Vitamin K antagonist (oral anticoagulant)
Warfarin with the birth control pill
Many women of reproductive age take a combined or progestogen-only oral contraceptive while also using a chronic medication such as Warfarin (Warfarin). The combination is generally fine at 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg, but a small number of medications can reduce contraceptive efficacy meaningfully and need either a backup method or a switch.
How Warfarin can affect contraceptive efficacy
Combined and progestogen-only contraceptives are metabolised through CYP3A4. Strong CYP3A4 inducers (some antiepileptics, rifampicin, St John's Wort) lower contraceptive plasma levels and reduce efficacy. Whether Warfarin acts on CYP3A4 determines whether Warfarin affects contraception. Most agents in Vitamin K antagonist (oral anticoagulant) have no clinically meaningful effect on the pill at 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg.
Practical guidance
According to the prescribing information for Warfarin, women on hormonal contraception should review Warfarin with the prescribing pharmacist or doctor. Where an interaction is documented, additional barrier contraception or switching to a non-oral method (IUD, implant) for the duration of Warfarin therapy is the standard mitigation.
Frequently asked questions
Will Warfarin make my pill less effective? ▾
Most Vitamin K antagonist (oral anticoagulant) medications at 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg do not affect oral contraceptive efficacy. The exceptions are CYP3A4-inducing drugs and a small number of others. The prescribing information for Warfarin states whether the interaction is meaningful.
Do I need a backup contraceptive on Warfarin? ▾
Backup contraception is needed only when there is a documented interaction between Warfarin and the contraceptive method. For most users at 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg, no backup is required. The pharmacist confirms whether Warfarin interacts with hormonal contraception.
Products containing Warfarin
More on Warfarin
- With alcoholWarfarin and alcohol — is it safe to drink?
- With foodShould Warfarin be taken with food?
- Side effectsWarfarin side effects: common, rare and warning signs
- Dosage guideWarfarin dosage guide: how much to take and when
- OnsetHow fast does Warfarin start working?
- DurationHow long does Warfarin last?
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.