Finasteride Liver Function Impact: What the Evidence Actually Shows

Does finasteride cause liver damage?

Neither label has a liver subsection under Warnings and Precautions, and neither carries a boxed warning [1][2]. The liver caution sits in the Hepatic Impairment subsection (8.6): both labels say caution should be exercised in patients with liver function abnormalities because finasteride is metabolized extensively in the liver [1][2].

The adverse reactions the labels tabulate from trials are mainly sexual. The Propecia table lists decreased libido, erectile dysfunction and ejaculation disorder; the Proscar table for the four-year PLESS trial lists impotence, decreased libido, decreased ejaculate volume, ejaculation disorder, breast enlargement, breast tenderness and rash [1][2]. No liver reaction appears in either table or in the labels' postmarketing lists [1][2].

What the liver does to finasteride

Finasteride is extensively metabolized in the liver, mainly by CYP3A4, into two metabolites with no more than 20 percent of its activity [1]. Mean terminal half-life is about 5 to 6 hours in men 18 to 60 and about 8 hours in men over 70; roughly 39 percent of a dose leaves in urine as metabolites and 57 percent in feces [1]. Finasteride does not appear to affect the cytochrome P450 system, and no clinically meaningful interactions were found with antipyrine, digoxin, propranolol, theophylline or warfarin [1]. The label's interaction section does not discuss CYP3A4 inhibitors or inducers [1]. Both labels also state that the effect of hepatic impairment on finasteride pharmacokinetics has not been studied [1][2].

Men who already have liver disease

Neither label gives an adjusted dose: Propecia is one 1 mg tablet once daily with or without meals [1], and Proscar is one 5 mg tablet once a day [2]. The caution about liver function abnormalities is the only liver-specific guidance either label offers [1][2].