Finasteride (Propecia, Proscar) is the most widely prescribed hair loss treatment in men. It works. But it fundamentally alters your hormonal environment, and that deserves honest discussion.
How Finasteride Actually Works
Finasteride is a type II 5-alpha reductase inhibitor. It blocks the enzyme that converts testosterone into DHT (dihydrotestosterone).
The pathway looks like this:
- Testosterone → (5-alpha reductase) → DHT
DHT is the more potent androgen. It's responsible for androgenetic alopecia (male pattern baldness), and it also drives prostate growth. Finasteride prevents this conversion, which is why it halts hair loss and shrinks enlarged prostates.
What happens to your hormones?
Total testosterone rises. When conversion to DHT is blocked, testosterone accumulates. Studies show approximately 10-15% elevation in total testosterone levels in men on finasteride.
DHT falls sharply. DHT drops by roughly 70% systemically (and 99% in the scalp).
Oestradiol may rise slightly. Without the DHT pathway as a metabolic outlet, some testosterone gets shunted toward aromatisation to oestradiol. Most men experience minimal change, but some notice modest E2 elevation.
What DHT Actually Does (Beyond Hair Loss)
This is where the debate gets honest. DHT isn't simply "the bad guy." It has genuine physiological roles:
Libido and erectile function. DHT contributes to sexual desire and penile sensitivity. It's not the only driver—testosterone matters more—but it's relevant.
Body hair and skin. DHT drives facial hair, body hair, and skin sebum production. Finasteride users often experience reduced body hair density.
Spatial cognition and competitive drive. Limited evidence suggests DHT contributes to competitive aggression and spatial reasoning, though this is not definitively established in humans.
Prostate function. Ironically, while finasteride shrinks an enlarged prostate (which is beneficial), DHT is required for normal prostate tissue maintenance. Long-term finasteride use does change prostate physiology.
Sexual Side Effects: What The Data Actually Shows
Finasteride's sexual side effect profile is controversial. Here's what the clinical trials showed:
Kaufman et al. (1998) — the landmark trial: sexual dysfunction in 5-8% of finasteride users, compared to 3% in placebo. The effect was dose-dependent (the dose matters). Notably, sexual dysfunction resolved in approximately 90% of men who discontinued finasteride.
Real-world reports suggest higher incidence than RCT data. Post-marketing surveillance and online forums report sexual dysfunction in 10-15% of users, though this likely includes confirmation bias.
Mechanism: Reduced DHT at the penile and neural level likely explains the effect. Some men experience decreased ejaculate volume, reduced libido, or erectile difficulties.
Timeline: Sexual side effects typically emerge within 2-4 weeks of starting finasteride, though some develop insidiously over months.
Post-Finasteride Syndrome: The Controversy
Post-Finasteride Syndrome (PFS) refers to persistent sexual, neurological, and psychological side effects that continue after discontinuing finasteride.
What the evidence shows:
Melcangi et al. (2020) published research suggesting finasteride may alter neurosteroid production in the brain, which could persist after stopping the drug. The hypothesis involves lasting changes to 5-alpha reductase expression in the central nervous system.
However, PFS remains scientifically controversial. Large population studies haven't consistently demonstrated PFS as a distinct syndrome. The FDA strengthened finasteride's label warnings in 2012 to include "persistent" sexual dysfunction, but evidence for permanent neurological damage is limited.
The honest take: Some men experience persistent side effects after stopping finasteride. Whether this represents a distinct syndrome or longer-term neuroadaptation remains unclear. The condition exists on a spectrum. For some, symptoms resolve within weeks of stopping. For others, they persist for months or years.
If you develop sexual side effects on finasteride, stopping the drug is reasonable and evidence-supported.
Dutasteride: The Alternative
Dutasteride (Avodart) blocks both type I and type II 5-alpha reductase, whereas finasteride blocks only type II.
Advantages: Slightly more effective for hair loss. DHT suppression is more complete (~99% systemically).
Disadvantages: Longer half-life (~5 weeks vs 6 hours for finasteride) means side effects take longer to resolve if discontinued. Sexual side effect incidence appears similar to finasteride, possibly slightly higher.
Dutasteride is not currently indicated for hair loss in the UK (off-label), whereas finasteride is licensed. Dutasteride is approved for benign prostatic hyperplasia.
Who Should Consider Finasteride, and Who Shouldn't
Reasonable candidates:
- Men with early-stage androgenetic alopecia who want to preserve existing hair
- Men with significant psychological distress from hair loss
- Men with no baseline sexual dysfunction or those willing to accept the risk
Consider alternatives if:
- You have a history of sexual dysfunction or low libido
- You're on other medications affecting sexual function
- You have a strong family history of PFS-like symptoms in finasteride users
- You have depression or mood instability (finasteride may worsen mood in susceptible individuals—mechanism unclear)
Alternatives exist: Minoxidil (topical) is less effective but has minimal systemic side effects. Newer medications like RU58841 and GT20029 are in development. Hair transplantation remains the most effective definitive option, though expensive.
Long-Term Considerations
Finasteride is often used indefinitely. Stopping the drug causes hair loss to resume within 3-6 months.
Long-term safety data extends beyond 20 years now. Cardiovascular risk doesn't appear elevated. Prostate cancer risk is actually reduced (paradoxically, finasteride use lowers prostate cancer incidence—likely because it reduces DHT-driven carcinogenesis in susceptible men). PSA levels drop roughly 50% on finasteride, which is important context when monitoring for prostate cancer.
The sexual side effect question remains the primary long-term concern. Most men tolerate finasteride well. Some don't, and discontinuation is appropriate for those individuals.
Summary
Finasteride is an effective, widely used medication with a clear mechanism of action and established safety profile. It does alter your hormonal environment meaningfully—DHT suppression is profound. Sexual side effects occur in a meaningful minority, and post-finasteride persistence of symptoms is a genuine concern for some, even if the prevalence and mechanism of PFS remain debated.
The decision to use finasteride should be informed and individualised. Hair loss is often cosmetically significant and psychologically important. The medication works. But so does understanding what you're accepting when you start it.