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).
Sources
- WHO: Medical eligibility criteria for contraceptive use (2015)– World Health Organization. Medical eligibility criteria for contraceptive use, 5th edition, 2015 (updated 2019).