BMA Concise Guide to Medicine & Drugs


Brand name Neo-Mercazole

Used in the following combined preparations None


Drug group Antithyroid drug

Overdose danger rating Medium

Dependence rating Low

Prescription needed Yes

Available as generic Yes


Carbimazole is an antithyroid drug that suppresses the formation of thyroid hormones and is used to manage an overactive thyroid gland (hyperthyroidism). In Graves’ disease (the most common cause of hyperthyroidism), a course of carbimazole alone or combined with thyroxine (so-called “block and replace” therapy) – usually given for 6–18 months – may cure the disorder. In other conditions, carbimazole is given until other treatments, such as surgery or radioiodine, take effect. If other treatments are not possible or are declined by the patient, carbimazole can be given long-term. The full effect of the drug may take several weeks, and beta blockers may be given during this period to control symptoms.

The most important adverse effect is a reduction in white blood cells (agranulocytosis), increasing the risk of infection. Although this is rare, if you develop a sore throat, mouth ulcers, or a fever, you should see your doctor immediately to have your white blood cell count checked.


Your drug prescription is tailored for you. Do not alter dosage without checking with your doctor.

How taken/used Tablets.

Frequency and timing of doses 1–3 x daily.

Adult dosage range 15–40mg daily (occasionally a larger dose may be needed). Once control is achieved, dosage is reduced gradually to a maintenance dose of 5–15mg for about 18 months.

Onset of effect Some improvement is usually felt within 1–3 weeks. Full beneficial effects usually take 4–8 weeks.

Duration of action 12–24 hours.

Diet advice Your doctor may advise you to avoid foods that are high in iodine, such as cod and mackerel.

Storage Keep in original container at room temperature out of the reach of children.

Missed dose Take as soon as you remember. If your next dose is due, take both doses together.

Stopping the drug Do not stop the drug without consulting your doctor; symptoms may recur.

Exceeding the dose An occasional unintentional extra dose is unlikely to cause problems. Large overdoses may cause nausea, vomiting, and headache. Notify your doctor.


The most serious side effect is a rare, life-threatening reduction in white blood cells (agranulocytosis). This may be indicated by sore throat, fever, or mouth ulcers and should be reported to your doctor immediately. If jaundice occurs, this should also be reported to your doctor without delay. Other possible side effects include headache, dizziness, joint pain, nausea, and loss of the sense of taste, which should be notified to your doctor if they are severe. You should also inform your doctor if you experience rash, itching, or hair loss.


Theophylline Blood levels of this drug may increase when taken with carbimazole.


Be sure to tell your doctor if:

· You have a long-term liver problem.

· You are pregnant.

· You are taking other medicines.

Pregnancy May be associated with defects in the baby. However, the risk to the baby of untreated hyperthyroidism is higher. Discuss with your doctor.

Breast-feeding The drug passes into the breast milk, but mothers may breast-feed as long as the lowest effective dose is used and the baby is carefully monitored. Discuss with your doctor.

Infants and children Reduced dose necessary.

Over 60 No special problems.

Driving and hazardous work Avoid such activities until you have learned how carbimazole affects you because the drug may cause dizziness.

Alcohol No known problems.


Carbimazole may rarely cause a reduction in the number of white blood cells.

Monitoring Periodic tests of thyroid function are usually required. If you have a sore throat, fever, or mouth ulcers, your white blood cell count must be checked.