A study of interaction of a low-dose combination oral contraceptive with anti-tubercular drugs.

Low-dose combination contraceptive (containing norethisterone acetate 1 mg and ethinyl estradiol 30 micrograms) was administered to women receiving concurrent therapy with either Rifampicin or "triple" antitubercular treatment consisting of paraaminosalicylic acid (PAS), isonicotinic acid hydrazide (INH) and streptomycin. Plasma levels of norethisterone (NET) and ethinyl estradiol (EE), PAS and INH were measured and the area under curve (AUC) was calculated for NET and EE. Rifampicin treatment (9 women) caused a statistically significant reduction of the plasma NET levels as well as the AUC of NET. In this group of women, though a trend for reduction in EE levels was observed in individual subjects, it was not statistically significant. Out of 7 regularly menstruating women on Rifampicin therapy, 2 showed a premenstrual rise of plasma progesterone (P) levels (> 4 ng/ml) suggesting an ovulatory cycle and 3 experienced menstrual irregularities. In contrast, plasma levels of NET and EE as well as their AUCs were not altered in 8 women receiving "triple" antitubercular therapy. Only one woman out of 8, had menstrual irregularity and all women had P levels in the anovulatory range. Furthermore, oral contraceptive treatment did not alter the plasma levels of PAS and INH.