Finasteride
Quick Definition
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.
In Depth
Finasteride was FDA-approved in 1992 for benign prostatic hyperplasia (BPH) and in 1997 for male pattern hair loss. It is the most-studied oral medication for androgenetic alopecia, with over 25 years of clinical follow-up data.
The 5-alpha-reductase enzyme converts testosterone to dihydrotestosterone (DHT). Finasteride selectively inhibits Type II 5-alpha-reductase, reducing serum DHT by approximately 70% and scalp DHT by a similar margin. DHT is the dominant androgen at the hair follicle in male pattern baldness; reducing DHT slows or reverses the miniaturization process.
The 5-year extension of the original Phase III trials reported that 90% of finasteride-treated men either improved or stabilized their hair count, vs 25% of placebo-treated men. Best responders are typically younger men with less advanced miniaturization.
Continued daily dosing is required to maintain results. Discontinuation generally produces reversion to the pre-treatment trajectory within 6-12 months.
The side effect profile that gets most online attention is sexual side effects (decreased libido, erectile dysfunction, ejaculation disorders) and the contested phenomenon of persistent post-finasteride syndrome (PFS). The original trials reported sexual side effects at low single-digit incidence with resolution on discontinuation in most cases. Post-marketing reports describe a smaller subset of patients with persistent symptoms after discontinuation; causation remains contested.
Topical finasteride is also available, primarily through compounding, and produces lower systemic DHT suppression while maintaining scalp DHT reduction.
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.
5-alpha-reductase Inhibitor
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).
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.