Body and biology
What Is 5-Alpha Reductase? Enzyme and Medicines Explained
5-alpha reductase · say it: fyve AL-fuh ree-DUK-taysAlso written: 5-alpha-reductase, 5α-reductase, 5-ARI, 5-alpha reductase inhibitor
Short answer
5-alpha reductase is an enzyme that turns testosterone into a stronger hormone, DHT. Some medicines block it to treat prostate growth and hair loss.
What it means
5-alpha reductase is an enzyme (a protein that speeds up a chemical reaction) found in the prostate, skin, hair follicles and liver. It converts testosterone into dihydrotestosterone (DHT), a more powerful androgen hormone. DHT plays a role in prostate growth and in pattern hair loss.
The term appears on labels for 5-alpha reductase inhibitors, such as finasteride and dutasteride. By blocking the enzyme, these medicines lower DHT levels, which can shrink an enlarged prostate (benign prostatic hyperplasia) or slow pattern hair loss. Their labels warn that they lower PSA blood test results, which matters for prostate cancer screening, list sexual side effects, and warn that pregnant people should not handle broken tablets because DHT is needed for a male baby's development.
5-alpha reductase inhibitors are different from alpha-blockers, which also treat prostate symptoms but work by relaxing muscles in the prostate and bladder neck.
Where you may see it
- The 'Mechanism of Action' section of finasteride and dutasteride labels
- Warnings about PSA test results and pregnancy handling
- Urology and dermatology clinic letters
📄 How it looks on a report
Finasteride is a competitive and specific inhibitor of Type II 5-alpha reductase, an enzyme that converts testosterone to DHT.
🙅 What it does not mean
- Blocking 5-alpha reductase does not stop testosterone; it lowers its conversion to DHT.
- A lower PSA result on these medicines does not automatically mean your prostate risk is lower; tell whoever checks your PSA that you take one.
- The handling warning is about pregnant people, not about you taking it as prescribed.
🗒️ Questions to ask your doctor
💊 Medicines whose labels mention “5-alpha reductase”
Plain-English explanations of each medicine, from its official FDA label.
- Finasteride
a 5-alpha reductase inhibitor, a medicine that blocks an enzyme involved in making a hormone called DHT
🔗 Read more from trusted sources
- “5-alpha reductase” on MedlinePlus ↗
US National Library of Medicine
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