Drug interactions
Methimazole + Phenindione
Severity: Moderate
Methimazole and Phenindione: 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
Correcting hyperthyroidism with methimazole slows the catabolism of vitamin K-dependent clotting factors. As a result the anticoagulant effect of vitamin K antagonists such as phenindione tends to fall as the patient becomes euthyroid. Starting or stopping methimazole can therefore change INR control.
What this interaction means in plain words
Methimazole and Phenindione 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.
Why it happens
Normalization of thyroid state reduces clotting factor turnover, which diminishes the response to vitamin K antagonists.
What to do
Monitor INR more frequently when methimazole is started, changed or stopped. Adjust the phenindione dose to maintain the target INR.
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?
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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 Phenindione?
MediHelp rates the interaction as moderate. Methimazole and Phenindione 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. Correcting hyperthyroidism with methimazole slows the catabolism of vitamin K-dependent clotting factors. As a result the anticoagulant effect of vitamin K antagonists such as phenindione tends to fall as the patient becomes euthyroid. Starting or stopping methimazole can therefore change INR control.
How serious is the interaction between Methimazole and Phenindione?
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 more frequently when methimazole is started, changed or stopped. Adjust the phenindione dose to maintain the target INR.
What other medicines interact with Methimazole?
MediHelp also lists interactions of Methimazole with, for example, Carbamazepine, Clozapine, Propylthiouracil. The full list is on the Methimazole page.
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