Finasteride reduces PSA levels by approximately 50%; PSA values should be doubled for comparison to baseline when screening for prostate cancer. Despite PSA reduction, finasteride does not increase risk of high-grade prostate cancer and may slightly reduce overall cancer risk.

Contraindications and warnings

Absolute Contraindications

  • Pregnancy and women of childbearing potential: Finasteride is contraindicated in pregnant women and those who may become pregnant; it is teratogenic (FDA Pregnancy Category X, now expressed as 'Contraindicated in pregnancy' under current labeling). Women should not handle crushed tablets due to risk of absorption through skin.
  • Known hypersensitivity to finasteride or excipients.

Warnings

  • PSA monitoring: Baseline PSA should be measured; interpret subsequent values in context of 50% reduction.
  • Prostate cancer risk: No increase in overall prostate cancer incidence; however, a slight increase in high-grade cancers was observed in the Prostate Cancer Prevention Trial (PCPT) — mechanism unclear and clinical significance debated.
  • Sexual dysfunction: Counsel patients on risk; effects usually reverse after stopping.
  • Hepatic impairment: Use with caution; no dose adjustment typically needed but metabolism may be altered.
  • Renal impairment: No significant dose adjustment required; finasteride is largely hepatically metabolized.

Drug interactions

  • CYP3A4 substrates: Finasteride is metabolized by CYP3A4; potential for clinically minor interactions with strong inhibitors (ketoconazole, ritonavir) or inducers (rifampin), though dose adjustment is rarely needed.
  • Alpha-blockers (doxazosin, tamsulosin, terazosin): Additive benefits for BPH without major pharmacokinetic interaction; often combined therapeutically.
  • Warfarin: No significant interaction; anticoagulation not altered.
  • Phosphodiesterase inhibitors (sildenafil, tadalafil): May be used concomitantly for combined ED and BPH treatment; additive benefits without major interactions.
  • NSAIDs, ACE inhibitors: No major interactions.
  • Hepatic enzyme inducers/inhibitors: May alter finasteride levels but rarely require dose changes.

Clinical pearls

  • "Finasteride takes time": Expect 3–6 months for BPH symptom improvement and 6–12 months for visible hair regrowth. Patients need counseling on delayed onset to improve adherence.
  • Combination therapy for BPH: Alpha-blockers work faster (~weeks); finasteride provides durable symptom reduction. Combination is more effective than either alone for moderate–severe BPH.
  • PSA paradox on exams: A patient on finasteride with a PSA of 4 ng/mL may actually have a higher cancer risk than the raw number suggests (since finasteride halves PSA). Always ask: "Is the patient on finasteride?" when interpreting PSA.
  • Hair loss counseling: Explain that finasteride halts progression rather than causing regrowth; response is modest and variable. Minoxidil can be added for synergistic effect.
  • Sexual side effects are dose-independent: Incidence is similar at 1 mg (hairloss) and 5 mg (BPH) doses.
  • Irreversible vs. reversible: Most sexual dysfunction reverses after stopping; "post-finasteride syndrome" (alleged persistent erectile dysfunction, depression, etc. after discontinuation) is rare and controversial.
  • Women and finasteride: Contraindicated in pregnancy; not indicated for female pattern hair loss (limited evidence). Spironolactone is preferred for androgenetic alopecia in women.
  • Black box consideration: While not technically a black box warning, pregnancy is a critical contraindication; emphasize to male patients not to expose pregnant partners to crushed tablets.
Mnemonic

"5-Alpha Stops DHT" — Finasteride blocks the 5-alpha reductase enzyme, stopping testosterone→DHT conversion.

Practice questions

1. A 62-year-old man with benign prostatic hyperplasia and lower urinary tract symptoms is started on finasteride 5 mg daily. After 2 weeks, his PSA level is 3.2 ng/mL, compared to a baseline PSA of 6.4 ng/mL drawn 6 months before finasteride. Which of the following best explains this PSA change?

Answer: B. Finasteride inhibits 5-alpha reductase, reducing DHT and shrinking the prostate. Since PSA is produced by prostate tissue, a ~50% reduction in PSA is expected and reflects prostate shrinkage, not cancer cure or treatment. The baseline PSA should be doubled when compared to current finasteride-era values. The other options misinterpret the mechanism or confuse PSA dynamics.

2. A 45-year-old man with male pattern baldness is prescribed finasteride 1 mg daily. He returns after 8 weeks and reports complete resolution of hair loss. Which of the following best describes what has occurred?

Answer: C. Finasteride typically requires 6–12 months for observable hair regrowth. At 8 weeks, meaningful regrowth is unlikely. The patient's response is likely placebo effect or subjective perception rather than true reversal of hair loss. This is a classic exam trap testing knowledge of the delayed onset of finasteride's clinical effects.

3. A couple presents for preconception counseling. The husband has been on finasteride 5 mg daily for 2 years for benign prostatic hyperplasia. The wife expresses concern about potential effects on fertility or fetal development. Which of the following statements is most accurate?

Answer: C. Finasteride is teratogenic (FDA Pregnancy Category X) and must be avoided during pregnancy due to risk of abnormalities of external genitalia in male fetuses, reflecting DHT's critical role in fetal androgen-dependent development. However, paternal use of finasteride does not impair male fertility or harm the fetus; only maternal exposure is harmful. The couple can conceive safely once the wife is not exposed to finasteride.

Frequently asked questions

How does finasteride work?

Finasteride selectively inhibits type II 5-alpha reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT). By reducing DHT levels by approximately 70%, it shrinks the prostate and slows hair loss. Notably, it does not significantly lower total serum testosterone, so systemic androgen effects are minimal.

What are the main side effects of finasteride?

The most common adverse effects are sexual: erectile dysfunction (3–5%), decreased libido (3%), and ejaculation disorders (2–3%). Gynecomastia occurs in 0.5–2% of patients. Most sexual side effects reverse within months after stopping the drug. Allergic reactions and headache are less common.

Is finasteride safe in pregnancy?

No. Finasteride is absolutely contraindicated in pregnancy and women who may become pregnant; it is teratogenic and can cause abnormalities of the external genitalia in male fetuses. Women should not handle crushed tablets due to risk of systemic absorption through the skin. Use effective contraception if a male partner is taking finasteride.

Why do I need to double a PSA value if a patient is on finasteride?

Finasteride reduces PSA levels by approximately 50% because PSA is produced by prostate tissue, and finasteride shrinks the prostate. When interpreting PSA for cancer screening, the measured value should be doubled to compare to age-matched reference ranges and to the patient's baseline before finasteride therapy began.

How long does it take finasteride to work?

Onset is slow: BPH symptom improvement typically requires 3–6 months, and visible hair regrowth for androgenetic alopecia may take 6–12 months. DHT levels drop within 2 weeks, but clinical benefit lags. Patients must be counseled on delayed onset to ensure compliance and realistic expectations.

Conditions it is used for

Benign prostatic hyperplasiaMale pattern baldness (androgenetic alopecia)Lower urinary tract symptomsUrinary hesitancyNocturiaUrinary frequencyHair loss in menProstate enlargement
DutasterideTamsulosinTerazosinDoxazosinAlfuzosinMinoxidilSpironolactoneSaw palmetto

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: D11AX10WHO ATC/DDD index: G04CB01

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.