Drug Interaction Report

Methimazole and Theophylline: Interaction Details

Name: Methimazole and Theophylline Interaction
Creator: Dr. Brian Staiger, PharmD, BCPS

A pharmacist-reviewed analysis of the Methimazole and Theophylline interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Theophylline

Accurbron® Aerolate® Aquaphyllin® Asbron® Bronkodyl® Duraphyl® Elixicon® Elixomin®

Methimazole

Felanorm Felimazole Reguzol Tapazole®

Medically reviewed by

Dr. Brian Staiger, PharmD, BCPS

Updated Aug 8, 2026

Editorial policy · Data sources

Interaction severity

Moderate

Can be significant — usually manageable with monitoring.

Minor

Moderate

Major

Contraindicated

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.

Of 45 documented Methimazole interactions, 37 are rated moderate — including this one.

Onset

delayed

Evidence

theoretical

Severity

Moderate

What happens

Altered theophylline metabolism

Interaction Deep Dive

Hyperthyroidism may increase the systemic clearance of theophylline. Once the patient becomes euthyroid, a dose reduction of theophylline may be necessary¹.

Why it happens (mechanism)

Change from thyroid to euthyroid state may alter theophylline clearance

Common questions

Can I take Methimazole and Theophylline together?

Altered theophylline metabolism Always confirm with your pharmacist or prescriber before making any change.

How serious is the Methimazole and Theophylline 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 "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.

Questions for your pharmacist

Does my dose of Methimazole or Theophylline 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 (1)

Product Information: methimazole oral tablets, methimazole oral tablets. Caraco Pharmaceutical Laboratories, Ltd, Detroit, MI, 2008. DailyMed

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Theophylline

Keep reading about Theophylline

All Theophylline interactions (46) Theophylline drug information

Also serious with: Peginterferon Alfa-2A Injection Medroxyprogesterone Capmatinib Estradiol Transdermal Patch

Methimazole

Keep reading about Methimazole

All Methimazole interactions (45) Methimazole drug information

Also serious with: Tegafur Methotrexate Injection Bupropion Levoketoconazole

Beyond drug–drug

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:

Theophylline + Activated Charcoal Ephedra Sida Cordifolia Adrafinil African Wild Potato Ashitaba Bacopa Baikal Skullcap + 118 more

Methimazole + Activated Charcoal Beer Siberian Cocklebur Wine Ascophyllum Nodosum Ashwagandha Babassu Baikal Skullcap + 56 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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A message from HelloPharmacist

About our interaction database: depth, not breadth

HelloPharmacist doesn't license one of the big commercial interaction databases, so what you'll find here is not a complete list of every known drug interaction. Instead, each entry is researched, analyzed, and written up individually — documented from the published literature, reviewed by a pharmacist, and cited so you can check every source yourself.

If a combination isn't listed here, it means we haven't documented it yet — never that it's safe. For screening a full medication list, we recommend pairing this site with a comprehensive tool like the Drugs.com Interaction Checker and, most importantly, a review by your own pharmacist. For a deep, plain-English, fully sourced explanation of a specific documented interaction — that's exactly what we built this for.

This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.