The prescribing information for Propecia (finasteride 1 mg) says Propecia is a 5α-reductase inhibitor. It says finasteride is a competitive and specific inhibitor of Type II 5α-reductase, an intracellular enzyme that converts the androgen testosterone into DHT. The label and a 1999 journal abstract each measured how far DHT fell. This guide says which number comes from which source.
What finasteride is
The Propecia label describes finasteride as a synthetic 4-azasteroid compound and states that each film-coated tablet contains 1 mg of finasteride. It states that Propecia is a 5α-reductase inhibitor indicated for the treatment of male pattern hair loss (androgenetic alopecia) in men only.
A generic label for finasteride tablets USP 1 mg (Sun Pharmaceutical) uses the same wording for the 1 mg tablets.
The enzyme and DHT
The Propecia label states that in men with male pattern hair loss, the balding scalp contains miniaturized hair follicles and increased amounts of DHT compared with hairy scalp.
It states that inhibition of Type II 5α-reductase blocks the peripheral conversion of testosterone to DHT, resulting in significant decreases in serum and tissue DHT concentrations. It adds that administration of finasteride decreases scalp and serum DHT concentrations in these men.
The label also says that the relative contributions of these reductions to the treatment effect have not been defined. This guide does not go beyond that statement.
Numbers from the label
The Propecia label states that finasteride produces a rapid reduction in serum DHT concentration, reaching 65% suppression within 24 hours of oral dosing with a 1-mg tablet.
It also states that mean circulating levels of testosterone and estradiol were increased by approximately 15% as compared to baseline, but remained within the physiologic range.
Numbers from a 1999 study abstract
The second source is a 1999 abstract in J Am Acad Dermatol. It describes a 42-day trial with a placebo group. Men with androgenetic alopecia (N = 249) underwent scalp biopsies before and after receiving 0.01, 0.05, 0.2, 1, or 5 mg daily of finasteride or placebo. The table shows what it reported for the 1 mg group.
| Question | What the source says |
|---|---|
| How far did serum DHT fall after oral dosing with a 1-mg tablet? (label) | The label states 65% suppression within 24 hours. |
| How far did scalp skin DHT fall in the 1 mg group? (study abstract, 42 days) | The abstract reports a significant decline of 64.1%, compared with 13.0% with placebo. |
| How far did serum DHT fall in the 1 mg group? (study abstract, 42 days) | The abstract reports a significant decline (P <.001) of 71.4%. |
| What happened to testosterone and estradiol? (label) | The label states mean circulating levels increased by approximately 15% as compared to baseline, within the physiologic range. |
The abstract also reports figures for the 0.01, 0.05, 0.2 and 5 mg groups. Its conclusion is that in this study, doses of finasteride as low as 0.2 mg per day maximally decreased both scalp skin and serum DHT levels. That is a statement about DHT measurements in one study. It is not dosing advice.
What this does not tell you
These figures are hormone levels. They are not hair measurements, and the label says the relative contributions of the DHT reductions to the treatment effect have not been defined.
The label and the abstract measured DHT in different ways and over different time frames. The 65%, 64.1% and 71.4% figures are separate results and are not added, averaged or interchanged here. The abstract describes one 42-day study, and only one of its dose groups was 1 mg.
The label states the tablet is indicated in men only. Whether finasteride suits a particular man, and at what dose, is for a licensed provider to decide.