Finasteride
Finasteride is a type II 5-alpha-reductase inhibitor that lowers dihydrotestosterone (DHT), used for male-pattern hair loss and benign prostatic hyperplasia. In the PED context it is used to reduce androgenic hair loss driven by DHT and DHT-derived compounds — but it does nothing against non-DHT androgens (e.g. nandrolone, trenbolone) and can worsen some, and it approximately halves PSA, which must be accounted for in prostate screening.
01 Overview
Finasteride inhibits the type II isoform of 5-alpha-reductase, cutting conversion of testosterone to the more potent androgen DHT by roughly 65-70% systemically. This slows or partially reverses androgenetic hair loss and shrinks the prostate. Its protection is specific to DHT-mediated effects: it does nothing for hair loss driven by androgens that are not 5-alpha-reduced substrates, and for 19-nor compounds 5-alpha-reduction actually yields weaker metabolites, so finasteride offers no benefit and can be counterproductive.
Two practical points matter. First, finasteride roughly halves serum PSA, so any PSA result should be doubled when screening for prostate cancer in men on the drug. Second, a subset of users report persistent sexual and mood side effects ("post-finasteride syndrome"), the mechanism and true incidence of which remain debated.
02 Mechanism
Competitive inhibitor of type II 5-alpha-reductase, reducing conversion of testosterone to dihydrotestosterone (DHT).
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Hair loss (label) | 1 mg/day | Oral | Standard Propecia dose for androgenetic alopecia; near-maximal DHT suppression already at 1 mg. |
| BPH (label) | 5 mg/day | Oral | Proscar dose for benign prostatic hyperplasia; little added DHT suppression over 1 mg. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Reduced DHT / slowed hair lossLowers DHT and slows or partially reverses androgenetic alopecia driven by DHT. | -65 to -70% serum DHT | Clinical | |
| Prostate shrinkageReduces prostate size and urinary symptoms in BPH. | reduced prostate volume | Clinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| No protection vs non-DHT androgensOnly DHT-mediated hair loss responds; 19-nor and other non-5AR-substrate androgens are unaffected, and finasteride can be counterproductive with them. | Moderate | Inherent limitation | Clinical | |
| Lowered PSA (screening confound)Finasteride roughly halves serum PSA, which can mask a rising PSA and delay prostate cancer detection. | Moderate | Universal | Clinical | |
| Sexual dysfunctionReduced libido, erectile dysfunction and reduced ejaculate; a subset report persistence after stopping ("post-finasteride syndrome"), which is debated. | Moderate | Uncommon | Clinical |