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Probable drug interaction between warfarin and hormonal contraceptives
Michelle M Zingone1, Alexander B Guirguis, Anita Airee
1College of Pharmacy, University of Tennessee, Knoxville Campus, Knoxville, TN 37920, USA. mzingone@uthsc.edu
The Annals of Pharmacotherapy
|November 26, 2009
Summary
Warfarin dose adjustments are necessary when hormonal contraceptives are initiated or altered. This case highlights a probable drug interaction between warfarin and multiple hormonal contraceptives, impacting anticoagulation control.
Area of Science:
- Pharmacology
- Drug Interactions
- Hormonal Contraceptives
Background:
- Warfarin is a critical anticoagulant requiring careful management.
- Hormonal contraceptives are widely used by women of reproductive age.
Observation:
- A patient on warfarin experienced significant International Normalized Ratio (INR) fluctuations after switching hormonal contraceptives.
- Transitioning from combined oral contraceptives to etonogestrel implant led to decreased warfarin efficacy.
- Discontinuation of etonogestrel and subsequent initiation of oral norethindrone also necessitated warfarin dose adjustments.
Findings:
- The patient required a 55.8% increase in warfarin dose after etonogestrel implant insertion.
- INR increased transiently to 6.5 upon etonogestrel removal.
- Oral norethindrone also influenced warfarin dosage requirements.
Implications:
- This case suggests a potential drug interaction between warfarin and various hormonal contraceptives.
- Ethinyl estradiol may inhibit CYP1A2 and 2C19, affecting warfarin metabolism.
- Intensified warfarin monitoring is crucial when hormonal contraceptive use changes.
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