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Felimazole 1.25 mg Tablets (Canada)

This page contains information on Felimazole 1.25 mg Tablets for veterinary use.
The information provided typically includes the following:
  • Felimazole 1.25 mg Tablets Indications
  • Warnings and cautions for Felimazole 1.25 mg Tablets
  • Direction and dosage information for Felimazole 1.25 mg Tablets

Felimazole 1.25 mg Tablets

This treatment applies to the following species:
Company: Dechra

Methimazole tablets for cats

For veterinary use only

DIN 02411512

DIN 02332655

DIN 02332663


Methimazole is a thioureylene antithyroid drug, which inhibits the synthesis of thyroid hormones. Methimazole (1-methylimidazole-2-thiol) is a white, crystalline substance that is freely soluble in water.

Indication: For the management of hyperthyroidism in cats.

Dosage and Administration

The starting dose of Felimazole tablets is 2.5 mg administered orally every 12 hours. Following 3 weeks of treatment, the dose should be titrated based on total T4 (TT4) concentrations; the usual goal is to have TT4 within the normal range as well as a good clinical response. Dose adjustments should be made in 1.25 mg or 2.5 mg increments. The maximum total dosage is 15 mg per day divided, not to exceed 10 mg as a single administration. Hematology, biochemistry, and TT4 should be evaluated prior to initiating treatment and monitored after 3 weeks and 6 weeks of treatment. Thereafter, bloodwork should be monitored every 3 months and the dose adjusted as necessary. Cats receiving doses greater than 10 mg per day should be monitored more frequently.


Do not use in cats with hypersensitivity to methimazole, carbimazole or the excipient, polyethylene glycol.

Do not use in cats with primary liver disease or renal failure.

Do not use in cats with autoimmune disease.

Do not use in cats with hematological disorders (such as anemia, neutropenia, lymphopenia, or thrombocytopenia) or coagulopathies.

Do not use in breeding, pregnant or lactating cats. Laboratory studies in rats and mice have shown evidence of teratogenic and embryotoxic effects of methimazole.

Cautions: Use of Felimazole in cats with renal dysfunction should be carefully evaluated. Reversal of hyperthyroidism may be associated with decreased glomerular filtration rate and a decline in renal function, unmasking the presence of underlying renal disease.

Due to potentially serious adverse reactions such as hepatopathy, immune-mediated anemia, thrombocytopenia, and agranulocytosis, cats on methimazole therapy should be monitored closely for any sign of illness including fever, lymphadenopathy or signs of anemia. If a cat becomes ill while on Felimazole tablets, the drug should be stopped and appropriate hematological and biochemical testing should be done.

Some hyperthyroid cats with concurrent diabetes mellitus treated with methimazole have decreases in their insulin requirements. It is recommended that the dose of insulin be reduced. Blood glucose, serum fructosamine, thyroid hormone concentrations and clinical signs of hypoglycemia should be closely monitored since the clinical course of the diabetes mellitus may be unpredictable.i

The disposition of other drugs may change as hyperthyroid cats become euthyroid. The use of concomitant medications has not been fully evaluated in cats treated with methimazole.

Methimazole does not address the underlying cause of feline hyperthyroidism.


Not for use in humans. Keep out of reach of children. For use in cats only.

Wash hands with soap and water after administration to avoid exposure to drug.

Do not break or crush tablets.

Methimazole is a human teratogen and crosses the placenta concentrating in the foetal thyroid gland. There is also a high rate of transfer into breast milk. Pregnant women or women who may become pregnant, and nursing mothers should wear gloves when handling tablets, litter or bodily fluids of treated cats.

Adverse Reactions

113 cats were treated in Phase 1 of a 10 centre, open label, 42 day study in the US. 101 cats continued into Phase 2 and were treated for a mean of 302 days with a range of 48 to 565 days. During the study the following adverse events, mostly mild and transient, were observed:

Clinical observations

Phase 1: Up to 42 days

Phase 2: After 42 days

Number of cats

% of cats

Number of cats

% of cats






Change in food consumption: Decreased










Change in food consumption: Increased





Diarrhea/loose stool





Skin lesions





Change in food consumption: Anorexia










Abnormal vocalization





Weight loss










Included: erythema, excoriation, lesions, abrasions, welts.

Hematology and Clinical Chemistry Changes: Over the course of the study, there was a decreasing trend in the mean counts of red blood cells, lymphocytes, neutrophils and monocytes; however, means remained within or near the normal limits. Ten cats had clinically significant increases of BUN and creatinine concurrently. Five other cats had clinically significant increases of BUN and a further five cats had clinically significant increases of creatinine. The mean ALT was increased above the reference range in the first 2 quarters of Phase 2 but was within the normal range at the following 2 visits. At the end of Phase 1, three cats had increased antinuclear antibody (ANA) titers, with fifteen cats having increased ANA titers at one or more points by the end of the study; the significance of this was not determined.

Early Withdrawals and Deaths: In Phase 1 of the study, three cats were withdrawn early; one because of the unmasking of latent renal disease and two due to the development of skin lesions. Eighteen cats died or were euthanized during Phase 2, four of which may have been related to Felimazole unmasking or accelerating chronic renal failure. Twelve cats developed neoplastic conditions unrelated to Felimazole treatment and four of the cats had concurrent hypertrophic cardiomyopathy.

Foreign Market Experience: The following events were reported voluntarily during post-approval use of Felimazole tablets in foreign markets: facial pruritus, self-induced excoriations of the head and neck, generalized lymphadenopathy, thrombocytopenia, anemia, hematemesis, epistaxis, bleeding diathesis and elevation of serum liver enzymes and bilirubin.

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

Clinical Pharmacology

Methimazole is an antithyroid drug that 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 catalyzed iodination of thyroglobulin and T3 and T4 synthesis.

Felimazole tablets are well absorbed following oral administration. Maximum plasma concentrations are achieved within 1 - 1 1/2 hours after dosing and methimazole is rapidly eliminated from the blood (T1/2 is approximately 3 hours). Administration of Felimazole tablets in a fasted state enhances absorption.

Safety and Efficacy Study Information:

Effectiveness: In Phase 1 of a US effectiveness field study with 113 cats, the product was considered effective if both the TT4 concentration was ≤ 51.5 nmol/L and the Investigator’s clinical assessment documented clinical improvement. Of the 111 evaluable cases, 73 (64.9%) were considered treatment successes. The decrease in TT4 concentration was significant from the pre-enrolment visit to the Day 42 visit. A TT4 of ≤ 51.5 nmol/L occurred in 69.1% and 64.1% of cats on Day 21 and Day 42, respectively. Investigators assessed 96.4% and 89.5% of cats as clinically improved on Days 21 and 42, respectively. 101 cats continued into Phase 2; effectiveness was based on a combination of Investigator’s clinical assessment, maintenance of TT4 concentrations at or near the laboratory reference range of 10.3-51.5 nmol/L and the presence or absence of adverse reactions. Mean TT4 concentrations were within or near the laboratory reference range during the first four quarterly visits. At the first quarterly visit, Investigators categorized 80.9% of cats as stable or improved relative to their baseline assessment. By the fourth quarterly visit, 75.8% were deemed to be stable or improved. The average maintenance dose required in Phase 2 was 2.5 mg twice daily, with a minimum of 2.5 mg per cat and a maximum of 15 mg per cat on a daily basis.

Animal Safety: In a 12-week safety study, healthy young cats were dosed with 0, 10, 20, and 30 mg Felimazole tablets per day, divided into two doses. Cats in all treated groups experienced anorexia, vomiting, loose stool and lethargy. Cats in the 20 and 30 mg/day groups also had facial excoriations, pruritus, and lymphadenopathy. The following hematological changes were seen: neutropenia, lymphopenia, anemia, and thrombocytopenia. The following biochemical changes were seen: increased globulin, increased magnesium, increased blood urea nitrogen, increased creatinine and decreased phosphorus. There was a dose-dependent occurrence of antinuclear antibodies. Most of the clinical pathology changes were mild in nature.

Gross necropsy findings in all treated groups included hepatomegaly, thymus atrophy and thyroid hyperplasia and darkening. Some treated males had delayed maturation of the testes.

The 30 mg/day dose was poorly tolerated and resulted in the clinical deterioration and euthanasia of four of the six cats in that group. Two of the cats showed signs of anemia, thrombocytopenia and severe clinical deterioration. One had been on the drug for 34 days, the other for 9 weeks. The drug was discontinued in a third cat treated with 30 mg/day while it received supportive care. It was euthanized on day 55 after becoming anorexic. This cat had anemia (HCT 21.6%) and red blood cell agglutination. Necropsy showed inflammation of the muscular layer of the stomach and a small erosion in the stomach. A fourth cat treated with 30 mg/day was euthanized after several days of anorexia when the decision was made to discontinue dosing in this group. All 30 mg/day cats that died had generalized lymphadenopathy. Necropsies revealed reactive lymph nodes and varying degrees of inflammation throughout the body. The remaining two cats in the 30 mg/day group were taken off Felimazole tablets at week 9 and fully recovered.


Store between 15°C and 25°C. Keep the container tightly closed in order to protect from moisture.

Methimazole is known to be sensitive to light. Protect the tablets from light if removed from the original container and not used immediately.


Felimazole 1.25 mg Tablets, code 435642

Felimazole 2.5 mg Tablets, code 992789

Felimazole 5 mg Tablets, code 992790

Each bottle contains a total of 100 tablets.

Dechra Ltd., Snaygill Industrial Estate, Keighley Road, Skipton, North Yorkshire, BD23 2RW, United Kingdom

Imported and distributed by: Dechra Veterinary Products Inc., 1 Holiday Avenue, East Tower, Suite 345, Pointe-Claire, Quebec, Canada, H9R 5N3

Felimazole is a registered trademark of Dechra Ltd.

iGraves TK, Taboada J (2007) The hyperthyroid cat with diabetes. Proceedings of the North American Veterinary Conference 2007, pp 364-5


CPN: 1786000.0

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