Phenprocoumon and Methimazole: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Methimazole
Phenprocoumon
No brand names on record
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
Decreased anticoagulant effectiveness
Interaction Deep Dive
In hyperthyroid patients, the metabolism of vitamin K clotting factors is increased, resulting in increased sensitivity to oral anticoagulants23. Antithyroid drugs, by reducing the extent of hyperthyroidism, decrease the metabolism of clotting factors and thus reduce the effects of oral anticoagulants 4. On the other hand, patients on anticoagulant therapy who are euthyroid due to antithyroid agents may develop marked hypoprothrombinemia if the antithyroid medications are ceased and they become thyrotoxic again 5. In one case report, treating patients with both methimazole and warfarin necessitates intensive and frequent monitoring, since alterations in thyroid function affect the response to anticoagulation 1.
Why it happens (mechanism)
Decreased metabolism of clotting factors
Literature reports
1 report — tap to read
a) One case report describes changes in thyroid hormone concentrations that significantly affected warfarin-induced anticoagulation in a patient with Graves' disease. A 54-year old man was treated with methimazole 30 mg/day for hyperthyroidism and enoxaparin then warfarin for atrial fibrillation. In the following 3 months after discharge, the patient's warfarin was repeatedly increased to 85 mg/week to achieve and maintain a therapeutic INR (range 2 - 3). The patient remained adequately anticoagulated for 12 weeks, when the methimazole dose was decreased to 10 mg/day. Continued hypothyroidism necessitated a stop of methimazole for 5 days, then resumed at a dose of 5 mg/day. This resulted in a supratherapeutic INR, and the warfarin was decreased to 60 mg/week. What followed was a pattern of methimazole dosage adjustments leading to significant changes in thyroid function, and subsequent changes in INR. After approximately 14 months of warfarin therapy, Two ECGs confirmed sustained normal sinus rhythm and anticoagulation was discontinued 1.
Common questions
Can I take Phenprocoumon and Methimazole together?
Decreased anticoagulant effectiveness Always confirm with your pharmacist or prescriber before making any change.
How serious is the Phenprocoumon and Methimazole interaction?
It is rated moderate. Can be significant — usually manageable with 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.
Questions for your pharmacist
- Does my dose of Phenprocoumon or Methimazole 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 anything you'd monitor while I'm on both?
References (5)
- Busenbark LA & Cushnie SA: Effect of graves' disease and methimazole on warfarin anticoagulation. Ann Pharmacother 2006; 40(6):1200-1203. PubMed
- Product Information: Tapazole(R), methimazole. Eli Lilly and Company, Indianapolis, IN, 2000. DailyMed
- Chute JP, Dahut WL, Shakir KM, et al: Enhancement of warfarin induced hypoprothrombinemia by hyperthyroidism. Blood 1994; 84(suppl 1):674a.
- Hansten PD: Oral anticoagulants and drugs which alter thyroid function. Drug Intell Clin Pharm 1980; 14:331-334. DOI
- Vagenakis AG, Cote R, Miller ME, et al: Enhancement of warfarin-induced hypoprothrombinemia by thyrotoxicosis. John Hopkins Med J 1972; 131:69-73.
Keep reading about Methimazole
Keep reading about Phenprocoumon
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
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