Neo-Mercazole (carbimazole): what it is, what it treats and how it works

Neo-Mercazole is a brand name for carbimazole, a prescription tablet that lowers thyroid hormone production in people with an overactive thyroid (hyperthyroidism). This guide is independent: it explains what the official product information and the research say, and it is not written by the company that makes or licenses Neo-Mercazole.

Key points

• Generic name: carbimazole. In the body it turns into methimazole (also called thiamazole), which does the work.
• Used for: an overactive thyroid, most often Graves’ disease or toxic nodular goitre; also to prepare for thyroid surgery and around radioactive iodine treatment.
• Not for an underactive thyroid. Neo-Mercazole does not contain thyroid hormone (T3 or T4); it reduces how much your thyroid makes.
• Most important warning: a sore throat, mouth ulcers, fever or other signs of infection can mean a rare drop in white blood cells. Stop the tablets and get a blood count the same day.
• Pregnancy: carbimazole can harm an unborn baby, especially in the first trimester. Use effective contraception and tell your doctor at once if you could be pregnant.

What Neo-Mercazole is

Neo-Mercazole tablets contain 5 mg of carbimazole. The New Zealand data sheet describes a pale pink tablet with a white core, marked “Neo 5”, sold in bottles of 100. The tablets contain lactose and sucrose, which matters if you have one of the rare hereditary sugar intolerances. Different companies hold the brand in different countries: in New Zealand the sponsor is AFT Pharmaceuticals, and in India Abbott sells Neo Mercazole 5 and 10 mg tablets. Many countries also sell plain “carbimazole” tablets from other makers.

In the UK, carbimazole comes as 5 mg, 10 mg, 15 mg and 20 mg tablets, mostly under the generic name. Whatever the box says, the active ingredient and the safety advice are the same.

Neo-Mercazole, carbimazole and methimazole: same thing?

Neo-Mercazole and carbimazole are the same medicine: one is the brand, the other is the ingredient. Searches such as “neomercazole generic name”, “neomercazole other name” or “is neomercazole the same as carbimazole” all have this answer.

Methimazole is closely related but not identical. Carbimazole is a prodrug: after you swallow it, it is rapidly and almost completely converted into methimazole (thiamazole), the active antithyroid compound. Methimazole is sold as a medicine in its own right, mainly as generic methimazole tablets in the US (the original US brand, Tapazole, has been discontinued); in the UK only carbimazole is licensed. A small 1983 study found that 15 mg of carbimazole gave the same methimazole blood levels as about 9.2 mg of methimazole, so the two are equally strong molecule for molecule, but the milligram doses differ (roughly 10 mg carbimazole to 6 mg methimazole). Your doctor should do any switch; see the dose conversion table.

What it is used for

The product information lists two kinds of hyperthyroidism and three ways the medicine is used:

• Graves’ disease (primary thyrotoxicosis), including during pregnancy when a specialist decides it is needed.
• Toxic nodular goitre (secondary thyrotoxicosis).
• As a course of treatment aimed at lasting remission, usually for 12 to 18 months.
• To bring hormone levels to normal before thyroid surgery.
• Before and after radioactive iodine treatment, with a pause around the iodine dose itself.

It should be started only after blood tests have confirmed hyperthyroidism. It is not used for hyperthyroidism caused by taking too much iodine.

How it works

The thyroid gland builds its hormones, T4 and T3, by attaching iodine to a protein. Methimazole blocks thyroid peroxidase, the enzyme that does this, so the gland makes less new hormone. It does not destroy hormone that is already stored or circulating. That is why the tablets take time to work.

How long until it works

Your thyroid keeps a stock of finished hormone, and the tablets do nothing to that stock. So the wait depends on how full it is. In a severe case with little in reserve, the data sheet says you can notice a change within three or four days. Emptying a normal store takes about a fortnight. A nodular goitre can hold so much that it takes weeks or months.

The NHS gives a rule of thumb: many people feel better within one to three weeks, and the medicine reaches full effect in one to two months. Your first check-up blood test usually comes at four to six weeks. Still no better at three months? The product information says the usual reason is tablets being missed.

Dose in brief

Typical adult starting doses are 20 mg a day for mild, 40 mg for moderate and 40 to 60 mg for severe cases, split into two or three doses. Once thyroid levels are normal the dose is lowered, usually to 10 to 15 mg a day, often taken once daily. You can take it with or without food. The details, including the “block and replace” approach and missed doses, are in Neo-Mercazole dosage.

Side effects in brief

Most side effects appear in the first eight weeks and are mild: nausea, headache, joint pain, mild stomach upset, rash and itching. The rare serious ones are a fall in white blood cells (agranulocytosis), liver problems and acute pancreatitis. See side effects and warning signs.

Who should not take it

The product information rules it out for people who have had a reaction to carbimazole, have a serious existing blood disorder, severe liver failure, a large goitre pressing on the windpipe, or had acute pancreatitis after carbimazole or methimazole before. It is also not for simple (non-overactive) goitre.

Pregnancy and breastfeeding

Here two risks pull against each other. An overactive thyroid left untreated can harm a pregnancy, yet the UK regulator (MHRA) links carbimazole with birth defects, mostly from use in the first trimester and at 15 mg a day or more. The practical answer is planning: reliable contraception while you take it, and a talk with your doctor before trying for a baby, because most specialists switch to propylthiouracil (PTU) for the early weeks.

Breastfeeding is less of a worry. Only small amounts reach the milk, and the NHS now says it is usually fine, although the older product information is more cautious. The full picture is on pregnancy and breastfeeding.

Alternatives

Neo-Mercazole is one of three antithyroid tablets, alongside methimazole and propylthiouracil. The other main treatments are radioactive iodine and surgery. Each has trade-offs in relapse rates, side effects and eye disease. See Neo-Mercazole compared and what the studies show.

Once you have a prescription, Neo-Mercazole and generic carbimazole can be ordered online: carbimazole from PockDose in Hong Kong, carbimazole from Yasabai in Thailand and Neo-Mercazole from VGR Australia in Australia. More pharmacies by country are on where to buy.

Read next

• Dosage and how to take it

  Starting and maintenance doses, block and replace, missed doses, stopping.

• Side effects and warning signs

  Common effects, agranulocytosis, liver, pancreatitis, pregnancy.

• Compared with methimazole, PTU, iodine and surgery

  Doses, safety and relapse rates side by side.

• What the studies show

  Trials and large cohort studies, in plain English.

• Pregnancy and breastfeeding

  Birth defect risk, switching to PTU, planning ahead.

• Blood tests

  Thyroid checks, blood counts, TRAb before stopping.

• Brand names worldwide

  Neo-Mercazole by country, Reduzol, Tapazole, Mercazole.

• Where to buy

  Pharmacies that stock carbimazole and methimazole.

Questions people ask

Is Neo-Mercazole for an overactive or an underactive thyroid?

Overactive. It lowers thyroid hormone. For an underactive thyroid, people take levothyroxine (thyroxine) instead. Some people take both in “block and replace” treatment, where carbimazole stops the gland and levothyroxine replaces the hormone.

Is Neo-Mercazole a T3 medicine?

No. It contains no hormone. It reduces production of both T4 and T3.

Should I take it before or after food?

Either. The NHS says you can take carbimazole with or without food. Swallow the tablets with water.

Can I drink alcohol while taking it?

The NHS says yes: alcohol does not change how carbimazole works. If the tablets make you dizzy, avoid alcohol because it can make dizziness worse.

Do I need a special diet?

Food does not affect carbimazole. Some doctors suggest limiting iodine-rich foods and iodised salt, because the thyroid uses iodine to make hormone.

Can I stop once I feel better?

No. Your thyroid levels will rise again, sometimes slowly. Doses are lowered and treatment stopped based on blood tests, usually after 12 to 18 months.

Do I need to stop it before radioactive iodine?

The product information says to stop Neo-Mercazole temporarily around the radioactive iodine dose. Your treatment centre will tell you how many days before and after.

Sources

Label and regulator sources checked 15 October 2026.

1. Neo-Mercazole New Zealand data sheet (revised 10 December 2020) — Medsafe (New Zealand)
2. Neo-Mercazole consumer medicine information — Medsafe (New Zealand)
3. Carbimazole — NHS (UK)
4. Carbimazole: increased risk of congenital malformations; strengthened advice on contraception — MHRA Drug Safety Update, February 2019
5. Comparative bioavailability of carbimazole and methimazole (Jansson et al., 1983) — Int J Clin Pharmacol Ther Toxicol
6. Overactive thyroid (hyperthyroidism) — NHS (UK)