Drug Mnemonic

Finasteride

5-ALPHA-REDUCTASE INHIBITOR · ↓ DIHYDROTESTOSTERONE · BPH / BALDNESS
April 6, 2026

A story built from the name finasteride itself - so the mechanism of action, key side effects, and the way Step 1 actually tests it stick on the first read.

RxMnemonic Antiandrogenics
Finasteride
5-ALPHA-REDUCTASE INHIBITOR · ↓ DIHYDROTESTOSTERONE · BPH / BALDNESS
FINA = five-a + STERIDE = steroid
Separate finasteride into fina (sounds like five-a) and steride (sounds like steroid). Now imagine a girl on the worst date of her life holding up her five-finger hand to block the male steroids across the table. Her five fingers remind you of five-a (fina), which helps you remember 5-alpha, and her blocking gesture represents an inhibitor, making finasteride a 5-alpha-reductase inhibitor.
Finasteride pharmacology mnemonic illustration
5-alpha reductase inhibitor FINA = five-a = 5-alpha
HOW STEP TESTS IT
1
Decreases DHT, not testosterone - competitively inhibits 5-alpha-reductase, blocking conversion of testosterone to dihydrotestosterone (DHT).
2
Treats benign prostate hyperplasia and baldness - DHT causes prostate enlargement and mediates androgenetic alopecia.
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Practice Question

A patient starts finasteride for benign prostatic hyperplasia. Which adverse effect is most likely?

← OxybutyninVasopressin →
Pharmacology Summary

Finasteride: Mechanism, Uses & High-Yield Facts for USMLE Step 1

Finasteride is a 5-alpha-reductase inhibitor that blocks the conversion of testosterone to dihydrotestosterone (DHT). DHT drives prostate growth and androgenetic alopecia, so finasteride is used for both benign prostatic hyperplasia and male pattern baldness. It also lowers PSA by roughly 50%, which must be accounted for when screening for prostate cancer. Side effects include decreased libido, erectile dysfunction, and gynecomastia.

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