Finasteride Metabolism and Energy Expenditure: What the Evidence Shows

How is finasteride absorbed and cleared?

The Propecia (1 mg) label gives the pharmacokinetic numbers. Bioavailability is about 65% and food does not change it; peak concentration arrives 1 to 2 hours after a dose, and about 90% is bound to plasma proteins [1]. The liver does most of the clearing, mainly by CYP3A4, into two metabolites that carry no more than 20% of its activity [1]. Mean terminal half-life is about 5 to 6 hours in men aged 18 to 60 and about 8 hours in men over 70 [1]. About 39% of a dose leaves in urine as metabolites and 57% in feces [1].

Why does the DHT effect outlast the half-life?

A 1 mg tablet suppresses serum DHT by 65% within 24 hours [1]. The label says the enzyme complex finasteride blocks turns over slowly, about 30 days for Type II 5-alpha reductase, and that the effect on DHT outlasts the plasma half-life [1]. The label directs one 1 mg tablet once daily with or without food, and says daily use for three months or more is usually needed before benefit is seen and that withdrawal leads to reversal of effect within 12 months [1].

What did the finasteride trials actually measure?

The finasteride trials measured other endpoints. The two 1-year hair-loss trials enrolled 1,553 men aged 18 to 41 and measured scalp hair counts, patient and investigator assessment and photo review; hair count in a 1-inch (5.1 cm2) circle of vertex scalp was 107 hairs above placebo at one year and 138 above at two years (P<0.001) [2]. The Prostate Cancer Prevention Trial (18,882 men aged 55 or older, 5 mg or placebo for seven years) had prostate cancer prevalence as its primary endpoint: 18.4% (803 of 4,368) versus 24.4% (1,147 of 4,692) [3].