5-alpha-reductase inhibitor
Also called 5-ARI, 5α-reductase inhibitor, DHT blocker
A 5-alpha-reductase inhibitor is a medicine that blocks 5-alpha-reductase, the enzyme that converts testosterone into dihydrotestosterone (DHT), a more potent male hormone. Blocking that conversion is how these drugs treat an enlarged prostate and, in the scalp, male pattern hair loss.
FDA-approved examples are finasteride (sold as Proscar for an enlarged prostate and as Propecia for male pattern hair loss) and dutasteride (sold as Avodart), used for an enlarged prostate. Topical and multi-ingredient finasteride preparations made by compounding pharmacies are a separate category: compounded medicines are not FDA-approved.
Why it matters after 45
Benign prostatic hyperplasia, an enlarged prostate, is the most common prostate condition after 50, which is when most men first meet this class, often alongside questions about thinning hair. It also matters for screening: these drugs lower PSA (prostate-specific antigen), so a clinician reading a PSA result needs to know one is being taken.
Where it comes up
- 4-in-1 Hair Loss Treatmentthe multi-agent preparation ACT 2 prescribes, which targets DHT signaling among other pathways
- Topical vs oral finasteridehow the route changes exposure
- Finasteride and sexual side effectswhat is known about the most-asked-about risk
- Hair thinning after 50why hair changes in later midlife and what drives it
References
- MedlinePlus. Finasteride. U.S. National Library of Medicine. https://medlineplus.gov/druginfo/meds/a698016.html
- MedlinePlus Genetics. SRD5A2 gene. U.S. National Library of Medicine. https://medlineplus.gov/genetics/gene/srd5a2/
- MedlinePlus. Enlarged Prostate (BPH). U.S. National Library of Medicine. https://medlineplus.gov/enlargedprostatebph.html
- National Cancer Institute. Prostate-Specific Antigen (PSA) Test. https://www.cancer.gov/types/prostate/psa-fact-sheet
Definitions here are educational and are not medical advice. They explain what a term means; the pages linked above cover what it means for you, and a clinician reviews your own results. Editorial & medical review policy · All glossary terms