Finasteride Dosing in Renal Impairment: What the Evidence Actually Shows

What the two labels say

Propecia (finasteride 1 mg) is indicated for male pattern hair loss in men only, at one 1 mg tablet once daily with or without meals [1]. Proscar (finasteride 5 mg) is indicated for symptomatic benign prostatic hyperplasia in men with an enlarged prostate, at one 5 mg tablet once a day [2]. Both labels state that no dose adjustment is necessary in renal impairment [1][2]. For male pattern hair loss, the labeled dose of finasteride 1 mg remains one tablet once daily in renal impairment [1].

The Propecia label describes a single-dose study of [14C]finasteride in people with chronic renal impairment, with creatinine clearances from 9.0 to 55 mL/min. AUC, maximum plasma concentration, half-life and protein binding were similar to healthy volunteers. Urinary excretion of metabolites was decreased, with an increase in fecal excretion, and plasma metabolite concentrations were significantly higher (a 60% increase in total radioactivity AUC). The label adds that finasteride has been tolerated at up to 80 mg/day for 12 weeks in men with normal renal function, where metabolite exposure would presumably be much greater [1].

Why little unchanged finasteride appears in urine

Finasteride is a competitive, specific inhibitor of Type II 5-alpha-reductase, roughly 100-fold selective for Type II over Type I, and it has no affinity for the androgen receptor [1]. It is extensively metabolized in the liver, mainly by CYP3A4, into two metabolites that keep no more than 20% of the parent drug's activity [1]. The label gives bioavailability of about 65%, peak plasma levels at 1 to 2 hours, protein binding of roughly 90%, and a terminal half-life of about 5 to 6 hours in men aged 18 to 60 and 8 hours in men over 70; about 39% of a dose is excreted in urine as metabolites and 57% in feces [1].

A disposition study gave six healthy volunteers 38.1 mg of radiolabeled finasteride. Mean recovery was 39.1% of the dose in urine and 56.8% in feces, while only 0.04% of the dose appeared in urine as unchanged drug [3]. Most of the urinary radioactivity was in an acidic metabolite fraction [3].

What the labels say to watch for

PSA. Proscar lowers serum PSA by approximately 50% within six months [2]. The label advises a new PSA baseline at least six months after starting treatment, doubling an isolated PSA value in men treated six months or more for comparison with normal ranges in untreated men, and evaluating any confirmed rise from the lowest on-treatment value, even if it stays within the normal range [2]. Propecia lowered mean PSA from 0.7 to 0.5 ng/mL at month 12 in men aged 18 to 41 [1].

Interactions. The labels state that no drug interactions of clinical importance have been identified and that finasteride does not appear to affect the cytochrome P450 system; antipyrine, digoxin, propranolol, theophylline and warfarin were tested with no clinically meaningful interactions found [1][2]. Proscar was also used in clinical studies alongside ACE inhibitors, diuretics, alpha-blockers, calcium channel blockers and NSAIDs without evidence of clinically significant adverse interactions [2].

Where the evidence stops

We found no study of finasteride in people on hemodialysis or peritoneal dialysis, and the labels' renal-impairment study did not report dialysis as a separate group [1][2].