Spotting, with oral contraceptives

In two studies rifabutin 300 mg daily for 10 or 14 days reduced the plasma levels of ethinylestradiol and norethisterone in women taking a combined oral contraceptive, but to a lesser extent than rifampicin. The AUC for ethinylestradiol decreased hy about 35% in both studies, and the AUC of norethisterone decreased by 17%. In one study, spotting occurred in 21.7% of women when they took rifabutin (compared with 3.7% in the control cycle and 36% with rifampicin). Ovulation did not occur with rifabutin or rifampicin in either study. - There appear to be no reports of contraceptive failure attributed to rifabutin. 

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