Carbimazole
Thioimidazole-derivative antithyroid drug
Also known as: Vidalta · Neo-Carbimazole · Carbazole · Basolest · Camazol · Carbistad · Neo Tomizol · NeoMercazole · Neo-Thyreostat · Thyrostat · Tyrazol · Neo-morphazole · Neo-Mercazole · carbimazolum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hyperthyroidism | 15 mg/animal (starting dose) or 10 mg/animal if total thyroxine <100 nmol/l. Adjust dose in 5 mg increments. | PO | q24h | Long-term | 🌍 EU |
- Hyperthyroidism: Do not break tablets. Note: There are no data on the use of the sustained-release formulation in dogs.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hyperthyroidism | 2.5-5 mg (total dose) per cat | PO | twice daily | — | 🌎 NA |
| Hyperthyroidism | 10-15 mg total dose daily per cat in divided doses | PO | divided doses | for 1-3 weeks | 🌎 NA |
| Hyperthyroidism | Initially, give 5 mg (total dose) | PO | q8h | for 2-3 weeks | 🌎 NA |
| Hyperthyroidism (Sustained-release tablets) | 15 mg | PO | once daily | — | 🌎 NA |
| Hyperthyroidism | 15 mg/animal (starting dose) or 10 mg/animal if total thyroxine <100 nmol/l. Adjust dose in 5 mg increments. | PO | q24h | Long-term | 🌍 EU |
- Hyperthyroidism: Will produce a euthyroid state for most patients. Then adjust dosage for the patient to the lowest effective dose. Most cats will need dosing at least once daily.
- Hyperthyroidism: Then adjust. May need to increase dose in approximately 10% of cats. Most cats require 5 mg PO q12h to maintain euthyroidism.
- Hyperthyroidism (Sustained-release tablets): Give at the same time each day. Do not break or crush tabs. Adjust dose upwards or downwards within a range of 10 mg-25 mg per day in 5 mg increments depending on clinical signs and TT4. If cat requires doses less than 10 mg per day, use alternative treatment.
- Hyperthyroidism: Do not break tablets.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Carbimazole is a thioimidazole-derivative anti-thyroid medication primarily used in veterinary medicine to manage feline hyperthyroidism.
- It acts as a prodrug and is rapidly converted in vivo to methimazole.
- Because it lacks the bitter taste of methimazole, it is often better tolerated by cats, reducing the incidence of gastrointestinal side effects.
- While not available in the USA or Canada, it is widely used in Europe and other regions.
- Clinical Pearl: Carbimazole requires approximately twice the milligram dose of methimazole to achieve equivalent serum levels due to differences in molecular weight.
Mechanism of action
Carbimazole is a prodrug that is almost entirely converted to methimazole in the body.
- Methimazole actively inhibits the enzyme thyroid peroxidase (TPO).
- Inhibition of TPO → blocks the oxidation of iodide to iodine → prevents iodine incorporation into tyrosyl residues of thyroglobulin.
- It also inhibits the coupling of iodinated tyrosyl residues (monoiodotyrosine and diiodotyrosine) → prevents formation of thyroxine (T4) and triiodothyronine (T3).
- Note: It does not affect the release or activity of pre-formed thyroid hormones already in circulation, which accounts for the 1-3 week lag time before clinical improvement is seen.
Safety & warnings
Contraindications
- Hypersensitivity to carbimazole or methimazole
- Not recommended in cats intolerant to methimazole
- History of or concurrent hematologic abnormalities
- Liver disease
- Autoimmune disease
Adverse effects
- Vomiting
- Anorexia
- Depression
- Eosinophilia (usually transient)
- Leukopenia (usually transient)
- Lymphocytosis (usually transient)
- Self-induced excoriations (rare but serious)
- Bleeding (rare but serious)
- Hepatopathy (rare but serious)
- Thrombocytopenia (rare but serious)
- Agranulocytosis (rare but serious)
- Positive direct antiglobulin test (rare but serious)
- Acquired myasthenia gravis (rare but serious)
- Positive ANA (with chronic use)
Precautions
Hypersensitivity: Contraindicated in patients hypersensitive to carbimazole or methimazole. Cross-Reactivity: Because carbimazole is a prodrug converted into methimazole, cats who have had prior serious reactions to methimazole should receive carbimazole with great caution. Pregnancy/Nursing: High levels of methimazole cross the placenta and may induce hypothyroidism in kittens. Levels higher than those found in plasma are found in breast milk. Kittens should be placed on a milk replacer after receiving colostrum from mothers on methimazole. FDA categorizes methimazole as category D for use during pregnancy.
Drug interactions
Potential for increased risk for hepatotoxicity; increased monitoring (LFT's) necessary
Carbimazole may decrease digoxin efficacy
Potential for decreased anticoagulant efficacy if carbimazole added
Carbimazole interferes with the uptake and efficacy of radioiodine therapy. It must be discontinued prior to treatment.
Concurrent use is not recommended as it may interfere with the dietary management of hyperthyroidism.
Monitoring
- CBC, platelet counts (baseline and every 2-3 weeks during first 3 months)
- Serum T4 (baseline and every 2-3 weeks during first 3 months)
- Liver function tests (if indicated by clinical signs)
- ANA (if indicated by clinical signs)
- T4 at 3-6 month intervals (after stabilized)
- Other diagnostic tests as dictated by adverse effects
Pharmacokinetics
Half-life
Absorption
Rapidly absorbed from the GI tract.
Distribution
In cats, the volume of distribution of methimazole is variable (0.12-0.84 L/kg). Methimazole apparently concentrates in thyroid tissue and biologic effects persist beyond measurable blood levels. Crosses the placenta and enters breast milk.
Metabolism
Rapidly and nearly totally converted to methimazole in vivo.
Elimination
Plasma elimination half-life of methimazole ranges from 2.3-10.2 hours in cats and 8-9 hours in dogs.
Overdose
Acute toxicity that may be seen with overdosage includes vomiting, anorexia, depression, and more serious effects such as agranulocytosis, hepatopathy, and thrombocytopenias.
Treatment: Consists of following standard protocols in handling an oral ingestion (empty stomach if not contraindicated, administer charcoal, etc.) and treating symptomatically and supportively.
Available products
Formulations
- Oral Tablets
- Oral Sustained-Release Tablets
Veterinary
- Carbimazole Oral Sustained-Release Tablets: 10 mg & 15 mg (Vidalta, Intervet-UK)
Human-labeled
- Carbimazole Oral Tablets: 5 mg & 20 mg (NeoCarbimazole, Carbazole, Neo Mercazole)
Regulatory status
Approved for feline hyperthyroidism.
POM-V (Prescription Only Medicine - Veterinarian)
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Unless otherwise labeled, carbimazole tablets should be stored at room temperature in well-closed containers.
Client information
- Purpose: This medication manages your cat's overactive thyroid gland (hyperthyroidism) by reducing the production of thyroid hormones. It is a management tool, not a cure.
- Patience is Key: It typically takes 1 to 3 weeks to see significant improvement in your cat's symptoms.
- Administration: Give exactly as prescribed. Do not crush or break sustained-release tablets (like Vidalta).
- Side Effects to Watch For: Contact your veterinarian if your cat experiences vomiting, loss of appetite, lethargy, or severe facial scratching/itching.
- Safety: Pregnant women or nursing mothers should handle this medication with caution (wear gloves), as it can cross the placenta and enter breast milk.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.