Why does St. Johns wort affect the oral contraceptive pill?

These women started taking St John’s wort and then had unplanned pregnancies. A total of 19 reports of suspected interactions between St John’s wort and hormonal contraceptives have been received through the Yellow Card scheme since 2000 (four for contraceptive implants and 15 for contraceptive pills). read more

The authors recommended that people taking oral contraceptives and St. John’s wort at the same time should back up their birth control with condoms. A study of 16 women found that taking St. John’s wort and birth control pills simultaneously led to a 13 to 15 percent reduction in the the levels of contraceptive hormones circulating in the body. read more

John's wort can make many other drugs less effective. There have been cases of unintended pregnancies in women taking St. John's wort and birth control pills and cases of organ rejection in those taking St John's wort with anti-rejection drugs after a transplant. read more