Carbimazole

What is carbimazole?

Carbimazole is a prescription-only medicine used to manage an overactive thyroid gland, a condition referred to as hyperthyroidism. This condition occurs when the thyroid secretes excessive thyroid hormone, which may result in symptoms such as weight loss, rapid heartbeat, sweating, and anxiety[1]. 

What is carbimazole used for?

Carbimazole is mainly used to treat hyperthyroidism (overactive thyroid). It may also be used for Graves’ disease, an autoimmune condition that causes the thyroid to become overactive, or before thyroid surgery or radioactive iodine treatment to help stabilise thyroid hormone levels[1]. 

How does carbimazole work?

Carbimazole operates by reducing the production of thyroid hormones in the thyroid gland. Once ingested, the body converts it into methimazole, a substance that inhibits an enzyme known as thyroid peroxidase, which is necessary for thyroid hormone synthesis[1]. 

Who can take carbimazole?

Carbimazole can be taken by most adults and children. However, it may not be suitable for people who have had an allergic reaction to carbimazole or other similar hyperthyroid medication, can’t have regular blood tests, or are pregnant, trying to get pregnant, or are breastfeeding. You should speak to your doctor or pharmacist first if you have[1]: 

• Serious liver problems 

• Severe pancreas problems (pancreatitis) after taking carbimazole previously 

• Received or are receiving radioactive iodine treatment for hyperthyroidism 

• Blood disorders or bone marrow suppression 

• A swollen thyroid 

Please tell your doctor or pharmacist about any conditions you may have or medication you take before you start treatment, and always follow the instructions in the Patient Information Leaflet[2]. 

How do you take carbimazole?

You can take carbimazole tablets whole with a glass of water, with or without food, at the same time each day. Tablets that have a score line may be split if you have difficulty swallowing. You should follow your doctor or pharmacist’s instructions carefully, with regular blood tests to check your thyroid hormone levels and any other tests your doctor may request to safely monitor your treatment[1]. 

What is the typical dosage of carbimazole?

Carbimazole comes as 5mg, 10mg, 15mg, and 20mg tablets. You may need to take more than 1 tablet for your dose. It’s important to know that you will need to have blood tests before you start taking carbimazole, and regularly during treatment, to check the thyroid hormone levels in your body as well as other tests such as full blood counts and liver function tests your doctor may request to monitor your treatment. Your doctor may increase or reduce your dose depending on your results to get them to the right levels, and once stabilised, dose may be reduced gradually as recommended by your doctor[1]. 

For adults, the usual starting dose is between 20mg and 40mg per day (higher doses may be prescribed under specialist supervision), divided into two or three doses spread evenly. Again, once your thyroid hormone levels become stable, your doctor may reduce the dose to a maintenance level – usually between 5 and 15mg per day. Children’s doses are based on age and weight, and will be decided by a specialist. This type of treatment typically lasts between 12 and 18 months, although some people may need to take it for longer[1]. 

Can you stop taking carbimazole suddenly?

You should not stop taking carbimazole without medical advice. This is because stopping suddenly can cause your thyroid hormone levels to rise again, which may result in symptoms returning or worsening[1]. 

What should you do if you take too much carbimazole?

Taking one extra dose of carbimazole by accident is unlikely to cause side-effects. However, if you take much more than you should, or are concerned about side effects, you should contact NHS 111 or go to your nearest accident and emergency department. Take the packaging with you when possible[1].  

What should you do if you forget a dose of carbimazole?

You can generally take a forgotten dose of carbimazole as soon as you remember. If it is almost time for your next dose, you can take both doses together. But if you frequently forget doses, consider setting a daily reminder or speaking to your pharmacist for advice[1]. 

What medicines should you avoid while taking carbimazole?

Carbimazole may interact with other medicines, including[1]: 

• Anticoagulants like warfarin 

• Heart medicine like digoxin 

• Antibiotics like erythromycin 

• Bladder medicines for urinary incontinence and other conditions 

• Beta blockers, which are often prescribed with carbimazole, but your doctor will tell you how much to take 

• Antipsychotics like clozapine 

• Theophylline, a medicine used to treat asthma and other breathing problems 

• Steroids like prednisolone 

• Any other thyroid medications 

• Any medication that could affect your white blood cell count  

• Some herbal medicines like black cohosh and biotin 

Please note this is not a complete list, always inform your doctor or pharmacist about all medications and supplements you take before you start a new medicine, and follow the instructions in the Patient Information Leaflet[2]. 

What are the side effects of carbimazole?

Carbimazole can have some common but mild side effects. You may notice nausea, vomiting, dizziness, thinning hair, headaches, joint pain, and skin rashes, which should all pass quite quickly[1]. 

There are potential serious side effects – these are rare but require urgent medical attention. They include[1]: 

• A sore throat, fever or mouth ulcers, which could be signs of low white blood cells 

• Unusual or increased bruising or bleeding 

• Fever 

• Significant fatigue 

• A feeling of being unwell or signs of an infection 

• Blistering of the skin, mouth, eyes and genitals  

• Swollen tongue, lips, face or throat 

• Swollen glands or lymph nodes 

• Fainting or excessive sweating 

• Nerve pain 

• Muscle pain or weakness 

• Kidney (and urinary) problems 

• Sudden severe stomach pain 

• Signs of liver problems, such as yellowing of the skin or eyes 

You should contact your GP if you experience any of these symptoms. In rare cases, the medicine may cause a serious allergic reaction (anaphylaxis). You should call 999 or visit A&E immediately if you believe this is happening[1]. 

Symptoms of anaphylaxis include[1]:  

• Difficulty breathing  

• Tightness in throat and chest  

• Swelling of the tongue or throat  

• Feeling faint or dizzy  

• Blue/grey skin or lips  

Loss of consciousness 

If you experience any side effects, including those not listed, you are encouraged to report them via the Yellow Card Scheme or contact your doctor, pharmacist, or nurse. This information is intended as a summary for general awareness and does not replace professional medical advice. 

The information provided in the side effects section is not a comprehensive list of all possible side effects associated with this medication. For a complete list of side effects, please refer to the Patient Information Leaflet (PIL) provided with your medication or consult your healthcare professional[2]. 

Frequently asked questions about carbimazole

How long does carbimazole take to work? 

Your carbimazole treatment usually starts working within a few weeks, but the full effect may take four to eight weeks[1]. 

Can I drink alcohol while taking carbimazole? 

Yes, you can drink alcohol in moderation while taking carbimazole, although it can potentially make side effects worse[1]. 

Can I take carbimazole during pregnancy? 

Carbimazole has been associated with a risk of birth defects when used during pregnancy, especially during the first trimester. You should only take carbimazole during pregnancy after thoroughly considering the risk and benefits, at the lowest effective dose, under specialist supervision and with regular monitoring[1]. 

Can I take carbimazole while breastfeeding? 

It is usually not recommended to take carbimazole while you’re also breastfeeding. Speak to your doctor or pharmacist for more advice[1]. 

Can I stop taking carbimazole once I feel better? 

No, for your safety, you should only stop taking carbimazole if your doctor or pharmacist tells you to, following their expert guidance[1]. 

Will I need blood tests while taking carbimazole? 

Regular blood tests are essential to monitor your thyroid hormone levels while you’re taking carbimazole[1]. Your doctor may also request full blood counts and liver function tests periodically to monitor treatment.  

What if I get a sore throat or fever while taking carbimazole? 

If you’re taking carbimazole and notice you’ve got a sore throat and/or a fever, stop taking the medicine and contact your doctor immediately. These may be signs of a potentially serious side effect[1]. 

Is carbimazole safe for long-term use? 

Yes, carbimazole can be taken long-term, but extended use requires regular monitoring for possible side effects[1]. 

For the official NHS guide to carbimazole, visit their website.