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Methimazole for cats: Felimazole, Felanorm, Thyronorm and ear gel

Methimazole (thiamazole) for hyperthyroid cats, from Felimazole tablets and Thyronorm oral solution to transdermal ear gel, with label doses, blood test schedule and side effects.

Illustration: an elderly tabby cat lying on a blanket beside an oral dosing syringe and a medicine bottle.

Methimazole, also called thiamazole, is the drug most hyperthyroid cats take. It is the same molecule as the human medicine, and the active form of carbimazole, but the products below are made and dosed for cats. Your vet chooses the product and the dose.

Key points

  • Products: Felimazole tablets, Felanorm oral solution and an FDA-approved generic in the US; Thyronorm oral solution in the UK.
  • Usual start: 2.5 mg twice a day (Felimazole label); Thyronorm starts at 5 mg a day split into two doses.
  • Blood tests: before starting, at 3 and 6 weeks, then every 3 months.
  • Watch for: poor appetite, vomiting, lethargy, intense itching of the face and neck, yellow gums, bleeding.

The products

  • Felimazole coated tablets, 2.5 mg and 5 mg. FDA-approved in 2009 for cats; prescription only.
  • Felanorm oral solution, the second FDA-approved methimazole product for cats.
  • Methimazole Coated Tablets (Felix Pharmaceuticals), 2.5 mg and 5 mg, approved by the FDA in July 2025 as the first generic of Felimazole.
  • Thyronorm 5 mg/ml oral solution, licensed in the UK. Its current product information (March 2026) names Virbac as the licence holder.
  • Transdermal gel, rubbed on the inside of the ear. It is made up by compounding pharmacies, not a licensed product.

Doses on the labels

Felimazole: 2.5 mg every 12 hours to start. After three weeks the dose is adjusted in 2.5 mg steps according to blood T4 and how the cat is doing. The label caps the total at 20 mg a day, with no single dose above 10 mg. In the US field study the average maintenance dose was 2.5 mg twice a day. The generic follows the same schedule.

Thyronorm: 5 mg a day (1 ml), split morning and evening, squirted into the mouth with the syringe rather than mixed into food. Once daily is allowed if twice is not practical, though the label notes 2.5 mg twice daily may work better in the short term. Keep the timing relative to meals the same each day.

Ear gel: in a trial of 44 cats, 14 of 16 on oral methimazole had normal T4 at two weeks, against 14 of 25 on the gel. The gel caused fewer stomach upsets (1 of 27 cats, against 4 of 17 on tablets). It can help cats that refuse tablets, but control may take longer.

Monitoring

Both the FDA and the labels set out the same pattern: a blood count, kidney and liver tests and total T4 before starting, again at 3 and 6 weeks, then every 3 months (Thyronorm’s label adds checks at 10 and 20 weeks). Cats on more than 10 mg a day need closer checks. Ask your vet about blood pressure too.

Side effects

In the classic study of 262 cats, 18.3% had clinical side effects, usually in the first month: poor appetite, vomiting, lethargy, scratching of the face and neck, bleeding, and jaundice from liver injury. Serious blood reactions (agranulocytosis or low platelets) occurred in 3.8%, and cleared within a week of stopping the drug.

The FDA’s most frequent reports are decreased appetite, vomiting, lethargy, itchy skin on the face, head and neck, hair loss, weight loss, raised liver values, anaemia, behaviour changes, raised kidney values and diarrhoea.

The Felimazole label rules it out in cats with primary liver disease, kidney failure, autoimmune disease, blood or clotting disorders, and in pregnant or nursing queens.

Safety for people

Wash your hands after every dose and wear gloves for the tablets, the litter tray, vomit and urine. Do not break or crush Felimazole tablets. Methimazole can harm an unborn baby, so pregnant people should take particular care or let someone else give the doses.

Methimazole or carbimazole?

Carbimazole turns into methimazole in the body, so the effect is the same. The main practical difference for cats is the once-daily prolonged-release carbimazole tablet, Vidalta. See carbimazole for cats and the overview of all treatments.

Sources

Checked 15 October 2026.

  1. Felimazole coated tablets (methimazole) package insert — Dechra Veterinary Products (US)
  2. Hyperthyroidism in cats, two FDA-approved drugs available to treat it — US Food and Drug Administration
  3. FDA approves first generic methimazole coated tablets for treating hyperthyroidism in cats (July 2025) — US Food and Drug Administration
  4. Thyronorm 5 mg/ml oral solution for cats, summary of product characteristics (March 2026) — Veterinary Medicines Directorate (UK)
  5. Methimazole treatment of 262 cats with hyperthyroidism (Peterson et al., 1988) — Journal of Veterinary Internal Medicine
  6. Efficacy and safety of transdermal methimazole in cats with hyperthyroidism (Sartor et al., 2004) — Journal of Veterinary Internal Medicine