Oral Contraceptives and St Johns Wort

Oral contraceptives are divided into two types: progestogen only and combined estrogen and progestogen. Most oral contraceptives are substrates for CYP3A4 (49). 17-Ethynylestradiol is a major component of oral contraceptive pill and is also used in hormonal replacement therapy in postmenopausal women. It is metabolized through hydroxylation in position 2 by CYP3A4 (50). St. John's wort has significant interaction with oral contraceptives (51). Muprhy et al. studied interaction between St. John's wort and oral contraceptives by investigating phramacokinetics of norethindrone and ethinyl estradiol using 16 healthy women. Treatment with St. John's wort (300mg three times a day for 28 days) resulted in a 13-15% reduction in dose exposure from oral contraceptives. Breakthrough bleeding increased in treatment cycle as did evidence of follicle growth and probable ovulation. Authors concluded that St. John's wort increased metabolism of norethindrone and ethinyl estradiol and thus interfered with contraceptive effectiveness (52).



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