5-alpha-reductase Inhibitor
Quick Definition
A 5-alpha-reductase inhibitor is a medication that blocks the conversion of testosterone to dihydrotestosterone (DHT). The class is FDA-approved for benign prostatic hyperplasia and male pattern hair loss. It includes finasteride (Type II selective) and dutasteride (Type I and Type II).
In Depth
5-alpha-reductase is the enzyme responsible for converting testosterone to dihydrotestosterone (DHT). DHT is the most potent androgen at certain target tissues, including the prostate gland and the hair follicle in male pattern baldness.
There are two major isoforms of the 5-alpha-reductase enzyme:
- Type I, expressed primarily in skin and liver. - Type II, expressed primarily in the prostate and hair follicle.
Finasteride is a selective Type II inhibitor. It reduces serum DHT by approximately 70% and scalp DHT by a similar margin.
Dutasteride inhibits both Type I and Type II isoforms. It produces deeper DHT suppression — approximately 90% reduction in serum DHT.
Both medications are FDA-approved for benign prostatic hyperplasia. Finasteride is FDA-approved at 1 mg daily for male pattern hair loss. Dutasteride is FDA-approved for hair loss in some other countries (notably South Korea) but not in the United States.
The clinical effect on male pattern hair loss: reducing the DHT signal at the scalp follicle slows or reverses the miniaturization process, allowing hairs to remain in the growing (anagen) phase longer. Best response is typically seen in younger men with less advanced miniaturization.
Side effects shared across the class include sexual side effects (decreased libido, erectile dysfunction, ejaculation disorders) at low single-digit incidence in original trials, with the contested phenomenon of persistent post-finasteride syndrome (PFS) reported in post-marketing literature.
5-alpha-reductase inhibitors are contraindicated in pregnancy due to potential effects on a male fetus. Even partner exposure to a topical compounded preparation should be managed with appropriate drying time before contact.
Related Terms
Dutasteride
Dutasteride is a dual 5-alpha reductase inhibitor that blocks both Type I and Type II isoforms of the enzyme responsible for converting testosterone into dihydrotestosterone (DHT). The FDA approved dutasteride (brand name Avodart) in 2001 for the treatment of benign prostatic hyperplasia (BPH) in adult males. By reducing DHT levels throughout the body by up to 90%, dutasteride has also been studied and used off-label for androgenetic alopecia (male pattern hair loss), though this indication has not received FDA approval. Dutasteride is available as a 0.5 mg capsule taken once daily, and compounded formulations may be prescribed by licensed providers for hair loss. Because it is more potent and has a longer half-life than finasteride, dutasteride requires careful patient selection and monitoring by a healthcare provider.
Minoxidil
Minoxidil is a vasodilator originally developed to treat high blood pressure, later found to stimulate hair growth when applied topically. The FDA approved topical minoxidil for androgenetic alopecia (pattern hair loss) in men in 1988 and in women in 1991. Compounded oral minoxidil is used off-label at low doses for hair loss, but this formulation is not FDA-approved. Minoxidil works by prolonging the anagen (growth) phase of hair follicles and improving blood flow to the scalp. It is available over-the-counter in 2% and 5% topical solutions and foams, and as a prescription oral tablet through licensed providers.
Finasteride
Finasteride is a Type II 5-alpha-reductase inhibitor FDA-approved for male pattern hair loss (1 mg, Propecia) and benign prostatic hyperplasia (5 mg, Proscar). It works by reducing dihydrotestosterone (DHT), the androgen most directly responsible for follicular miniaturization in male pattern baldness.
Anti-androgen
An anti-androgen is a medication that reduces or blocks androgen activity in the body. Mechanisms include reducing androgen production, blocking the androgen receptor, and inhibiting the conversion of testosterone to DHT. The category includes 5-alpha-reductase inhibitors, androgen receptor blockers, and some other agents.