# Carbimazole: veterinary dose and monograph > Carbimazole is a veterinary drug. An antithyroid drug for FELINE HYPERTHYROIDISM — carbimazole is a PRO-DRUG that is rapidly converted in the body to METHIMAZOLE (the active drug already in this formulary), so the two share mechanism, efficacy and side effects. Indications include feline hyperthyroidism — long-term medical management; pre-treatment stabilisation before radioiodine therapy or thyroidectomy; trial of reversible thyroid control to assess renal function before definitive treatment. This monograph lists 2 reference doses for cats, each with route, frequency and source. Source: https://vetmasterji.com/drugs/carbimazole · Last reviewed: 2026-10-01 ## At a glance - **Drug class:** Veterinary drug - **Also known as:** Vidalta, Neo-Mercazole, Carbimazole - **Species with doses:** Cats - **Routes:** PO - **Last reviewed:** 1 October 2026 ## Indications - Feline hyperthyroidism — long-term medical management - Pre-treatment stabilisation before radioiodine therapy or thyroidectomy - Trial of reversible thyroid control to assess renal function before definitive treatment ## Carbimazole dose by species Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them. ### Cats *Carbimazole doses for cats* | Indication | Dose | Route | Frequency | Duration | | --- | --- | --- | --- | --- | | Hyperthyroidism (controlled-release) | 15 mg per cat PO once daily; adjust within 10–25 mg/day in 5 mg steps | PO | q24h | — | | Hyperthyroidism (plain carbimazole) | 5 mg per cat PO q8h for 2–3 weeks, then 5 mg q12h | PO | q8–12h | — | - Hyperthyroidism (controlled-release): Papich 5e and Plumb’s p.194 (Vidalta label): 15 mg once daily at the same time each day, with food; maintenance 10–25 mg once daily by clinical signs and total T4. Do not split or crush. If less than 10 mg/day is needed, use another treatment. - Hyperthyroidism (plain carbimazole): Papich 5e and Plumb’s p.194 (Peterson 2000): 5 mg q8h induction, then most cats need 5 mg q12h; 2.5–5 mg q12h (Trepanier). Carbimazole doses are about twice methimazole doses. Titrate to total T4. ## Contraindications Pre-existing significant hepatic disease, blood dyscrasias, or autoimmune disease. Known thioamide (methimazole/carbimazole) hypersensitivity. Pregnancy (crosses placenta — fetal goitre/hypothyroidism). Monitor for the methimazole adverse-effect profile. Handle with care (gloves); pregnant women should avoid handling. ## Species-specific warnings > **Caution:** Cats: Monitor CBC and liver values for idiosyncratic toxicity, and renal values as euthyroidism is restored (azotaemia may emerge). Avoid over-treatment into hypothyroidism. ## Adverse effects - GI upset (vomiting, anorexia) - Facial/neck excoriations (self-trauma — immune-mediated) - Hepatotoxicity - Haematologic: thrombocytopenia, neutropenia/agranulocytosis (uncommon but serious) - Unmasking of underlying CKD as thyroid levels normalise ## Drug interactions - Beta-blockers (atenolol/propranolol): often co-used initially to control tachycardia - Drugs affected by altered metabolic rate (e.g. those cleared faster when hyperthyroid) — re-evaluate as euthyroidism is restored - Other bone-marrow-suppressing/hepatotoxic drugs: additive risk ## Pregnancy and lactation Crosses the placenta and can cause fetal goitre/hypothyroidism — avoid in pregnancy. Pregnant human handlers should not handle the tablets. ## Monitoring - Total T4 (efficacy/titration) — recheck ~2–3 weeks after starting/dose changes, then periodically - CBC (thrombocytopenia/neutropenia) and liver enzymes, especially in the first months - Renal values (azotaemia may be unmasked as hyperthyroidism is controlled) - Facial pruritus/excoriation and GI tolerance ## Toxicity and overdose Shares methimazole's adverse profile: GI upset, hepatotoxicity, facial excoriations, and uncommon but serious haematologic toxicity (thrombocytopenia, agranulocytosis). Over-treatment causes iatrogenic HYPOTHYROIDISM (which can worsen renal function). These are idiosyncratic/dose-related effects rather than acute overdose toxicity. Management: No specific antidote. Discontinue/reduce dose; supportive care; monitor CBC (for cytopenias) and liver values, and check thyroid levels (manage iatrogenic hypothyroidism by dose reduction). Treat hepatotoxicity supportively. Decontaminate large recent ingestions. ## Clinical pearls - Effectively "methimazole, once daily" — carbimazole converts to methimazole, and the controlled-release Vidalta improves compliance over twice-daily methimazole - It controls but does not cure — radioiodine or thyroidectomy are the definitive options; a medical trial first checks how the kidneys cope with euthyroidism - Watch for the serious-but-uncommon toxicities (agranulocytosis, hepatotoxicity, facial excoriations) — monitor CBC/liver early on - Restoring euthyroidism can unmask CKD (GFR falls) — recheck renal values and avoid over-treatment (iatrogenic hypothyroidism worsens kidneys) - Swallow controlled-release tablets whole; handle with gloves ## Mechanism of action Carbimazole is metabolised to METHIMAZOLE, a thioamide that inhibits thyroid peroxidase — the enzyme that iodinates tyrosine residues and couples them to form thyroid hormones (T4/T3). Blocking this halts new thyroid hormone synthesis (it does not destroy the gland or block release of pre-formed hormone, so the effect takes days to manifest and is fully reversible on discontinuation). ## Formulations and products - Controlled-release 10 / 15 mg tablet (Vidalta) - 5 mg tablet (Neo-Mercazole, plain) - Vidalta (MSD Animal Health) — 10 / 15 mg controlled-release tablet, Controlled-release tablet - Neo-Mercazole — 5 mg tablet, Tablet (human-label) ## Storage Tablets: 15–30 °C, dry, protect from light; handle with gloves (pregnant women should not handle). Swallow controlled-release tablets whole. ## Before you use this dose > **Caution:** Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision. ## Other hormones, emergency drugs, supplements and other drugs - [Cabergoline](https://vetmasterji.com/drugs/cabergoline) - [Calcitriol](https://vetmasterji.com/drugs/calcitriol) - [Calcium Borogluconate (IV Calcium)](https://vetmasterji.com/drugs/calcium-borogluconate) - [Calcium EDTA (Calcium Disodium Edetate)](https://vetmasterji.com/drugs/calcium-edta) - [Carboplatin](https://vetmasterji.com/drugs/carboplatin) - [Cetirizine](https://vetmasterji.com/drugs/cetirizine) - [Chlorambucil](https://vetmasterji.com/drugs/chlorambucil) - [Chlorpheniramine Maleate](https://vetmasterji.com/drugs/chlorpheniramine) ## Frequently asked questions ### What is the dose of Carbimazole for cats? Reference doses for Carbimazole in cats: 15 mg per cat PO once daily; adjust within 10–25 mg/day in 5 mg steps PO q24h (Hyperthyroidism (controlled-release)); 5 mg per cat PO q8h for 2–3 weeks, then 5 mg q12h PO q8–12h (Hyperthyroidism (plain carbimazole)). Confirm against the label and the patient before use. ### When should Carbimazole not be used? Pre-existing significant hepatic disease, blood dyscrasias, or autoimmune disease. Known thioamide (methimazole/carbimazole) hypersensitivity. Pregnancy (crosses placenta — fetal goitre/hypothyroidism). Monitor for the methimazole adverse-effect profile. Handle with care (gloves); pregnant women should avoid handling. ### What are the side effects of Carbimazole in animals? GI upset (vomiting, anorexia); Facial/neck excoriations (self-trauma — immune-mediated); Hepatotoxicity; Haematologic: thrombocytopenia, neutropenia/agranulocytosis (uncommon but serious); Unmasking of underlying CKD as thyroid levels normalise. ## Sources - Plumb's Veterinary Drug Handbook - Merck Veterinary Manual, 11th edition - Papich, Papich Handbook of Veterinary Drugs, 5th edition --- VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.