The metabolism of generic tadalafil is via the hepatic enzyme cytochrome P450 34A (CYP34A) to a catechol metabolite, which undergoes further metabolism to its primary circulating metabolite, methylcatechol glucuronide, a 10,000-fold less potent molecule than tadalafil. The CYP34A metabolism pathway of tadalafil was verified with studies using a CYP34A inhibitor ketoconazole and a CYP34A inducer rifampin. Tadalafil is excreted largely as metabolites, approximately two-thirds into the feces and one-third into the urine.