Like all medicines, carbimazole can cause side effects, although not everyone gets them. If you do have side effects, these usually happen in the first 8 weeks of taking carbimazole.
Side effects of carbimazole
Side effects usually improve as your body gets used to carbimazole. There are things you can do to help cope with them:
Feeling or being sick sick (nausea or vomiting)
Stick to simple meals and try not to eat rich or spicy food. It may help to take your carbimazole with food or just after eating to avoid feeling sick.
If you're being sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having strong-smelling pee.
Contact a doctor if feeling or being sick lasts for more than a few days.
Tell your doctor if you are vomiting due to a stomach bug or illness that lasts for more than 2 days.
If you take the combined pill or progestogen-only pill and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Feeling dizzy
Lie down until the dizziness passes, then get up slowly. Move slowly and carefully. Avoid coffee, cigarettes, alcohol and recreational drugs. Do not drive, cycle or operate machinery until you feel OK again.
If the dizziness does not get better, or this keeps happening, speak to a doctor.
Headaches
Make sure you rest and drink plenty of fluids. Do not drink too much alcohol. Ask your pharmacist to recommend a painkiller.
Headaches should usually go away. Talk to your doctor if they do not go away or are severe.
Painful joints
Cooling treatments can help with joint pain. Try putting an ice pack wrapped in a cloth on the joint for about 15 to 20 minutes, and repeat several times a day. Avoid keeping the joint still and take gentle exercise to stop it becoming stiff. Ask a pharmacist to recommend a painkiller.
Speak to your doctor if the pain does not go away or is severe.
Itchy or mild rash
Use unperfumed moisturiser or skin lotion, wear loose cotton clothing, keep your nails clean, short and smooth, and pat or tap the skin instead of scratching it. Ask your pharmacist to recommend an antihistamine.
Thinning hair
No one knows if taking carbimazole causes this or if it's the changes in your thyroid hormone levels. Hair usually grows back, but this may take a little while because the natural cycle of hair growth takes several months.
Talk to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.
Serious side effects
Serious side effects are uncommon.
Stop taking carbimazole and call your doctor or 111 now if:
- you have a high temperature, sore throat, mouth ulcers, severe tiredness or flu-like symptoms – these can be signs of a low white blood cell count
- you feel generally unwell or think you may have an infection
- you notice you are bruising or bleeding more easily
Call your doctor or 111 straight away if:
- the whites of your eyes turn yellow, or your skin turns yellow (this may be less obvious on brown or black skin), or you have dark pee – this can be a sign of liver problems
- you have swollen glands
- you feel faint, and sweaty – these can be signs of low blood sugar
- you have muscle pain or weakness – these can be signs of muscle problems
- you have nerve pain – this could be burning, stabbing or shooting pain
- you have kidney problems, such as passing less pee (urine), water retention, or blood in your pee
- you have sudden severe stomach (abdominal) pain – this can be a sign of an inflamed pancreas (acute pancreatitis)
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to carbimazole.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Do not drive yourself to A&E.
The person you speak to at 999 will give you advice about what to do.
Other side effects
These are not all the side effects of carbimazole. For a full list see the leaflet inside your medicine packet.
Reporting side effects
You can report any suspected side effect using the Yellow Card safety scheme.
Visit the Yellow Card website for more information about reporting side effects.