5-alpha reductase inhibitor (Type II)

Finasteride

Brand names: Proscar (BPH), Propecia (alopecia)

Finasteride is a 5-alpha-reductase inhibitor used for benign prostatic enlargement (to shrink the prostate and reduce symptoms and complications) and, at lower dose, for male-pattern hair loss.

Dosing — not shown for this use

The product information we hold does not support a dose for this page's specific use, route or population, so none is shown. Follow your local or national guideline, and confirm against the product SmPC.

Clinical monograph

How it works

It inhibits 5-alpha-reductase, reducing conversion of testosterone to the more potent dihydrotestosterone, shrinking prostatic tissue over months.

Prescribing in practice

The benefit develops over several months; it suits larger prostates and reduces the risk of acute retention and surgery.
It roughly halves PSA — interpret PSA results with this in mind when screening for prostate cancer.
Sexual adverse effects and rare mood changes occur; women who are or may become pregnant must not handle crushed or broken tablets (teratogenic to a male fetus).

Monitoring

Review urinary symptoms over months; account for the PSA reduction in any prostate-cancer surveillance.

Counselling the patient

It can take 3-6 months to work.
It lowers your PSA blood test — remind whoever interprets it.
Sexual side effects can occur; pregnant partners should not handle broken tablets.

Evidence & guidelines

Reduces prostate size, symptoms and complications in benign prostatic enlargement, especially larger glands (NICE NG97).

Reference: NICE NG118; EAU BPH Guidelines; MTOPS Trial; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways