Finasteride
Also known as: Propecia, Proscar
5-Alpha Reductase Inhibitor | DHT Blocker for Hair Loss & BPH
Overview
Finasteride is an FDA-approved 5-alpha reductase inhibitor that blocks the conversion of testosterone to dihydrotestosterone (DHT), the primary androgen responsible for androgenetic alopecia (male pattern hair loss) and benign prostatic hyperplasia (BPH). At 1mg daily, it reduces scalp DHT levels by approximately 66%, slowing or halting hair loss in roughly 90% of men and producing visible regrowth in about 48% over one to two years. Originally developed for prostate enlargement at 5mg (Proscar), the lower 1mg dose (Propecia) was approved specifically for hair loss in 1997. It remains one of only two FDA-approved oral treatments for androgenetic alopecia and has decades of long-term safety data.
Key benefits
- Reduces scalp DHT by approximately 66% at 1mg daily
- Slows or stops hair loss progression in roughly 90% of men
- Produces visible hair regrowth in approximately 48% of men within 1-2 years
- FDA-approved with over 25 years of clinical use and long-term safety data
- Convenient once-daily oral dosing with no injections required
- Well-characterized side effect profile with low incidence of adverse events
- Can be combined with minoxidil for enhanced efficacy
Mechanism of action
Finasteride competitively and selectively inhibits the Type II isoform of the enzyme 5-alpha reductase, which is predominantly found in hair follicles, the prostate, and the liver. This enzyme converts testosterone into dihydrotestosterone (DHT). By blocking this conversion, finasteride reduces serum DHT levels by approximately 70% and scalp DHT by roughly 66% at the 1mg dose. Since DHT is the primary androgen driving miniaturization of hair follicles in genetically susceptible individuals, lowering DHT levels allows affected follicles to recover, extend the anagen (growth) phase, and produce thicker terminal hairs. Notably, finasteride does not block androgen receptors and has minimal effect on testosterone levels, which may increase slightly (by about 10-15%) as a compensatory response to reduced DHT.
Molecular data
- Type
- Synthetic 4-azasteroid compound
- Half-life
- 6-8 hours (DHT suppression persists ~24 hours)
Indications
What the research community uses this compound for, with self-reported effectiveness.
Hair Loss
Prostate Health
Dosing protocols
Common protocols by delivery method. Adjust the curve below to model accumulation in your own cycle.
Delivery method
| Protocol | Dose | Frequency | Duration |
|---|---|---|---|
| — | 1mg | Once daily | — |
| — | 1.25mg (quarter of 5mg Proscar tablet) | Once daily | — |
| — | 1mg | 3x per week (Mon/Wed/Fri) | — |
| — | 5mg | Once daily | — |
Delivery method
| Protocol | Dose | Frequency | Duration |
|---|---|---|---|
| — | 0.025-0.1% solution, 1mL | Once daily | — |
Safety
Reported adverse effects, contraindications, and what to monitor on cycle.
Common side effects
- Decreased libido (reported in 1.8% of men in clinical trials vs 1.3% placebo)
- Erectile dysfunction (reported in 1.3% vs 0.7% placebo)
- Decreased ejaculate volume (reported in 0.8% vs 0.4% placebo)
Rare side effects
- Gynecomastia (breast tissue enlargement or tenderness)
- Testicular pain
- Depression or mood changes
- Skin rash or allergic reaction
- Elevated liver enzymes (very rare)
Contraindications
- Women who are pregnant or may become pregnant (finasteride is teratogenic and can cause abnormalities of external genitalia in a male fetus; even handling crushed tablets poses a risk)
- Women who are breastfeeding
- Known hypersensitivity to finasteride or any component of the formulation
- Severe hepatic impairment (finasteride is metabolized by the liver)
- Pediatric patients (not indicated for use in children)
References
Primary literature and clinical-trial data informing this entry.