The effect of rifampicin on norethisterone pharmacokinetics.

The pharmacokinetics of norethisterone have been studied in 8 women during and one month after treatment with rifampicin (450--600 mg/day). Rifampicin caused a significant reduction in the A.U.C. of a single dose of 1 mg norethisterone from 37.8 +/- 13.1 to 21.9 +/- 5.9 ng/ml X h (p less than 0.01). The plasma norethisterone half life (beta-phase) was also reduced from 6.2 +/- 1.7 to 3.2 +/- 1.0 h (p less than 0.0025). In one additional woman on long term oral contraceptive therapy the 12 hour plasma norethisterone concentration was reduced by rifampicin from 12.3 ng/ml to 2.3 ng/ml. Rifampicin caused a significant increase in antipyrine clearance, 6 beta-hydroxycortisol excretion and plasma gamma-glutamyltranspeptidase activity but there was no significant correlations between changes in these indices of liver microsomal enzyme induction. There was a significant correlation between the percentage increase in antipyrine clearance and the percentage decrease in norethisterone A.U.C. during rifampicin. The changes in norethisterone pharmacokinetics during rifampicin therapy are compatible with the known enzyme inducing effect of rifampicin.