Antithyroid Drug

Carbimazole

Brand names: Neo-Mercazole

Carbimazole is an antithyroid drug used to treat hyperthyroidism, particularly Graves' disease, to restore euthyroidism or to prepare for definitive treatment.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

Clinical monograph

How it works

It is converted to methimazole, which inhibits thyroid peroxidase and so reduces thyroid hormone synthesis.

Prescribing in practice

Agranulocytosis is the key risk — advise stopping and seeking an urgent full blood count for sore throat, fever, mouth ulcers or other infection.
It is teratogenic (especially the first trimester) — use effective contraception and seek specialist advice in pregnancy (propylthiouracil may be preferred early).
Its effect on thyroid levels is delayed; a beta-blocker is often added initially for symptom control.

Monitoring

Monitor thyroid function to titrate the dose; check FBC urgently with infection symptoms, and review liver function if hepatotoxicity is suspected.

Counselling the patient

Stop it and get an urgent blood test if you develop a sore throat, fever or mouth ulcers.
It takes some weeks to control symptoms.
Use reliable contraception and tell your clinician if you might be pregnant.

Evidence & guidelines

First-line antithyroid drug for hyperthyroidism, with mandatory counselling about agranulocytosis.

Reference: BTA Hyperthyroidism Guidelines 2019; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways