Finasteride
Type II 5-alpha reductase inhibitor that blocks the conversion of testosterone to DHT. Approved at 1 mg for male pattern hair loss (Propecia) and 5 mg for benign prostatic hyperplasia (Proscar). The most common hair-protection drug for men on TRT.
Overview
Type II 5-alpha reductase inhibitor that blocks the conversion of testosterone to DHT. Approved at 1 mg for male pattern hair loss (Propecia) and 5 mg for benign prostatic hyperplasia (Proscar). The most common hair-protection drug for men on TRT.
At 1 mg/day lowers scalp DHT by about 64% and serum DHT by about 71%. Serum testosterone rises slightly. Shrinks the prostate and lowers PSA by roughly half, which matters when interpreting PSA results. Does not blunt the lean mass, strength or bone gains of testosterone at replacement doses. Small increase in sexual side effects compared with placebo.
Compound Guide
Structure: 4-azasteroid. A competitive, slowly reversible inhibitor of type II (and to a lesser extent type III) 5-alpha reductase. It does not block the type I enzyme, which is why dutasteride lowers serum DHT further.
Dosage:
- Hair loss: 1 mg once daily. Doses as low as 0.2 mg give near-maximal DHT suppression.
- Prostate: 5 mg once daily
- Topical: 0.25% spray formulations exist in some markets.
Evidence snapshot:
- In the pivotal trials of 1,553 men, 1 mg added 107 hairs over placebo in a 5.1 cm² vertex area at one year and 138 at two years (Kaufman 1998).
- In a 52-week trial of hypogonadal men aged 60+ on testosterone enanthate 125 mg/week, adding finasteride 5 mg did not change the strength, fat-free mass or bone density gains, and it prevented the prostate growth (Borst 2014).
Key Notes:
- Only works on androgens that need 5-alpha reductase. That means testosterone. Masteron, Primobolan, Anavar and Winstrol are already DHT-derived, so finasteride only reduces the DHT coming from your testosterone base.
- Do not combine with Nandrolone. 5-alpha reductase converts nandrolone to the weaker androgen DHN (rat and cell data). Blocking that step should, in theory, leave more of the stronger parent hormone acting at the scalp and prostate. No human trial has tested it.
- Sexual side effects: the pooled risk of erectile dysfunction is roughly doubled versus placebo (number needed to harm about 82). Expectation alone raises reporting, so start it at a time when nothing else in your protocol is changing.
- PSA: finasteride roughly halves PSA. Tell whoever reads your PSA that you take it, or a rising PSA can be missed.
- Pairs with Minoxidil, which works on a separate, androgen-independent pathway.
Bloodwork Monitoring:
- DHT at baseline and 6-8 weeks after starting, if you want to confirm suppression.
- PSA before starting if you are over 40, then interpret later results knowing the drug halves it.
- Testosterone and Estradiol may drift up slightly because less testosterone is converted to DHT.
How finasteride, dutasteride and minoxidil fit together on TRT: Does TRT cause hair loss? and How much hair oral minoxidil regrew in Phase 3.
Usage History
Markers to Monitor
Frequently Asked Questions
Quick Reference
Category
Ancillary
Half-Life
About 5-6 hours, but DHT suppression persists because the enzyme inhibition is long-lasting. Dosed once daily.
Detection Time
N/A