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Neo-Mercazole side effects: common effects and warning signs

Side effects of Neo-Mercazole (carbimazole), from nausea, rash and hair thinning to rare agranulocytosis, liver injury and pancreatitis, plus pregnancy risks and interactions.

Illustration: a thermometer, a strip of throat lozenges and a bottle of pale pink tablets.

Most people who take Neo-Mercazole (carbimazole) have no side effects or only mild ones that settle. A few rare reactions are serious, and knowing the warning signs is the most useful safety step you can take.

Key points

  • Common and usually mild: nausea, headache, joint pain, mild stomach upset, rash, itching, dizziness. They usually start in the first eight weeks.
  • Rare but serious: agranulocytosis (very low white blood cells), liver injury, acute pancreatitis, severe skin reactions.
  • Red flags: sore throat, mouth ulcers, fever or flu-like illness; yellow skin or eyes; sudden severe stomach pain.
  • Pregnancy: linked with birth defects, mainly in the first trimester and at 15 mg a day or more.

Common side effects

The product information lists nausea, headache, joint pain (often in the thumb joint), mild stomach upset, skin rash, hives and itching as the most frequent. Dizziness and tingling (paraesthesia) are also listed as common. These reactions usually go away on their own and often do not mean stopping the medicine. The NHS suggests:

  • Nausea: simple meals; taking the tablets with or just after food.
  • Rash or itching: unperfumed moisturiser, loose cotton clothing; a pharmacist can suggest an antihistamine. A severe or spreading rash needs a doctor.
  • Joint pain: a wrapped ice pack and gentle movement; ask a pharmacist about painkillers.
  • Headache and dizziness: rest and fluids; do not drive until dizziness passes.

Hair loss and weight gain

Some people notice thinner hair. Nobody knows for sure whether carbimazole does this or the swing in thyroid levels does, the NHS says, and the hair usually grows back, slowly, because hair grows in cycles lasting months.

Weight worries come up even more. Too much thyroid hormone burns weight off, so most people lost some before diagnosis. Carbimazole stops that loss, and some of the weight comes back; that is the thyroid settling, not a side effect. Once your levels are normal the tablets do not change your weight. Putting on weight steadily while feeling tired and cold is different: the dose may now be too high, leaving the thyroid underactive. A blood test settles it.

Agranulocytosis: the warning everyone should know

Agranulocytosis is a sudden, severe fall in neutrophils, the white blood cells that fight bacterial infection. It is rare. A Japanese analysis of 754 cases estimated it at 0.1% to 0.15% of people treated, with 84.5% of cases starting within 90 days of beginning treatment. In another Japanese hospital study it was more common at a starting methimazole dose of 30 mg a day (0.81%) than 15 mg a day (0.22%).

It usually comes on abruptly, so a normal blood count one week does not rule it out the next. That is why the advice centres on symptoms: if you develop a sore throat, fever, mouth ulcers or other signs of infection, stop the tablets and get a blood count the same day. Caught early and with the medicine stopped, the blood count usually recovers. The risk of a fatal outcome may be higher in people aged 65 and over.

The product information also lists rarer blood effects: aplastic anaemia, low platelets, haemolytic anaemia and low prothrombin (which can increase bleeding).

Liver problems

Carbimazole can cause abnormal liver tests, hepatitis and cholestatic jaundice; liver necrosis is rare. Jaundice may last for several weeks after stopping. Warning signs include upper stomach pain, loss of appetite, generalised itching, yellow skin or eyes and dark urine. The medicine is stopped and liver tests are done straight away. Severe liver injury is a much bigger concern with propylthiouracil (PTU) than with carbimazole or methimazole; see the comparison.

Acute pancreatitis

There have been post-marketing reports of acute pancreatitis with carbimazole and methimazole. If it happens, carbimazole is stopped immediately and must never be restarted, because re-exposure can bring it back sooner. Report fever or stomach pain during treatment promptly.

Other rare effects

  • Severe skin reactions, including Stevens-Johnson syndrome (very rare).
  • Angioedema and multi-system hypersensitivity, such as vasculitis affecting the skin, liver, lungs or kidneys.
  • Low blood sugar from insulin autoimmune syndrome, which can be severe.
  • Muscle pain or weakness (myopathy): creatine kinase should be checked.
  • Nerve problems (neuritis, polyneuropathy) and loss of taste.
  • Swollen lymph glands and salivary gland inflammation.

Pregnancy and breastfeeding

The UK medicines regulator (MHRA) concluded in 2019 that carbimazole increases the risk of congenital malformations, particularly in the first trimester and at 15 mg a day or more. Reported defects include a scalp defect (aplasia cutis), facial and nasal passage malformations, abdominal wall and gut defects, and a heart wall defect. Untreated hyperthyroidism also harms pregnancies, so stopping the tablets on your own is not the answer. Planning is.

If you could get pregnant, use contraception that reliably works for you for as long as you take carbimazole. Tell your doctor when you start thinking about a pregnancy, or as soon as you suspect you are pregnant. That gives time to change treatment: most specialists move people to propylthiouracil for the first trimester, and some go back to carbimazole or methimazole later in pregnancy at the lowest dose that keeps the thyroid under control.

Carbimazole gets into breast milk in small amounts. The NHS says breastfeeding is usually fine; the older product information gives three routes: stop breastfeeding, change to PTU (far less of it reaches the milk), or carry on with the baby’s thyroid checked. More on pregnancy and breastfeeding.

Interactions

Tell your doctor or pharmacist about all your medicines; the full list, including iodine, biotin and herbal remedies, is on interactions. The product information highlights:

  • Warfarin and other anticoagulants: their effect can change as thyroid levels normalise; carbimazole can itself lower clotting factors. More INR checks are advised.
  • Theophylline, digoxin and beta blockers: blood levels or effects can rise as you become euthyroid, so their doses may need lowering.
  • Prednisolone: may be cleared faster.
  • Erythromycin: may be cleared more slowly.
  • Other medicines that can lower white blood cells: need extra care.
  • Iodine: large amounts of iodine (including iodine-containing contrast or supplements) blunt carbimazole’s effect.

Do not run out

Stopping because a pack ran out lets thyroid levels climb back, and restarting means another round of dose finding. Order repeat carbimazole in good time: your local pharmacy, or online from Yasabai or VGR Australia if they deliver to you.

Reporting side effects

You can report suspected side effects to your national regulator: the Yellow Card scheme in the UK, the TGA in Australia, Medsafe’s CARM in New Zealand, or MedWatch in the US.

Sources

Checked 15 October 2026.

  1. Neo-Mercazole New Zealand data sheet, sections 4.4 to 4.8 — Medsafe (New Zealand)
  2. Side effects of carbimazole — NHS (UK)
  3. Carbimazole: increased risk of congenital malformations; strengthened advice on contraception — MHRA Drug Safety Update, February 2019
  4. Analysis of 754 cases of antithyroid drug-induced agranulocytosis over 30 years in Japan (Nakamura et al., 2013) — Journal of Clinical Endocrinology & Metabolism
  5. Methimazole-induced agranulocytosis is more frequent with an initial dose of 30 mg daily than with 15 mg daily (Takata et al., 2009) — Thyroid