Why can doxycycline affect combined oral contraception?
Doxycycline is a non-liver enzyme inducing antibiotic. As with other antibiotics doxycycline may temporarily decrease colonic bacteria thus inhibiting the enterohepatic circulation of ethinylestradiol, the semisynthetic oestrogen contained in all combined hormonal contraceptives. The importance of the enterohepatic circulation varies from woman to woman. There is no secondary re-absorption of progestogens via the enterohepatic circulation and therefore doxycycline (as for other antibiotics) has no effect on the efficacy of progestogen-only contraceptives (including the pill, injectables, implants, the levonorgestrel releasing intrauterine system or progestogen-only emergency contraception). Therefore this advice concentrates on those using the combined oral contraceptive pill for contraception.
