Carbimazole is used to control an overactive thyroid gland (hyperthyroidism). Find out how to take it safely and possible side effects. Carbimazole is also called Neo-Mercazole.

Type of medicine Also called
  • Medicine for thyroid disorders
  • Neo-Mercazole®

What is carbimazole?

Carbimazole is used to control an overactive thyroid gland (hyperthyroidism). It stops your thyroid gland from making too much thyroid hormone. In New Zealand, carbimazole is available as tablets (5 mg). 


  • The dose of carbimazole will be different for different people. It will depend on the severity of your condition and could be adjusted during the course of treatment.
  • Treatment with carbimazole is usually started at high doses (15 to 40 mg per day) for a few weeks until your thyroid returns to normal activity. The dose is then slowly reduced (to 5 to 15 mg per day)  so that your thyroid gland can maintain its normal activity.
  • Each day's tablets may be divided evenly into two doses (morning and evening), or three doses (morning, noon and night).
  • Your doctor will advise you how long to take carbimazole for (usually for 12 to 18 months).
  • Always take your carbimazole exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much to take, how often to take it and any special instructions.

My dose

Make sure you know exactly how many tablets to take each day. If you are unsure, ask your pharmacist.

Date Morning Noon Evening

How to take carbimazole

  • Timing: Take each day's tablets divided evenly into two doses (morning and evening), or three doses (morning, noon and night). Try taking your carbimazole tablets at the same times each day. You can take carbimazole with or without food. 
  • Missed dose: If you forget to take your dose, take it as soon as you remember. But, if it is nearly time for your next dose, just take the next dose at the right time. Do not take double the dose.
  • Keep taking carbimazole regularly. It may take 4 to 8 weeks before you notice an improvement.  

Precautions before starting carbimazole

  • Are you pregnant, planning a pregnancy or breastfeeding?
  • Do you have problems with your liver?
  • Do you have a blood or bone-marrow disorder?
  • Are taking any other medicines? This includes any medicines you are taking which you can buy without a prescription, as well as herbal and complementary medicines.

If so, it’s important that you tell your doctor or pharmacist before you start carbimazole. Sometimes a medicine isn’t suitable for a person with certain conditions, or it can only be used with extra care.

What are the side effects of carbimazole?

Like all medicines, carbimazole can cause side effects, although not everyone gets them. Often side effects improve as your body gets used to the new medicine.  

Sore throat or flu symptoms

Carbimazole can cause a drop in your white blood cells, which fight infection – a condition is called neutropenia. This can put you at increased risk of getting infections. At the first sign of infection (such as sore throat, fever, mouth ulcers, poor appetite, bruising, cold or flu symptoms, muscle aches or headache) you need to contact your doctor for an urgent blood test to check your white blood cells. Read more about neutropenia.

Other side effects

Side effects What should I do?
  • Stomach upset
  • Nausea (feeling sick) 
  • These are quite common when you first start carbimazole.
  • Try taking carbimazole with food.
  • Let your doctor know if troublesome. 
  • Headache
  • Pain in your joints, bones and muscles
  • Itchy skin rash
  • These are quite common when you first start carbimazole and settle after a few weeks.
  • Let your doctor know if troublesome or if you are worried. 
  • Signs of problems with your liver, such as yellowing of your skin or eyes, dark urine, pain in your abdomen (tummy)
  • Tell your doctor immediately or ring HealthLine 0800 611 116.
Did you know that you can report a side effect to a medicine to CARM (Centre for Adverse Reactions Monitoring)? Report a side effect to a product


Carbimazole can interact with other medications and herbal supplements, so check with your doctor or pharmacist before starting carbimazole or before starting any new medicines.

Learn more

Neo-Mercazole Medsafe Consumer Information Sheet 
Carbimazole New Zealand Formulary


  1. Carbimazole New Zealand Formulary
  2. Management of thyroid dysfunction in adults BPAC, NZ, 2010

Additional resources for healthcare professionals

Neo-Mercazole Medsafe, NZ

Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 30 Jan 2019