The mean oral bioavailability of finasteride is approximately 65%.  Its volume of distribution is 76 L/kg.  The plasma protein binding of finasteride is 90%.  The drug has been found to cross the blood–brain barrier , whereas levels in semen were found to be undetectable.  Finasteride is extensively metabolized in the liver , first by hydroxylation via CYP3A4 and then by aldehyde dehydrogenase .  The metabolites of finasteride have about 20% of its potency in inhibiting 5α-reductase and hence its metabolites are not particularly active.  The drug has a terminal half-life of 5 to 6 hours in adult men (18–60 years of age) and a terminal half-life of 8 hours or more in elderly men (greater than 70 years of age).  It is eliminated as its metabolites 57% in the feces and 40% in the urine .