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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 — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

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).

Related

Curated clinical cross-links plus same-class fallbacks.

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