Finasteride
A 5-alpha reductase inhibitor used for an enlarged prostate (BPH) and, at a lower dose, for male pattern hair loss.
Drug card
- How it works
- Blocking conversion of testosterone to DHT, so the prostate shrinks
- Used for
- Benign prostatic hyperplasia (enlarged prostate)
- Male pattern hair loss
- With doxazosin, to slow BPH getting worse
- Known for
- Low libido, erection or ejaculation problems
- Breast enlargement and tenderness
- Genital defects in a male fetus exposed in pregnancy
- Its name
- -steride means a 5-alpha reductase inhibitor
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does finasteride work?
A 4-azasteroid and competitive inhibitor of the type II enzyme, the main form in the prostate, liver and hair follicles; it forms an enzyme complex that breaks up only slowly. Serum DHT falls by about 70%, and it does not bind androgen receptors itself. Its effect on the prostate builds over about 6 months; the plasma half-life is about 6 hours.
Uses
What is finasteride used for?
- Benign prostatic hyperplasia (enlarged prostate)
- Male pattern hair loss
- With doxazosin, to slow BPH getting worse
Side effects
Side effects of finasteride
- Low libido, erection or ejaculation problemsKnown for
- Breast enlargement and tendernessKnown for
- Genital defects in a male fetus exposed in pregnancyKnown forSerious
- DepressionSerious
- Suicidal thoughtsSerious
- Higher risk of high-grade prostate cancerSerious
- Male breast cancer (rare)Serious
- Reduced fertility and semen quality (rare)
- Allergic reactions, including lip and face swellingSerious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- It halves PSA: set a new baseline after 6 months, and treat any confirmed rise as a warning sign of prostate cancer.
- Contraindicated in pregnancy; women who are or may be pregnant must not handle crushed or broken tablets.
- Not for women or children; check for other urinary conditions, including prostate cancer, before treating BPH.
- Ask about low mood: depression and suicidal thoughts have been reported, and sexual effects can outlast treatment.
Good to know: Unlike alpha blockers it shrinks the prostate rather than relaxing its muscle, so benefit takes up to 6 months; dutasteride, its relative, blocks both type I and type II enzymes.
Remember it
A way to remember it
DHT
- D
- Decreased libido, erection and ejaculation problems
- H
- Halves the PSA level
- T
- Teratogenic to a male fetus: pregnant women must not handle broken tablets
Written for biofideX
Its name
-steride means 5-alpha reductase inhibitor
Finasteride and dutasteride block 5-alpha reductase, which turns testosterone into dihydrotestosterone. WHO lists the stem under -ster- (steroids).
- Every drug name stem
Its class
5-alpha reductase inhibitors
They block 5-alpha reductase, the enzyme that turns testosterone into dihydrotestosterone (DHT), the androgen that drives prostate growth and male pattern hair loss. Finasteride blocks mainly the type II enzyme, dutasteride types I and II. With less DHT an enlarged prostate slowly shrinks over months and PSA falls by about half, while testosterone rises a little.
Easily mixed up with alpha-1 blockers and aromatase inhibitors.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Finasteride 5 mg tablets: label (DailyMed)
- Finasteride 1 mg tablets: label (DailyMed)
- Finasteride (NCI Thesaurus)
- Finasteride (NHS)
- Common questions about finasteride (NHS)
- Side effects of finasteride (NHS)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Reproductive and urinary health
Finasteride is one of 9 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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