Effect of subacute dosing and phenobarbital and 3-methylcholanthrene pretreatment on the metabolism of acetaminophen in rats.

The metabolism of 14C-ring-labelled acetaminophen was studied in male Wistar rats. Pretreatment with phenobarbital increased the initial rate of elimination of 14C from the blood and increased the amount of acetaminophen glucuronide excreted in the urine. Pretreatment with 3-methylcholanthrene did not significantly affect the rate of elimination from the blood and decreased the amount of acetaminophen glucuronide in the urine. Daily dosing with acetaminophen for up to 3 weeks increased the rate of elimination of 14C from the blood after 4 h, and increased the urinary excretion of both total 14C and the glucuronide and sulfate conjugates. Subacute dosing with acetaminophen had a diuretic effect but this did not correlate with the increased excretion of the drug. It is concluded that acetaminophen elimination is increased by phenobarbital pretreatment and by subacute dosing with acetaminophen, but by different mechanisms.