Methimazole (thiamazole): uses, doses, side effects and how it relates to Neo-Mercazole

Methimazole is the antithyroid medicine most people with an overactive thyroid take worldwide. It is also called thiamazole, the name used in much of Europe. And it is what Neo-Mercazole becomes: carbimazole is converted into methimazole soon after it is absorbed. So nearly everything on this site about Neo-Mercazole applies to methimazole too, except the milligram doses.

Key points

• Drug class: thionamide antithyroid drug. It slows the thyroid’s production of T4 and T3.
• Prescribed for: Graves’ disease and toxic multinodular goitre, and to calm hyperthyroidism before surgery or radioactive iodine.
• US label doses: 15 mg a day (mild), 30 to 40 mg (moderately severe), 60 mg (severe); maintenance 5 to 15 mg a day.
• Tablets: 5 mg and 10 mg generics in the US. The original brand, Tapazole, is discontinued.
• Same red flags as carbimazole: sore throat or fever means stop and get a blood count the same day.

What it is prescribed for

The US label lists two uses: Graves’ disease with hyperthyroidism or toxic multinodular goitre when surgery or radioactive iodine are not the right choice, and short-term control of symptoms before thyroid surgery or radioiodine. In practice, guidelines put it first. The European Thyroid Association recommends methimazole as the preferred drug for a 12 to 18 month first course in Graves’ disease, and the US label calls it the preferred choice for children because propylthiouracil carries a liver risk.

Brand names: Tapazole and others

Tapazole was the original US brand. FDA records list both Tapazole 5 mg and 10 mg as discontinued, with a note that this was not for safety or effectiveness reasons; generic methimazole 5 mg and 10 mg tablets remain on the market from many makers. Pharmacies still use “Tapazole” as a search name, which is why it appears on product pages. Outside the US the drug may be sold as thiamazole, and in countries such as the UK it is not licensed at all; carbimazole is used instead.

Dose

Doses on the US label are split into three, about eight hours apart. Once levels settle, many doctors move to once a day, because the active drug stays in the thyroid longer than in the blood.

Lower starting doses tend to be safer. In a Japanese randomised trial, 15 mg a day was enough for mild and moderate Graves’ disease, and 30 mg worked better only for severe cases. In a large Japanese hospital comparison, agranulocytosis affected 0.81% of patients started on 30 mg a day and 0.22% of those started on 15 mg.

Methimazole 5 mg or 10 mg

The tablets differ only in strength. Most maintenance doses are made from 5 mg tablets; 10 mg tablets cut the pill count at higher starting doses. The US 5 mg tablet in our source label is scored to break into two even halves.

Converting from carbimazole

A pharmacokinetic study found 15 mg of carbimazole gave the same methimazole levels as 9.2 mg of methimazole. That is the basis for the usual rule of about 10 mg carbimazole to 6 mg methimazole. Anyone moving between countries should have the doctor convert the dose and check thyroid blood tests a few weeks after the switch.

How long it takes and how long you take it

Methimazole stops new hormone being made but does not remove hormone already stored, so improvement builds over weeks. Blood tests come roughly every six weeks while the dose is being adjusted. For Graves’ disease a course usually runs 12 to 18 months; longer low-dose treatment is an option some people choose instead of radioiodine or surgery (see the studies).

Side effects

The label separates rare major reactions from more common minor ones.

Minor: rash, hives, nausea, vomiting, stomach discomfort, joint and muscle pain, tingling, loss of taste, hair loss, headache, itching, drowsiness, swelling, dizziness, skin pigmentation and swollen glands.

Major (rare): agranulocytosis and other bone marrow suppression, drug fever, a lupus-like syndrome, low blood sugar from insulin autoimmune syndrome, hepatitis (jaundice can persist for weeks after stopping), low clotting factors, kidney inflammation, acute pancreatitis, and vasculitis, usually ANCA-positive, which can affect the skin, kidneys, lungs and nerves.

More detail, including how often each problem occurs, is on Neo-Mercazole side effects; the risks are the same because the active drug is the same.

Pregnancy and breastfeeding

Methimazole crosses the placenta. The label lists rare birth defects after first-trimester use, among them a scalp defect (aplasia cutis), facial and nasal passage malformations, oesophageal atresia and abdominal wall defects, and says other drugs may be more appropriate early in pregnancy. Guidelines advise switching to propylthiouracil when planning pregnancy and through the first trimester. If methimazole is used, the lowest dose that controls the mother’s thyroid is the rule.

Methimazole does reach breast milk, but the label cites a long-term study of 139 breastfeeding mothers on methimazole that found no harm to their babies, and asks for the baby’s thyroid function to be checked weekly or every two weeks.

Interactions

The label highlights four groups. Warfarin and other oral anticoagulants may work more strongly, so INR needs checking, especially before surgery. Beta blockers, digoxin and theophylline may need lower doses once the thyroid is back to normal, because an overactive thyroid speeds up their clearance. Extra care is needed with any other medicine that can lower white blood cells.

Where to get methimazole

Methimazole is prescription only. With a prescription, methimazole 5 mg and 10 mg tablets are listed by PockDose in Hong Kong, and thiamazole tablets by Yasabai in Thailand. If you are prescribed carbimazole instead, VGR Australia stocks Neo-Mercazole. Other countries are on where to buy.

Sources

Checked 15 October 2026.

1. Methimazole tablets prescribing information — DailyMed, US National Library of Medicine
2. Drugs@FDA: Tapazole (NDA 007517), marketing status — US Food and Drug Administration
3. 2018 European Thyroid Association guideline for the management of Graves' hyperthyroidism — European Thyroid Journal
4. Comparison of methimazole and propylthiouracil in Graves' disease (Nakamura et al., 2007) — Journal of Clinical Endocrinology & Metabolism
5. Methimazole-induced agranulocytosis is more frequent with 30 mg than 15 mg daily (Takata et al., 2009) — Thyroid
6. Comparative bioavailability of carbimazole and methimazole (Jansson et al., 1983) — Int J Clin Pharmacol Ther Toxicol