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# 5-α-reductase inhibitors

Drugs that block the conversion of testosterone into DHT, so the prostate shrinks slowly. Used in benign prostatic hyperplasia.

Updated Nov 1, 2026

## What they are

Inhibitors of steroid 5-α-reductase, the enzyme that turns testosterone into the more potent androgen dihydrotestosterone (DHT) in the prostate, skin and hair follicles. Over months they shrink an enlarged prostate, improve urinary symptoms and lower the risk of acute urinary retention and surgery. Finasteride also slows male-pattern hair loss.

## Mechanism of action

5-α-reductase type 2 predominates in the prostate and genital skin, type 1 in non-genital skin, liver and bone. Finasteride inhibits type 2; dutasteride inhibits both. With less DHT the androgen-driven growth of prostate tissue reverses gradually, while circulating testosterone stays normal or rises slightly.

Target: Steroid 5-α-reductase (SRD5A2; dutasteride also SRD5A1)

## Key points

- They lower PSA by about half, so a new baseline is set on treatment and any confirmed rise is investigated for prostate cancer.
- Teratogenic for a male fetus (abnormal genital development): pregnant women do not handle crushed tablets or leaking capsules.
- Benefit takes months; they are combined with an α1-blocker (tamsulosin, doxazosin) for faster symptom relief and less progression.
- Side effects are mostly sexual: lower libido, erectile dysfunction, less ejaculate; also breast tenderness or enlargement.
- A small rise in high-grade prostate cancer was seen in prevention trials; they are not approved to prevent prostate cancer.

## Drugs in this class

- [**Finasteride** An oral 5-α-reductase type 2 inhibitor that shrinks an enlarged prostate over months and, at a lower strength, slows… Steroid 5-α-reductase type 2 (SRD5A2)](https://durgical.com/en/pharmacology/finasteride/)
- [**Dutasteride** An oral inhibitor of both 5-α-reductase types, used in benign prostatic hyperplasia. It stays in the body for months… Steroid 5-α-reductase types 1 and 2 (SRD5A1 and SRD5A2)](https://durgical.com/en/pharmacology/dutasteride/)

## Test yourself

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

## Which hormone do 5-α-reductase inhibitors reduce, and what happens to testosterone?

They reduce dihydrotestosterone (DHT); circulating testosterone stays normal or rises a little.

## How is PSA interpreted in a man taking finasteride?

Finasteride roughly halves PSA, so a new baseline is set and any confirmed rise from the lowest value is evaluated for prostate cancer.

## Why are 5-α-reductase inhibitors often combined with an α1-blocker in BPH?

The α1-blocker relaxes prostate smooth muscle within days, while the 5-α-reductase inhibitor shrinks the gland over months; together they lower progression more.

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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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=adfb8f6b-a7bb-4ac2-b2aa-fec6d56630ad) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - DUTASTERIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=e537c60d-9ba2-4e7f-a405-2b915dcd7ead) (DailyMed (U.S. National Library of Medicine), 2026); [Finasteride: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a698016.html) (MedlinePlus (U.S. National Library of Medicine), 2026).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/classes/5-reductase-inhibitors/