Carbimazole

Key facts

• Carbimazole (brand Neo-Mercazole) is an antithyroid drug that treats hyperthyroidism, including Graves' disease, by slowing how much hormone your thyroid makes.
• It is taken daily; symptoms improve over 1 to 3 weeks as existing hormone stores are used up, though full control can take longer.
• Its most serious risk is agranulocytosis, a sudden, severe drop in infection-fighting white blood cells. If you develop a fever, sore throat, or mouth ulcers, stop carbimazole and get urgent medical care and a blood test the same day.
• Seek care too for yellowing of the skin or eyes, or a severe rash.

What carbimazole treats

Carbimazole treats hyperthyroidism, an overactive thyroid that causes weight loss, anxiety, heat intolerance, tremor and a fast heartbeat. It is a mainstay treatment for Graves' disease, and it is also used to stabilise thyroid hormone levels before surgery or radioactive iodine treatment, and if hyperthyroidism returns after earlier therapy.

How carbimazole works

Carbimazole is converted in the body to methimazole, which blocks an enzyme called thyroid peroxidase. This enzyme is needed to build thyroid hormone from iodine, so blocking it slows new hormone production while your body uses up the hormone already stored in the gland.

Before you take it

• Tell your prescriber if you have ever had agranulocytosis or a serious blood reaction to carbimazole or methimazole; this is a reason to avoid the drug.
• Pregnancy needs specialist input: carbimazole crosses the placenta and, at higher doses, can affect the baby's thyroid, so your team may adjust the dose or medicine.
• Liver disease and a history of vasculitis need extra monitoring.
• Iodine-rich supplements and some heart medicines can interact; tell your pharmacist about everything you take.

Side effects

Common effects include nausea, itching or rash, a metallic taste, mild joint aches, and a low-grade fever.

Stop and seek urgent medical care for any of these:

• Fever, sore throat, or mouth ulcers (possible agranulocytosis).
• Yellowing of the skin or eyes, or dark urine (possible liver injury).
• Severe rash or blistering skin.
• Unusual bruising or bleeding.

Safety essentials

• The agranulocytosis rule is absolute: fever, sore throat or mouth ulcers mean you stop the medicine and get a same-day blood count, even if it is the middle of the night.
• Regular blood tests check your thyroid hormone levels and liver function throughout treatment; do not skip them.
• Do not stop carbimazole abruptly without medical advice, since hyperthyroidism can rebound once the drug is withdrawn.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.