Felimazole 5 mg Coated Tablets for Cats

Composition

Each tablet contains:

Active substance:

Thiamazole 5mg

Excipients:

Orange sugar-coated biconvex tablets, 5.5 mm diameter.

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Pharmacological properties

4.1 ATCvet code: QH03BB02.

4.2 Pharmacodynamics

Thiamazole acts by blocking the biosynthesis of thyroid hormone in vivo. The primary action is to inhibit binding of iodide to the enzyme thyroid peroxidase, thereby preventing the catalysed iodination of thyroglobulin and T3 and T4 synthesis.

4.3 Pharmacokinetics

Following oral dosing in healthy cats, thiamazole is rapidly and completely absorbed with a bioavailability of >75 %. However, there is a considerable variation between animals. Elimination of the drug from cat plasma is rapid with a half-life of 4.5-5.0 hours. Peak plasma levels occur approximately 1-2 hours after dosing. Cmax is between 1.6-1.9 μg/ml.

In rats thiamazole has been shown to be poorly bound to plasma protein (5 %); 40 % was bound to red blood cells.

The metabolism of thiamazole in cats has not been investigated, however, in rats thiamazole is rapidly metabolised in the thyroid gland. About 64 % of the administered dose being eliminated in the urine and only 7.8 % excreted in faeces. This is in contrast with man where the liver is important for the metabolic degradation of the compound. The drug residence time in the thyroid gland is assumed to be longer than in the plasma.

From man and rats it is known that the drug can cross the placenta and concentrates in the foetal thyroid gland. There is also a high rate of transfer into breast milk.

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Target species

Cats.

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Indications

For the stabilisation of hyperthyroidism in cats prior to surgical thyroidectomy.

For the long-term treatment of feline hyperthyroidism.

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Dosage

Oral use.

For the stabilisation of feline hyperthyroidism prior to surgical thyroidectomy and for the long term treatment of feline hyperthyroidism, the recommended starting dose is 5 mg per day.

Wherever possible, the total daily dose should be divided into two and administered morning and evening. Tablets should not be split.

If, for reasons of compliance, once daily dosing with a 5 mg tablet is preferable, then this is acceptable although the 2.5 mg tablet given twice daily may be more efficacious in the short term. The 5 mg tablet is also suitable for cats requiring higher dose rates.

Haematology, biochemistry and serum total T4 should be assessed before initiating treatment and after 3 weeks, 6 weeks, 10 weeks, 20 weeks, and thereafter every 3 months. At each of the recommended monitoring intervals, the dose should be titrated to effect according to the total T4 and to clinical response to treatment. Dose adjustments should be made in increments of 2.5 mg and the aim should be to achieve the lowest possible dose rate.

If more than 10 mg per day is required animals should be monitored particularly carefully.

The dose administered should not exceed 20 mg/day.

For long term treatment of hyperthyroidism, the animal should be treated for life.

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Contraindications

Do not use in cats suffering from systemic disease such as primary liver disease or diabetes mellitus.

Do not use in cats showing signs of autoimmune disease.

Do not use in animals with disorders of white blood cells, such as neutropenia and lymphopenia.

Do not use in animals with platelet disorders and coagulopathies (particularly thrombocytopenia).

Do not use in cases of hypersensitivity to the active substance or to any of the excipients.

Do not use in pregnant or lactating females (please refer to section 3.7).

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Adverse reactions

Cats:

Uncommon (1 to 10 animals / 1,000 animals treated):

Vomitinga; Anorexiaa, Inappetencea, Lethargya; Pruritusa,b, Excoriationa,b; Prolonged bleedinga,c,d; Hepatopathya, Icterusa,d; Eosinophiliaa, Lymphocytosisa, Neutropeniaa, Lymphopeniaa, Leucopeniaa,e, Agranulocytosisa, Thrombocytopeniaa,g, Haemolytic anaemiaa.

Rare (1 to 10 animals / 10,000 animals treated):

Serum anti-nuclear antibodiesf,h, Anaemiaf,h.

Very rare (<1 animal / 10,000 animals treated, including isolated reports):

Lymphadenopathyf,h.

a Resolves within 7-45 days after cessation of thiamazole therapy.

b Severe. Of the head and neck.

c Sign of bleeding diathesis.

d Associated with hepatopathy.

e Slight.

f Immunological side effect.

g Occurs uncommonly as a haematological abnormality and rarely as an immunological side effect.

h Treatment should be stopped immediately and alternative therapy considered following a suitable period for recovery.

Adverse events have been reported following long term control of hyperthyroidism. In many cases, signs may be mild and transitory and not a reason for withdrawal of treatment. The more serious effects are mainly reversible when medication is stopped.

Following long-term treatment with thiamazole in rodents, an increased risk of neoplasia in the thyroid gland has been shown to occur, but no evidence is available in cats.

Reporting adverse events is important. It allows continuous safety monitoring of a veterinary medicinal product. Reports should be sent, preferably via a veterinarian, to either the marketing authorisation holder or its local representative or the national competent authority via the national reporting system. See the package leaflet for respective contact details.

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Warnings

Special precautions for safe use in the target species:

If more than 10 mg per day is required animals should be monitored particularly carefully.

Use of the veterinary medicinal product in cats with renal dysfunction should be subject to careful benefit-risk assessment by the clinician. Due to the effect thiamazole can have on reducing the glomerular filtration rate, the effect of therapy on renal function should be monitored closely as deterioration of an underlying condition may occur.

Haematology must be monitored due to risk of leucopenia or haemolytic anaemia. Any animal that suddenly appears unwell during therapy, particularly if they are febrile, should have a blood sample taken for routine haematology and biochemistry. Neutropenic animals (neutrophil counts <2.5 x 109/l) should be treated with prophylactic bactericidal antibacterial drugs and supportive therapy.

As thiamazole can cause haemoconcentration, cats should always have access to drinking water.

Please refer to section 3.9 for monitoring instructions.

Special precautions to be taken by the person administering the veterinary medicinal product to animals

Wash hands after use.

In case of accidental ingestion, seek medical advice immediately and show the package leaflet or the label to the physician.

Thiamazole may cause vomiting, epigastric distress, headache, fever, arthralgia, pruritus and pancytopaenia. Treatment is symptomatic.

Wash hands with soap and water after handling litter used by treated animals. Do not eat, drink or smoke while handling the tablet or used litter.

Do not handle this veterinary medicinal product if you are allergic to antithyroid products. If allergic symptoms develop, such as a skin rash, swelling of the face, lips or eyes or difficulty in breathing, you should seek medical attention immediately and show the package leaflet or the label to the physician.

Do not break or crush tablets.

As thiamazole is a suspected human teratogen, women of child-bearing age and pregnant women should wear gloves when handling litter of treated cats.

Pregnant women should wear gloves when handling the veterinary medicinal product.

Special precautions for the protection of the environment:

Not applicable.

3.11 Special restrictions for use and special conditions for use, including restrictions on the use of antimicrobial and antiparasitic veterinary medicinal products in order to limit the risk of development of resistance

Not applicable.

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Use during pregnancy, lactation or lay

Pregnancy and lactation:

Laboratory studies in rats and mice have shown evidence of teratogenic and embryotoxic effects of thiamazole. The safety of the veterinary medicinal product was not assessed in pregnant or lactating cats. Do not use in pregnant or lactating females.

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Interactions

Concurrent treatment with phenobarbital may reduce the clinical efficacy of thiamazole.

Thiamazole is known to reduce the hepatic oxidation of benzimidazole wormers and may lead to increases in their plasma concentrations when given concurrently. Thiamazole is immunomodulatory, therefore this should be taken into account when considering vaccination programmes.

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Overdose

In tolerance studies in young healthy cats, the following dose-related clinical signs occurred at doses of up to 30 mg/animal/day: anorexia, vomiting, lethargy, pruritus and haematological and biochemical abnormalities such as neutropenia, lymphopenia, reduced serum potassium and phosphorus levels, increased magnesium and creatinine levels and the occurrence of anti-nuclear antibodies. At a dose of 30 mg/day some cats showed signs of haemolytic anaemia and severe clinical deterioration. Some of these signs may also occur in hyperthyroid cats treated at doses of up to 20 mg per day.

Excessive doses in hyperthyroid cats may result in signs of hypothyroidism. This is however unlikely, as hypothyroidism is usually corrected by negative feedback mechanisms. Please refer to Section 3.6: Adverse events.

If overdosage occurs, stop treatment and give symptomatic and supportive care.

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Special warnings

None

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Withdrawal period

Not applicable.

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Incompatibilities

Not applicable.

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Immediate packaging

Tablet container: White polypropylene tub with white LDPE/HDPE tamper-evident lid containing 100 tablets in a cardboard box.

Blister: Transparent PVC/Aclar – aluminium blister. Blister strips contain 25 tablets. Each cardboard box contains 4 blister strips.

Not all pack sizes may be marketed.

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Storage

Do not store above 25°C.

Tablet container: Keep the container tightly closed in order to protect from moisture. Keep the container in the outer carton.

Blister: Keep the blister strips in the outer carton.

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Shelf life

Tablet container: Shelf life of the veterinary medicinal product as packaged for sale: 5 years.

Blister: Shelf life of the veterinary medicinal product as packaged for sale: 3 years.

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Disposal of unused product

Medicines should not be disposed of via wastewater.

Use take-back schemes for the disposal of any unused veterinary medicinal product or waste materials derived thereof in accordance with local requirements and with any national collection systems applicable to the veterinary medicinal product concerned.

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Additional information

Vm 50406/5051

Vm 50406/3045

8 DATE OF FIRST AUTHORISATION

22 January 2002

9 DATE OF THE LAST REVISION OF THE SUMMARY OF THE PRODUCT CHARACTERISTICS

January 2026

10 CLASSIFICATION OF VETERINARY MEDICINAL PRODUCT

Veterinary medicinal product subject to prescription.

Find more product information by searching for the ‘Product Information Database’ on www.gov.uk.

Approved: 24 February 2026

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Registration holder

Dechra Regulatory B.V.

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