Contraceptive hormones can interact with certain other drugs, reducing the effectiveness of OCs or modifying the effects of the other drugs. Pregnancies and breakthrough bleeding due to interference with contraceptive hormones have been reported in OC users taking:

• The anticonvulsant drugs carbamazepine, ethosuximide, methylphenobarbital, paramethadione, phenobarbital, phenytoin, primadone, and topirimate;
• The antitubercular antibiotic rifampicin; an
• The antitubercular antibiotic rifampicin; an

If a user of low-dose OCs is taking any of these drugs, she can increase her contraceptive protection by using an additional method of contraception while continuing with her daily pill, or she can change to an OC with 50 µg estrogen. If she uses the drug for less than a month, she should continue using her back-up contraceptive method or different pill regimen for at least a week after stopping the drug. If her cycle of 21 contraceptive pills ends during this week, for best protection she can start the next cycle of pills immediately. If she is using 28-day pill cycles, she can skip the seven placebos or iron tablets and start the next cycle of pills immediately (178, 189).

Women who must take any of these drugs for a long time, such as women being treated for tuberculosis, may want to consider another contraceptive method. Alternatively, they may increase their contraceptive protection by taking OCs with 50 µg estrogen for extended periods without interruption (179).

Oral contraceptives can speed up the metabolism of certain other drugs. Increased clinical effects have been observed in OC users taking anti-anxiety benzodiazepine drugs, corticosteriods used against inflammations, and theophylline (a drug used for asthma and some circulatory conditions). Thus OC users may require lower doses of these drugs than other women (18, 189, 418). Low-estrogen OCs generally speed metabolism less than high-dose pills.