Time-dependent change in renal clearance of bethanidine in humans.

Blood levels and urinary excretion rates of bethanidine were determined in three normal human subjects following oral administration of a single dose of the drug. The postabsorptive decline of blood concentration with time was noticeably slower than the corresponding decline in the urinary excretion rate. The discrepancy can be attributed to a continual decrease in the renal clearance of bethanidine throughout the study. Therefore, pharmacokinetic modeling of urinary excretion data alone would lead to erroneous conclusions concerning the persistence of drug in the blood.