Neo-Mercazole interactions: medicines, supplements, iodine and blood tests
Medicines that interact with Neo-Mercazole (carbimazole) and methimazole, including warfarin, digoxin, beta blockers, theophylline, prednisolone, erythromycin and clozapine, plus iodine, biotin, herbal remedies and alcohol.
Compared with a lot of tablets, Neo-Mercazole (carbimazole) mixes with other medicines fairly peacefully. Its product information admits that “little is known about interactions”. The ones that do matter are mostly knock-on effects. An overactive thyroid speeds up how fast you clear some drugs; as carbimazole slows the thyroid down, those drugs hang around longer, and a dose that suited you last month can become too much. Then there are a handful of things, iodine and biotin especially, that interfere with the treatment or the blood tests.
Key points
- Doses may need lowering as your thyroid normalises: digoxin, theophylline, beta blockers.
- Warfarin can act more strongly; more INR checks are needed.
- Extra care with other medicines that can lower white blood cells, such as clozapine.
- Iodine (including amiodarone and some supplements) weakens carbimazole’s effect.
- Biotin supplements can make thyroid blood tests misleading.
Medicines to tell your doctor about
The NHS asks you to mention these before starting carbimazole; the product information and the US methimazole label explain why.
| Medicine | What can happen | What is usually done |
|---|---|---|
| Warfarin and other anticoagulants | Bleeding risk rises: carbimazole can reduce vitamin K-dependent clotting factors, and clotting changes as the thyroid normalises | More INR or prothrombin checks, especially before surgery |
| Digoxin | Blood levels rise as you become euthyroid | Dose may need reducing |
| Beta blockers (such as propranolol) | Cleared more slowly once the thyroid is normal | Dose may need reducing; often prescribed alongside carbimazole at first |
| Theophylline | Levels rise if the dose is not reduced | Dose reduction, level monitoring |
| Prednisolone and other steroids | Prednisolone may be cleared faster | Dose review |
| Erythromycin | Carbimazole may slow its clearance | Dose review |
| Clozapine | Both can lower white blood cells | Extra care and blood count monitoring |
| Bladder medicines for incontinence | Listed by the NHS as needing a review | Tell your doctor |
The table is not the whole story, so check the leaflet in your pack as well.
Iodine
Iodine is the raw material your thyroid builds hormone from, which makes it the part of your diet that matters most for carbimazole. Flood the gland with it and the tablets work less well; run short and they work more strongly, the product information explains. Iodine treatment in the past also dulls the response.
Where would a large dose come from? Supplements that contain iodine are one source. Another is amiodarone, a heart rhythm drug packed with iodine, which the NHS notes can trigger an overactive thyroid on its own. Some doctors also ask patients on carbimazole to cut down on iodine in food: iodised salt, some processed foods, cheese and eggs, according to the NHS.
Supplements and herbal remedies
- Biotin (vitamin B7, common in hair and nail products) can make thyroid function tests inaccurate. The NHS says not to take it without talking to your doctor, and the British Thyroid Foundation asks people to mention any supplement containing iodine or biotin before a thyroid test.
- Black cohosh, taken for menopausal symptoms, is named by the NHS as possibly affecting carbimazole.
- Other herbal remedies are generally untested with carbimazole, so the NHS cannot say they are safe.
Medicines that affect thyroid tests
The British Thyroid Foundation lists the contraceptive pill, steroid hormones, anticonvulsants, anti-inflammatory drugs, lithium and amiodarone as medicines that can affect thyroid blood tests. Tell whoever orders your test what you take.
Alcohol and food
The NHS says alcohol does not affect how carbimazole works, though it can make dizziness worse. Food does not change carbimazole’s effect; you can take it with or without meals.
Methimazole and PTU
Methimazole is the active form of carbimazole and has the same interactions. Propylthiouracil (PTU) has a similar list on its US label: warfarin, beta blockers, digoxin and theophylline. See methimazole and propylthiouracil.
Getting carbimazole
Carbimazole is listed by PockDose, Yasabai and VGR Australia. For side effects see side effects.
Sources
Checked 15 October 2026.
- Taking carbimazole with other medicines and herbal supplements (reviewed May 2025) — NHS (UK)
- Neo-Mercazole New Zealand data sheet, sections 4.4 and 4.5 — Medsafe (New Zealand)
- Methimazole tablets prescribing information (drug interactions) — DailyMed, US National Library of Medicine
- Thyroid function tests (patient leaflet) — British Thyroid Foundation
- Overactive thyroid (hyperthyroidism), causes — NHS (UK)
- Common questions about carbimazole — NHS (UK)