[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [5-α-reductase inhibitors](https://durgical.com/en/pharmacology/classes/5-reductase-inhibitors/) › Finasteride

# 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](https://durgical.com/en/pharmacology/classes/5-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.

On this page

- The basics Mechanism of action
- Indications
- Adverse effects
- Pharmacokinetics
- Memory aid

- Clinical What the labels mean

- Safety Contraindications
- Key interactions
- What is monitored
- Administration and safety
- Pregnancy and breastfeeding
- Nursing care

- Study Test yourself

- Connections Connections

## 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](https://www.rcsb.org/structure/7BW1) · Drag to rotate · pinch to zoom

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 →](https://durgical.com/en/conditions/benign-prostatic-hyperplasia/)
- Reduction of the risk of acute urinary retention and prostate surgery; with doxazosin, less progression [Benign prostatic hyperplasia →](https://durgical.com/en/conditions/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

### Serious (rare, important)

- High-grade prostate cancer (small increase in prevention trials)
- Breast lumps, pain or nipple discharge (male breast cancer has been reported)
- New or worse depression, or thoughts of self-harm
- Severe allergic reactions

## 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

## Memory aid

FINAsteride = «FINAl stop for DHT»: one enzyme (type 2), PSA ×2, pregnant hands off.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/finasteride/)

[More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology)

## What the labels mean

**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).

## Connections

## Related drugs 3

- [Dutasteride](https://durgical.com/en/pharmacology/dutasteride/)
- [Tamsulosin](https://durgical.com/en/pharmacology/tamsulosin/)
- [Doxazosin](https://durgical.com/en/pharmacology/doxazosin/)

## Related conditions 1

- [Benign prostatic hyperplasia](https://durgical.com/en/conditions/benign-prostatic-hyperplasia/)

[← Back to the class: 5-α-reductase inhibitors](https://durgical.com/en/pharmacology/classes/5-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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=adfb8f6b-a7bb-4ac2-b2aa-fec6d56630ad) (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); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=G04CB01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/finasteride/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/finasteride/