Estradiol and Warfarin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Warfarin
Estradiol
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Altered warfarin exposure, reduced or increased anticoagulant effectiveness and altered INR
Interaction Deep Dive
Coadministration of warfarin and oral contraceptives (inhibitors of CYP1A2 and CYP3A4) may increase warfarin exposure and may increase INR. Consider the CYP1A2/CYP3A4 inhibition potential when starting, stopping, or changing dose of concomitant medications. Closely monitor INR if a concomitant hormonal contraceptive is used with warfarin3. Concomitant use of a combination contraceptive and warfarin may result in enhanced or reduced anticoagulant efficacy of warfarin 2. One study of 12 patients demonstrated an enhanced response to anticoagulant therapy when given concurrently with oral contraceptive 1. In another case report, emergency contraception with progestogen only in a patient receiving warfarin resulted in an enhanced anticoagulant effect evident by an INR of 8.1 4.
Why it happens (mechanism)
Inhibition of CYP1A2-mediated metabolism of warfarin; inhibition of CYP3A4-mediated metabolism of warfarin
Literature reports
3 reports — tap to read
a) Oral contraceptives potentiated anticoagulant efficacy, as measured by prothrombin time ratio, in 12 women (mean age, 34.5 yr) when treated concomitantly with a combination contraceptive (11, oral; 1, parenteral depot) and an anticoagulant (nicoumalone) compared with an anticoagulant alone. Anticoagulation was being used for Bjork-Shiley valvular prosthesis (n=9) and embolic mitral valve disease (n=3). Patients were followed for a total of 374 patient-months of which 230 months and 144 months were concomitant use with a mean anticoagulant dose of 2.05 mg (phase A) and anticoagulant alone at a mean dose of 2.53 mg (phase B), respectively. Although the anticoagulant dose requirement was lower during phase A (p less than 0.01), prothrombin time ratio was higher at 1.67 during phase A compared with 1.5 during phase B (p less than 0.01). It is postulated that the estrogens in the oral contraceptives may inhibit hepatic cell microsome enzymes. This may enhance the anticoagulant effect due to slowed breakdown of the anticoagulant 1.
b) Warfarin dose requirements were altered when 3 different hormonal contraceptives were used by a 33-year-old woman initially maintained on warfarin 38.5 mg/wk (historical max dose, 42 mg/wk) for long-term anticoagulation after aortic valve replacement. Monophasic ethinyl estradiol 0.02 mg/norethindrone 1 mg/day was also being given. Because of increased thrombosis risk, her oral contraceptive was replaced with an etonogestrel subdermal implant which required a 55.8% warfarin dose increase (60 mg/wk) to obtain goal INR (range, 2.5 to 3.5). After 10 months, the implant was removed due to increased menstrual bleeding. Nine days later, her INR increased to 6.5. During the next 48 days, no systemic contraceptives were used and her warfarin dose was titrated to 55.5 mg/wk. Oral norethindrone 0.35 mg/day was initiated and the warfarin dose stabilized at 53.5 mg/wk; however, norethindrone was discontinued 39 days later. Subsequently, no further warfarin dose adjustments were needed and the patient decided to avoid hormonal contraception. Ethinyl estradiol inhibition of CYP1A2- and CYP2C19-mediated warfarin metabolism is the postulated primary mechanism of this drug interaction which is considered probable based on the Horn Interaction Probability Scale 2.
c) A case report describes an enhanced anticoagulant effect of warfarin after giving a 35-year-old woman levonorgestrel for emergency contraception. The patient had familial type I (quantitative) antithrombin deficiency and a history of deep venous thrombosis and pulmonary thromboembolism. She had been stable on warfarin 7 mg per day with an INR) of 2.1. Three days after receiving emergency contraception, her INR was reported to be 8.1. Warfarin treatment was discontinued for two days and the patient's INR dropped to 2.5. No hemorrhagic complications occurred 4.
Common questions
Can I take Estradiol and Warfarin together?
Altered warfarin exposure, reduced or increased anticoagulant effectiveness and altered INR Always confirm with your pharmacist or prescriber before making any change.
How serious is the Estradiol and Warfarin interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Estradiol or Warfarin need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (4)
- de Teresa E, Vera A, Ortigosa J, et al: Interaction between anticoagulants and contraceptives: an unexpected finding. Br Med J 1979; 2:1260-1261.
- Zingone MM, Guirguis AB, Airee A, et al: Probable drug interaction between warfarin and hormonal contraceptives. Ann Pharmacother 2009; 43(12):2096-2102. PubMed
- Product Information: WARFARIN SODIUM oral tablets, warfarin sodium oral tablets. Teva Pharmaceuticals (per Dailymed), Parsippany, NJ, 2023. DailyMed
- Ellison J, Thomson A, & Greer I: Apparent interaction between warfarin and levonorgestrel used for emergency contraception. BMJ 2000; 321:1382.
Keep reading about Warfarin
Keep reading about Estradiol
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Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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