Finasteride is a 5-Alpha-Reductase Inhibitor
Finasteride is a synthetic 5-alpha-reductase inhibitor (5-ARI), specifically a Type II 5α-reductase inhibitor, that blocks the conversion of testosterone to dihydrotestosterone (DHT). 1
Mechanism of Action
Finasteride works by competitively and specifically inhibiting the Type II isoenzyme of 5α-reductase, an intracellular enzyme present in the prostate, scalp, and genital skin 2, 1. This inhibition prevents the conversion of testosterone into 5α-dihydrotestosterone (DHT), which is the primary androgen responsible for:
- Prostatic growth and enlargement in benign prostatic hyperplasia (BPH)
- Hair follicle miniaturization in androgenetic alopecia (male pattern baldness)
Classification Context
As a hormonal therapy, finasteride represents the sole hormonal treatment that demonstrates both efficacy and acceptable safety for BPH treatment 2. It differs from other BPH medications like alpha-blockers (e.g., tamsulosin, doxazosin) which work through different mechanisms on smooth muscle receptors.
Key Distinguishing Features:
- Enzyme specificity: Inhibits Type II 5α-reductase (unlike dutasteride, which inhibits both Type I and Type II isoenzymes) 2
- Lipophilic properties: Can cross the blood-brain barrier and affect neurosteroid metabolism 3
- Systemic effects: Reduces DHT levels in both serum and target tissues (prostate, scalp) 2, 4
Clinical Applications
The drug class designation as a 5-alpha-reductase inhibitor directly relates to its FDA-approved indications:
Important Caveat:
While finasteride is classified as a 5-ARI and sometimes discussed alongside antiandrogens in hair loss treatment 6, it is not a true antiandrogen—it does not block androgen receptors but rather prevents DHT formation through enzyme inhibition.