Major
Ethinylestradiol × Rifampicin
Estrogens. Natural and semisynthetic estrogens, plain×Antituberculous antibiotic
Mechanism
Rifampicin is the most potent CYP3A4 inducer in clinical practice. Ethinylestradiol plasma levels fall by 40–60% within the first week. Combined oral contraceptive efficacy drops to chance.
Symptoms
Breakthrough bleeding, missed periods, unplanned pregnancy. The patient feels no symptoms at the moment of dosing – the problem becomes apparent only later.
Management
Switch hormonal contraception to non-hormonal methods (copper IUD or barrier methods) for the duration of rifampicin and an additional 4 weeks after stopping. Alternatives: depot progesterone or a levonorgestrel IUD (minimal CYP3A4 effect).
Check the full regimen, not just this pair
Opens the checker with these two drugs prefilled. Add the rest of the regimen and recompute additive risks.
Sources
- WHO: Medical eligibility criteria for contraceptive use (2015)– World Health Organization. Medical eligibility criteria for contraceptive use, 5th edition, 2015 (updated 2019).