Influence of aspirin on the pharmacokinetics of norethindrone.

The influence of a single dose of aspirin on the pharmacokinetics of the synthetic progestogen norethindrone was studied in rabbits. It was found that neither the 24-hr plasma levels nor the pharmacokinetic parameters of norethindrone following intravenous dosing were significantly altered by aspirin. However, after oral administration of norethindrone, the area under the plasma norethindrone versus time curve was significantly decreased by aspirin from 0.72 +/- 0.058 ng/ml X hr to 0.49 +/- 0.046 ng/ml X hr (mean +/- S.E) and the oral bioavailability was reduced from 56 +/- 4.2% to 38 +/- 3.6%, perhaps due to increased gut wall metabolism of norethindrone by aspirin.