Methimazole

Indications

Felanorm (methimazole) is indicated for the treatment of hyperthyroidism in cats.

Dosage and Administration

Always include the Instructions for Preparing a Dose of Felanorm (methimazole) Oral Solution and the Information for Cat Owners with the prescription. The starting dose of Felanorm is 2.5 mg administered every 12 hours. Following 3 weeks of treatment, the dose should be titrated to effect based on individual serum total T4 (TT4) levels and clinical response. Dose adjustments should be made in 2.5 mg increments. The maximum total dosage is 20 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. The product should be administered orally using the enclosed graduated dosing syringe. The syringe has measurements representing the mg dose of the product. Rinse/clean the oral dosing syringe between uses. (See the Instructions for Preparing a Dose of Felanorm (methimazole) Oral Solution).

Contraindications

Do not use in cats with hypersensitivity to methimazole or carbimazole. Do not use in cats with primary liver disease or renal failure. Do not use in cats with autoimmune disease. See ADVERSE REACTIONS. Do not use in cats with hematological disorders (such as anemia, neutropenia, lymphopenia, or thrombocytopenia) or coagulopathies. See ADVERSE REACTIONS. Do not use in pregnant or lactating queens. Laboratory studies in rats and mice have shown evidence of teratogenic and embryotoxic effects of methimazole.

Warnings

Methimazole has anti-vitamin K activity and may induce bleeding diathesis without evidence of thrombocytopenia. See ADVERSE REACTIONS. Keep Felanorm in a secure location out of reach of dogs, cats, and other animals to prevent accidental ingestion or overdose. HUMAN WARNINGS: Not for use in humans. Keep out of reach of children. For use in cats only. Wear protective single use, impermeable (e.g., latex or nitrile) gloves when administering the solution. Wash hands with soap and water after administration to avoid exposure to drug. Wear protective gloves to prevent direct contact with litter, feces, urine, or vomit of treated cats, and the solution. Wash hands after contact with the litter of treated cats. Methimazole is a human teratogen and crosses the placenta concentrating in the fetal 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 the solution, litter or bodily fluids of treated cats. Individuals with an endocrine disorder that could be impacted by methimazole should use similar precautions. Methimazole may cause vomiting, gastric distress, headache, fever, arthralgia, pruritus, and pancytopenia. In the event of accidental ingestion/overdose, seek medical advice immediately and show the product label to the physician. Avoid skin and oral exposure, including hand-to-mouth contact. Wash any spillages or splatter from the skin immediately. Do not eat, drink, smoke/vape, or use smokeless tobacco while handling the product or used litter. Felanorm may cause skin or eye irritation. Avoid eye contact, including hand-to-eye contact. In case of accidental eye contact, rinse eyes immediately with clean running water. If irritation develops, seek medical advice.

Precautions

Use of Felanorm 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 anorexia, vomiting, head/facial pruritus or edema, depression/lethargy, weight loss, anemia, skin lesions, diarrhea, fever, or lymphadenopathy. If a cat becomes ill while on Felanorm, the drug should be stopped and appropriate hematological and biochemical testing should be done (see ANIMAL SAFETY and POST APPROVAL EXPERIENCE). Anticoagulants may be potentiated by the anti-vitamin K activity of Felanorm. Concurrent use of phenobarbital may reduce the clinical effectiveness of Felanorm. A reduction in dose of certain drugs (β-adrenergic blocking agents, digitalis glycosides, and theophylline) may be needed when the patient becomes euthyroid. Methimazole is known to reduce the hepatic oxidation of benzimidazole anthelmintics (e.g. fenbendazole), leading to increased plasma concentration of these anthelmintics when administered concurrently. Methimazole caused delayed maturation of the testes in young male cats in the 12-week safety study. See ANIMAL SAFETY. The safety of Felanorm has not been evaluated in male cats intended for breeding.

Adverse Reactions

In a US field study with 113 cats, the most common adverse reactions included change in food consumption (increase or decrease), lethargy, vomiting, diarrhea/loose stool, skin lesions, and abnormal vocalization. Three cats were withdrawn early from the study, one due to unmasking of latent renal disease and two due to the development of skin lesions. 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 normal ranges for the testing laboratory. In the extended use phase of the US field study with 101 cats, the most common adverse reactions reported in the study above (lethargy, anorexia) were also observed. Additional signs occurring more frequently in the long-term study were: depression/withdrawn behavior, weight loss, hair coat abnormalities, increased blood urea nitrogen (BUN), weakness, agitation and diarrhea. Most of the adverse reactions reported were mild and transient. Serum chemistry and hematology results in the extended use study were consistent with the trends noted during the field study. The mean alanine transaminase (ALT) was above the reference range at the first two quarterly visits, but within the normal reference range (10-100 U/L) through the next two quarterly visits. Mean lymphocyte counts decreased consistently during the study period, to slightly below the reference range (1200-8000 cells/mcL) at the fourth quarterly visit. Sixteen cats experienced elevated antinuclear antibody (ANA) titers at one or more points during long-term therapy with methimazole , but the significance was not determined. Eighteen cats died or were euthanized during the extended use study, four of which may have been related to methimazole due to the unmasking/acceleration of chronic renal failure. See PRECAUTIONS. In a foreign field study with 26 cats using a starting dose of 5 mg twice daily (twice the recommended starting dose), one cat was withdrawn due to lethargy, vomiting and facial excoriations. Marked thrombocytopenia was reported in two cats; the platelet count returned to normal in one cat when methimazole tablets were discontinued. Two cats collapsed and died within 12 days of starting methimazole at a dose of 5 mg twice daily. Both cats were reported with lethargy, vomiting, anorexia, and bloody diarrhea; one cat also had pallor. In a second foreign field study with 78 cats using a starting dose of 2.5 mg twice daily, 4 cats were withdrawn due to suspected adverse reactions to methimazole including anemia, cholangiohepatitis, excoriations, vomiting, lethargy, jaundice, and anorexia. One cat receiving 2.5 mg three times daily collapsed and died after 8 weeks of treatment. Adverse reactions included pallor, anorexia, dehydration, jaundice, bleeding diathesis, and anemia. The most frequently reported adverse reactions included mild, transient, self-limiting vomiting, lethargy, and anorexia. Foreign Market Experience: The following events were reported voluntarily during post-approval use of methimazole in foreign markets: facial pruritus, self-induced excoriations of the head and neck, generalized lymphadenopathy, thrombocytopenia, hematemesis, epistaxis, and elevation of serum liver enzymes and bilirubin. If overdosage occurs, stop treatment and give symptomatic and supportive care. POST APPROVAL EXPERIENCE (revised 2015): The following adverse events are based on voluntary, post approval reporting. Not all adverse events are reported to FDA/CVM. It is not always possible to reliably estimate the adverse event frequency or establish a causal relationship to product exposure using these data. The following adverse events are listed in decreasing order of reporting frequency: Anorexia, vomiting, head/facial pruritus or edema, depression/lethargy, weight loss, anemia, elevated liver enzymes, skin lesions, elevated BUN, diarrhea, and thrombocytopenia. In some reported cases, the patients recovered after adverse signs were recognized, the drug was withdrawn, and veterinary care was applied. In some cases, death (or euthanasia) has been reported as an outcome of the adverse reactions listed above.

Description

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. The chemical formula is C 4 H 6 N 2 S. Molecular weight is 114.16. Methimazole Chemical Structure: image description

Information for Owners

This summary contains important information about Felanorm. You should read this information before starting your cat on Felanorm and review it each time the prescription is refilled. This sheet is provided only as a summary and does not take the place of instructions from your veterinarian. Talk to your veterinarian if you do not understand any of this information or if you want to know more about Felanorm. What is Felanorm? Felanorm is a prescription oral solution containing methimazole and is used for the treatment of hyperthyroidism in cats. Hyperthyroidism occurs when the thyroid gland over-produces thyroid hormone and is a lifelong condition that requires treatment. Felanorm reduces the production of thyroid hormone. Common signs of hyperthyroidism include weight loss, and an increase in appetite, thirst, and urination. Other signs may include vomiting, diarrhea, hyperactivity, and unkempt coat. What kind of results can I expect when my cat takes Felanorm for the control of hyperthyroidism? Felanorm controls hyperthyroidism but does not cure the disease. It may take several weeks for the medication to normalize the thyroid hormone levels. Improvement in signs may include: weight gain or cessation of weight loss, a decrease in excessive appetite, water consumption, and urination, and a decrease in vomiting, diarrhea, and hyperactivity. Your veterinarian will evaluate your cat several weeks after starting treatment and on a regular basis thereafter to determine if dose adjustments are needed. Dose adjustment should only be carried out in consultation with your veterinarian. What cats should not take Felanorm? Your cat should not be given Felanorm if s/he: Is hypersensitive to methimazole Has been diagnosed with primary liver disease or kidney failure Has been diagnosed with an autoimmune disease Has been diagnosed with a blood disorder (such as anemia or low blood cell counts) or a clotting disorder Is pregnant or nursing What to discuss with your veterinarian before giving Felanorm to your cat. Tell your veterinarian about: Any serious disease or health conditions your cat has had Any previous treatments for hyperthyroidism, including methimazole treatment or radioactive iodine, and any side effects related to those treatments Any allergies that your cat has now or has had All medications that you are giving your cat or plan to give your cat, including those you can get without prescription (over-the-counter) and any dietary supplements If you plan to breed your cat, or if your cat is pregnant or nursing Talk to your veterinarian about: What tests might be done before Felanorm is prescribed The potential side effects your cat may experience while taking Felanorm How often your cat may need to be examined by your veterinarian The risks and benefits of using Felanorm, including human user safety risks What are the possible side effects that may occur in my cat during therapy with Felanorm? Felanorm, like all other drugs, may cause side effects in individual cats. These are normally mild, but serious side effects have been reported in cats taking methimazole. Serious side effects can, in rare situations, result in death. It is important to stop the medication and contact your veterinarian immediately if you think your cat may have a medical problem or side effect while on Felanorm. Possible side effects include loss of appetite, lethargy, vomiting, diarrhea/loose stool, head and facial itchiness or swelling, skin lesions, weight loss, elevated liver enzymes, elevated kidney enzymes, abnormal vocalization, and low counts of red cells, white cells, and platelets. Methimazole may cause your cat to bleed or bruise more easily. In some cases, patients recovered after side effects from methimazole were recognized, methimazole was withdrawn, and veterinary care was administered. In some cases, death (or euthanasia) was the outcome of the side effects. To report suspected adverse drug events, for technical assistance or to obtain a copy of the Safety Data Sheet (SDS), contact Virbac at 1-800-338-3659. For additional information about adverse drug experience reporting for animal drugs, contact FDA at 1-888-FDA-VETS or http://www.fda.gov/reportanimalae. Felanorm should only be given to cats. People should not take Felanorm. Keep Felanorm and all medication out of reach of children. Call your physician immediately if you accidentally swallow Felanorm. How to give Felanorm to your cat. Felanorm should be given as prescribed by your veterinarian and according to your veterinarian's instructions. Your veterinarian will tell you what amount of Felanorm is right for your cat. Whenever possible, you should administer Felanorm on a consistent schedule with regard to time of day. Do not change the way you give Felanorm to your cat without first speaking with your veterinarian. If a dose is missed, resume dosing based on your cat's regular dosing schedule. Do not give double the dose the next time. If your cat is due to have a monitoring blood test within a couple of days of missing a dose, make sure you tell your veterinarian as it could affect the blood results. Advice on Correct Administration. See Instructions for Preparing a Dose of Felanorm (methimazole) Oral Solution . How to Store Felanorm. Felanorm should be stored in the original container at room temperature (77°F), however temperatures between 59°-86°F are permitted. Special precautions to be taken when administering Felanorm. Methimazole can cause or increase the risk of birth defects in human babies. Always wear protective single use, impermeable (e.g., latex or nitrile) gloves when administering Felanorm or to prevent direct contact with litter, feces, urine, or vomit of treated cats. Wash your hands with soap and water after administration of Felanorm or after contact with the litter of treated cats to avoid exposure to Felanorm. This is especially important if you are pregnant, become pregnant, are nursing, or have an endocrine disorder that could be impacted by Felanorm. Avoid skin and oral exposure, including hand-to-mouth contact. Wash any spillages or splatter from the skin immediately. Do not eat, drink, smoke/vape, or use smokeless tobacco while handling the product or used litter. In the event of accidental ingestion, seek medical advice immediately and show the product label to the physician. Felanorm may cause skin or eye irritation. Avoid eye contact, including hand-to-eye contact. In case of accidental eye contact, rinse eyes immediately with clean running water. If irritation develops, seek medical advice. Can Felanorm be given with other medications? Interactions with certain medications are possible. Always tell your veterinarian about all medications, including those you can get without a prescription (over-the-counter), that you have given your cat in the past and all medications that you are planning to give with Felanorm. What can I do if my cat gets more than the prescribed amount of Felanorm? Contact your veterinarian immediately if your cat gets more than the prescribed amount of Felanorm. What else should I know about Felanorm? This sheet provides a summary of information about Felanorm. If you have any questions or concerns about Felanorm or hyperthyroidism in cats, talk to your veterinarian. As with all prescribed medications, Felanorm should only be given to the cat and condition for which it was prescribed, at the prescribed dose, and as directed by your veterinarian. Principal Display Panel Felanorm ® (methimazole) Oral Solution 5 mg of methimazole/mL For oral use in cats only CAUTION: Federal law restricts this drug to use by or on the order of a licensed veterinarian. Approved by FDA under ANADA # 200-771 image description image description

Storage

Store at controlled room temperature 25°C (77°F) with excursions between 15°-30°C (59°-86°F) permitted. Keep the container tightly closed.

Other labels for this ingredient

These labels are also stored for the same active ingredient. They are not identical to the one reprinted above, and each is listed by product and set id rather than merged with it.

• Methimazole Coated Tablets (methimazole tablets) For oral use in cats only., FELIX PHARMACEUTICALS PRIVATE LIMITED. Set id 6139b0d2-d947-4a56-9c09-b5cff3345127.
• Methimazole Coated Tablets (methimazole tablets) For oral use in cats only., BUTLER ANIMAL HEALTH SUPPLY, LLC DBA COVETRUS NORTH AMERICA. Set id 76d06942-9677-4fa2-bf4c-276e28409eab.
• Felimazole ® Coated Tablets (methimazole tablets), Dechra Veterinary Products. Set id 2d70b747-314c-4baa-95c4-1734855a1556.
• ReguZol Coated Tablets (methimazole tablets) For oral use in cats only., MWI/Vetone. Set id 63afd6cd-0909-48f9-90fe-9a16af613022.

FDA adverse event reports

The FDA Center for Veterinary Medicine publishes the adverse event reports it receives through openFDA. The counts below are the reports naming methimazole as an active ingredient for each species this label names, as returned on 2026-09-16, coded by the VeDDRA reaction term the reporter chose. Reactions are counted per report, so one report can appear under several terms.

These are counts, not rates. Reporting is voluntary, the number of animals treated is not known, and a widely used product accumulates reports for that reason alone. A report records that an event followed a dose; it does not establish that the product caused it. The counts say nothing about how one product compares with another.