Hormonal contraceptives warfarin, interactions

Hormonal contraceptives An interaction of warfarin with hormonal contraceptives has been described in a 33-year-old woman [44 ]. While taking warfarin 38.5 mg/week she switched from a monophasic combined oral contraceptive (ethinylestradiol + nor-ethindrone) to an implantable progestogen-only contraceptive (etonogestrel) 19 days later her INR fell to 1.8 and she required an increase in warfarin dose to 60 mg/week. After 10 months she decided to have the implant removed, and 9 days later her INR rose to 6.5. The warfarin dose was reduced to 55.5 mg/week. She then started to use an oral progestogen-only contraceptive (norethindrone) and the effective warfarin dose was 53.5 mg/ week. The authors hypothesized that the predominant mechanism of this interaction was inhibition of CYP1A2 and CYP2C19 by ethinylestradiol. 

The authors considered that the altered warfarin requirements observed in this patient may have been a result of multiple factors. They suggested that the predominant mechanism of interaction could have been inhibited by ethinylestradiol of CYP1A2 and CYP2C19. They recommended that further studies be undertaken to confirm the mechanism of the interaction. It is necessary to intensify warfarin monitoring when hormonal contraceptives are introduced or regimens are changed. 

Zingone MM, Guirguis AB, Airee A, Cobb D. Probable drug interaction between warfarin and hormonal contraceptives. Ann Pharmacother 2009 40 2096-102. 

Drug-drug interactions Watfarin In a single case an interaction of hormonal contraceptives with warfarin made dosage changes necessary [36 ]. 

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