Contraception failure oral contraceptives + rifampicin

Theophylline is a narrow therapeutic index drug with significant difference in bioavailability following oral administration. The half-life of the drug is increased by heart failure, cirrhosis and viral infections, in elderly patients, and by certain drugs, such as cimetidine, ciprofloxacin, oral contraceptives and fluvoxamine. The half-life is decreased in smokers, chronic alcoholism, and by certain drugs, such as phenytoin, rifampicin and carbamazepine. 

Sustained-release formulations can produce stable serum concentrations with once or twice daily dosage. Therapeutic effects occur at blood levels > 5 mg/1, and side effects increase considerably at levels > 15 mg/1. Smoking, alcohol, anticonvulsants, and rifampicin induce the drug-metabolizing enzyme system in liver and reduce the half-life of theophylline. On the other hand, heart and liver failure, sustained fever, old age and drugs such as cimeti-dine, ciprofloxacin, and oral contraceptives reduce theophylline clearance and thereby increase serum concentrations. 

Gupta KC, Ali MY (1980) Failure of oral contraceptive with rifampicin. Med... 

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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