Carbimazole tablets - Neo-Mercazole

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Keep your regular appointments with your doctor so that your condition can be monitored and any dose adjustments can be made. You will need to have regular blood tests.

Common side-effects tend to be minor, but on occasions, carbimazole can cause serious blood disorders. If you get an infection, a sore throat, mouth ulcers, a high temperature, any unusual bruising, or if you just feel generally unwell, contact your doctor for advice straightaway.

Type of medicine Antithyroid medicine
Used for Hyperthyroidism
Also called Neo-Mercazole®
Available as Tablets

Antithyroid medicines like carbimazole are used to treat hyperthyroidism (an overactive thyroid), which is also known as thyrotoxicosis. When your thyroid gland produces too much thyroid hormone, it can cause many of your body's functions to speed up, and this leads to symptoms such as losing weight, sweating, feeling irritable or shaky, and diarrhoea. You also may feel tired or worried, and develop menstrual problems. Carbimazole is the most commonly used medicine for hyperthyroidism. It works by reducing the amount of thyroid hormones which your thyroid gland makes.

Carbimazole may be used alone or together with other treatments for hyperthyroidism. It is also a treatment used to prepare a person for thyroidectomy (surgical removal of part or all of the thyroid gland), and before radio-iodine treatment.

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking carbimazole it is important that your doctor or pharmacist knows:

  • If you are pregnant, trying for a baby, or breast-feeding.
  • If you have been told you have a blood or bone-marrow disorder.
  • If you have any problems with the way your liver works.
  • If you are taking any other medicines. This includes any medicines you are taking which are available to buy without a prescription, such as herbal and complementary medicines.
  • If you have ever had an allergic reaction to a medicine.
  • Before you start this treatment, read the manufacturer's printed information leaflet from inside your pack. The manufacturer's leaflet will give you more information about carbimazole and a full list of the side-effects which you may experience from taking it.
  • Take this medicine exactly as your doctor tells you to. It is likely you will be asked to take a higher dose to start with, and then your dose may be reduced as your thyroid levels become normal. Your doctor will tell you how many tablets to take each day and when to take them, and this will also be printed on the label of the pack to remind you.
  • It may help to swallow the tablets with a drink of water. You can take carbimazole before or after meals.
  • Try to take your doses at the same times each day, as this will help you to remember to take them regularly.
  • If you forget to take a dose, take it as soon as you remember. If when you remember, it is time for you to take your next dose then take both doses together.
  • Try to keep your regular appointments with your doctor. This is so your doctor can check on your progress and any dose adjustments can be made. You will need to have regular blood tests to check how you are responding to the treatment.
  • Getting the right balance of thyroid hormone in your blood can take time. Carbimazole reduces the production of thyroid hormones, but it does not affect the hormones which have already been made and stored by your body. Your symptoms will start to improve after a week or so, but it can take 4 to 8 weeks of treatment for your thyroid hormone levels to come down to normal and for you to feel the full benefit of the treatment.
  • There are two methods of taking carbimazole. You will be started on a dose of 15-40 mg daily, taken divided into 2-3 doses a day. This may then be reduced after 4-8 weeks to a lower maintenance dose of 5-15 mg, taken once daily. This is called the 'titration' method. Alternatively, after 4-8 weeks, you may be prescribed a medicine called thyroxine to take in addition to the carbimazole tablets which you are already taking. This is called the 'block and replace' method. This method often gives a better control of thyroid hormone levels. Ask your doctor if you are not sure whether you are using the 'titration' method, or the 'block and replace' method.
  • In order to control your thyroid gland, you may need to take carbimazole tablets for several months if you are using the 'block and replace' method, or for a year or so if you are using the 'titration' method. Continue to take the tablets until you are advised otherwise by your doctor.
  • Before having any kind of medical treatment, you should tell the person carrying out the treatment that you are taking carbimazole.

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with carbimazole. You will find a full list in the manufacturer's information leaflet supplied with your medicine. The unwanted effects often improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following continue or become troublesome.

Common carbimazole side-effects What can I do if I experience this?
Feeling sick, stomach upset Stick to simple foods - avoid rich or spicy meals
Headache, muscle and joint pain Ask your pharmacist to recommend a suitable painkiller
Skin rash, itching Let your doctor know about this, as treatment with an antihistamine medicine may help
Changes to the way things taste, hair loss These should pass after the first few weeks

Important: carbimazole can cause a rare but serious side-effect which reduces the numbers of blood cells which fight infection and help to stop bleeding. If you develop any of the following, let your doctor know straightaway:

  • A sore throat.
  • Mouth ulcers.
  • A high temperature.
  • Any unusual bruising or bleeding.
  • If you feel unusually tired, or if you feel you are getting an infection.

If you experience any other symptoms, which you think may be due to this medicine, discuss them with your doctor or pharmacist.

  • Keep all medicines out of the reach and sight of children.
  • Store in a cool, dry place, away from direct heat and light.

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. Take the container with you, even if it is empty.

If you buy any medicines, check with a pharmacist that they are safe to take with your other medicines.

This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.

If you have any questions about this medicine, ask your pharmacist.

Further reading & references

  • Manufacturer's PIL, Carbimazole 5 mg and 20 mg tablets, Amdipharm Mercury Company Limited, The electronic Medicines Compendium. Dated February 2013.
  • British National Formulary; 66th Edition (September 2013) British Medical Association and Royal Pharmaceutical Society of Great Britain, London

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Helen Allen
Current Version:
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
3575 (v25)
Last Checked:
19/11/2013
Next Review:
18/11/2016
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