Finasteride

5-α-reductase inhibitors
5-α-reductase inhibitors
Pregnancy: contraindicated
Contraindicated in pregnancy: by lowering DHT it can disturb the development of the external genitalia of a male fetus (e.g. hypospadias in animal studies).

An oral 5-α-reductase type 2 inhibitor that shrinks an enlarged prostate over months and, at a lower strength, slows male-pattern hair loss.

Updated Nov 1, 2026

Quick facts

Group: 5-α-reductase inhibitors
Target: Steroid 5-α-reductase type 2 (SRD5A2)
Used for: Reduction of the risk · Male-pattern hair loss
Key effects: Decreased libido · Erectile dysfunction · Rash
Monitor: PSA · Urinary symptoms and flow over months · Mood
ATC: G04CB01

No doses: the clinician sets them by the current guideline.

Mechanism of action

Steroid 5-α-reductase type 2 (SRD5A2)Finasteride

The figure shows human 5-α-reductase type 2 with finasteride joined to its cofactor NADPH, the adduct the drug forms in the active site.

Target in 3D: Steroid 5-α-reductase type 2 (SRD5A2). PDB structure 7BW1 (RCSB PDB, public domain). View the structure at RCSB PDB

Finasteride is a competitive, specific inhibitor of 5-α-reductase type 2. In the active site it forms a tight adduct with the cofactor NADPH, so the enzyme can no longer turn testosterone into dihydrotestosterone (DHT). Serum and prostatic DHT fall markedly, prostate volume shrinks over months, urinary flow improves and the risk of acute retention and surgery falls. In the scalp, less DHT slows the shrinking of hair follicles.

Finasteride enters prostate and follicle cells
It locks 5-α-reductase type 2 with its cofactor NADPH
Testosterone is no longer converted to DHT
Over months the prostate shrinks and urine flows better

Indications

Symptomatic benign prostatic hyperplasia with an enlarged prostate Benign prostatic hyperplasia →
Reduction of the risk of acute urinary retention and prostate surgery; with doxazosin, less progression Benign prostatic hyperplasia →
Male-pattern hair loss (lower-strength product)

Adverse effects

Common

Decreased libido
Erectile dysfunction
Lower ejaculate volume and other ejaculation problems
Breast tenderness or enlargement
Rash

Contraindications

Women who are or may be pregnant
Hypersensitivity to any component
Not indicated in women or children

Key interactions

α1-blockers (doxazosin, tamsulosin): Intended combination: faster symptom relief and less BPH progression than either alone

What is monitored

PSA: a new baseline after starting; any confirmed rise is investigated
Urinary symptoms and flow over months
Mood
Breast changes

Administration and safety

Oral tablet once a day, with or without food
Tablets are coated; crushed or broken tablets are not handled by women who are or may be pregnant
If skin contact occurs, the area is washed with soap and water
The BPH and hair-loss products have different strengths and are not interchangeable

Pregnancy and breastfeeding

Contraindicated in pregnancy: by lowering DHT it can disturb the development of the external genitalia of a male fetus (e.g. hypospadias in animal studies).

Nursing care

Teach that benefit in BPH takes months and the drug is not stopped without advice
Tell lab staff and the doctor about the drug when PSA is measured
Teach women in the household not to handle broken or crushed tablets
Ask about sexual side effects and mood changes
Report breast lumps, pain or nipple discharge

Pharmacokinetics

Absorption: Oral, with or without food
Metabolism: Extensively in the liver, mainly by CYP3A4
Elimination: As metabolites in urine and faeces
Half-life: About 6 hours, a little longer in older men; the enzyme effect outlasts the drug
Onset and duration: Symptom benefit in BPH takes months

Test yourself

Think of the answer, then tap the question to see it.

Why must a pregnant woman not handle crushed finasteride tablets?

Absorbed finasteride lowers DHT and can cause abnormal development of the external genitalia in a male fetus.

How long does finasteride take to improve BPH symptoms?

Months; the prostate shrinks gradually, which is why α1-blockers are used for quicker relief.

What does finasteride do to PSA, and why does it matter?

It lowers PSA by about half; without adjusting for this, a prostate cancer could be missed, so a new baseline is used and rises are investigated.

Which isoform of 5-α-reductase does finasteride inhibit?

Type 2, the form that predominates in the prostate and genital skin.

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Connections

← Back to the class: 5-α-reductase inhibitors

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - FINASTERIDE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2026); Finasteride: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.