Acetaminophen pharmacokinetics: comparison between pregnant and nonpregnant women.

Acetaminophen is the most commonly taken drug during pregnancy, but knowledge about its absorption and disposition is lacking. Six healthy women volunteered to ingest a standard 1000 mg dose at 36 weeks' gestation and 6 weeks post partum. Mean maternal serum concentrations of acetaminophen were consistently less than but not significantly different from the postpartum values. The mean half-life of acetaminophen during pregnancy (3.7 hours) was not significantly different from the nonpregnant value (3.1 hours). The maximum plasma concentration occurred at 0.8 hours and was 20.8 +/- 6.9 micrograms/ml during pregnancy and 23.7 +/- 6.0 micrograms/ml in the nonpregnant state. The absorption, metabolism, and renal clearance of acetaminophen were unchanged. The decrease in the mean area under the curve during pregnancy may be explained by the increase in volume of distribution of acetaminophen. Potentially hepatotoxic metabolites were not measurable in the maternal serum. We conclude that the absorption and disposition of acetaminophen, when used in a standard oral dose, are not affected by pregnancy.