Does Methimazole Interact With Warfarin?

Drug Interactions Anticoagulants (oral) Due to potential inhibition of vitamin K activity by methimazole, the activity of oral anticoagulants (e.g., warfarin) may be increased; additional monitoring of PT/INR should be considered, especially before surgical procedures.

Severity: Moderate

Severity basis: drug interactions section (keywords: monitor, consider). Combining Methimazole (Antithyroid agent) and Warfarin (Vitamin K antagonist anticoagulant) is classified as a moderate interaction.

What to Do

Use this combination with caution. Your prescriber may monitor you more closely or adjust doses.

From the FDA Label

Drug Interactions Anticoagulants (oral) Due to potential inhibition of vitamin K activity by methimazole, the activity of oral anticoagulants (e.g., warfarin) may be increased; additional monitoring of PT/INR should be considered, especially before surgical procedures.

Source: FDA structured product labeling (set_id 05a8fed7-f805-4c59-8840-630174b6d130).

Methimazole

Antithyroid agent

Brands: Tapazole

View Methimazole safety profile

Warfarin

Vitamin K antagonist anticoagulant

Brands: Coumadin, Jantoven

View Warfarin safety profile

Check these against your full medication list in our free Interaction Checker

This information is educational — not medical advice.

This page is provided for general educational purposes and summarizes publicly available data from sources such as the U.S. Food & Drug Administration. It is not a substitute for the judgment of a licensed clinician and should not be used to start, stop, or change any medication. It may be incomplete or out of date, and individual circumstances vary. Always talk with your prescriber or pharmacist about your specific medications and health conditions. If you think you may have a medical emergency, call 911.

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