Drug interactions
Methimazole + Warfarin
Severity: Moderate
Methimazole and Warfarin: moderate interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Treating hyperthyroidism with methimazole reduces warfarin sensitivity reversal; as the patient becomes euthyroid, warfarin requirements rise and the INR may fall. Without adjustment this risks subtherapeutic anticoagulation.
What this interaction means in plain words
Methimazole and Warfarin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Methimazole belongs to Endocrine; Warfarin belongs to Cardiovascular.
Why it happens
Restoring euthyroid status slows catabolism of vitamin K-dependent clotting factors, which reduces the enhanced warfarin effect seen in hyperthyroidism.
What to do
Monitor INR frequently as thyroid function normalizes and increase the warfarin dose as needed. Recheck INR if methimazole is later stopped or the dose changed.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
Pro+ members
Unlock the full page
Create an account and start a Pro+ trial to unlock the full management advice for Methimazole and Warfarin, the additional notes, the evidence detail and the whole-prescription interaction checker.
- Full clinical management advice
- Additional notes
- Evidence detail
- Interaction checker for a whole prescription
Try Pro+ free for 7 days on the web. No card needed.
Other interactions of Warfarin
- Defibrotide + Warfarin Contraindicated
- Ketorolac + Warfarin Contraindicated
- Abciximab + Warfarin Major
- Acetylsalicylic acid + Warfarin Major
- Adagrasib + Warfarin Major
- Amiodarone + Warfarin Major
Other interactions of Methimazole
- Carbamazepine + Methimazole Major
- Clozapine + Methimazole Major
- Methimazole + Propylthiouracil Major
- Methimazole + Sodium iodide i-131 Major
- Iodinated glycerol + Methimazole Moderate
- Iodine + Methimazole Moderate
Frequently asked questions
Can I take Methimazole with Warfarin?
MediHelp rates the interaction as moderate. Methimazole and Warfarin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Treating hyperthyroidism with methimazole reduces warfarin sensitivity reversal; as the patient becomes euthyroid, warfarin requirements rise and the INR may fall. Without adjustment this risks subtherapeutic anticoagulation.
How serious is the interaction between Methimazole and Warfarin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor INR frequently as thyroid function normalizes and increase the warfarin dose as needed. Recheck INR if methimazole is later stopped or the dose changed.
What other medicines interact with Warfarin?
MediHelp also lists interactions of Warfarin with, for example, Defibrotide, Ketorolac, Abciximab. The full list is on the Warfarin page.
Full monographs, dosing and an interaction checker in the MediHelp app
Open the complete reference, check multi-drug interactions and use dosing tools in the MediHelp app.
