Felimazole is a prescription, coated methimazole tablet approved in the United States to treat hyperthyroidism in cats. Methimazole reduces the thyroid’s production of thyroid hormone. The U.S. prescribing information starts treatment at 2.5 mg every 12 hours, then adjusts the dose after three weeks based on the cat’s serum total T4 (TT4) and clinical response. Bloodwork is part of the plan from the start, and the cat’s veterinarian, not the owner, decides every dose and any change to it. Serious blood and liver reactions are possible, so the monitoring schedule matters as much as the tablet itself.
What Felimazole treats and how it works
Hyperthyroidism occurs when the thyroid gland produces too much thyroid hormone. The U.S. Food and Drug Administration (FDA) describes it as the most common endocrine disease in cats older than eight years. Typical signs include increased appetite with weight loss, increased thirst and urination, vomiting, diarrhea, hyperactivity, and a rough, unkempt coat. A veterinarian may suspect the condition from these signs and an enlarged thyroid gland, then confirm it with blood tests that measure thyroid hormone.
Felimazole contains methimazole, an antithyroid drug. It is sold in 2.5 mg and 5 mg coated oral tablets. In the United States, its approved use is treatment of hyperthyroidism in cats, and it may be dispensed only by or on the order of a licensed veterinarian. The FDA’s consumer overview of the two approved hyperthyroidism drugs is at FDA: Hyperthyroidism in Cats, Two FDA-Approved Drugs Available to Treat It.
Labeled starting dose and how it is adjusted
The figures below come from the Dechra U.S. prescribing information (revised November 2021). They are reference information about the label, not instructions. A cat’s actual dose is set by its veterinarian.
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- Starting dose: 2.5 mg every 12 hours. The prescribing information states: “The starting dose of Felimazole Coated Tablets is 2.5 mg administered every 12 hours.”
- First adjustment: after three weeks, the dose is titrated to effect based on that cat’s serum TT4 and clinical response.
- Adjustment step: changes are made in 2.5 mg increments.
- Maximum: 20 mg per day in divided doses, with no more than 10 mg given as a single administration.
Because the dose depends on individual lab values and how the cat looks and behaves, two cats on the same label can need very different amounts. The full prescribing information is available from Dechra: Felimazole U.S. prescribing information, revised November 2021, and the product page is at Dechra: Felimazole Coated Tablets.
Monitoring schedule
Methimazole treatment is not a one-time prescription. The label calls for hematology, biochemistry, and TT4 testing on the following schedule:
| Timing | What the label calls for |
|---|---|
| Before the first dose | Hematology, biochemistry, and TT4 as a baseline |
| Three weeks after starting | Testing and dose adjustment as needed based on TT4 and clinical response |
| Six weeks after starting | Hematology, biochemistry, and TT4 testing |
| Every three months after that | Hematology, biochemistry, and TT4 testing, with adjustment as needed |
| Doses above 10 mg per day | More frequent checks, as the label advises |
The label does not set a specific interval for “more frequent” checks at higher doses, so the veterinarian chooses that timing. FDA guidance similarly calls for a complete blood count, a chemistry panel, thyroid hormone testing, and a discussion of blood pressure monitoring. The FDA overview linked above covers these recommendations.
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Why kidney checks are part of the plan
Hyperthyroidism can increase blood flow to the kidneys, which can mask underlying kidney disease. When thyroid hormone is lowered, kidney function may worsen, or kidney disease that was hidden may become apparent. FDA recommends careful monitoring of cats with kidney disease, along with regular blood tests and urine samples to assess kidney status. This is one reason a cat’s kidney values can change after treatment starts, even when the thyroid is improving.
Side effects and what to do about them
Side effects are described in three groups: those FDA lists as commonly reported with methimazole, those listed in the manufacturer’s product information, and less frequent but serious reactions that the label warns about.
Commonly reported effects
- Decreased appetite (anorexia) and vomiting
- Decreased activity, depression, or lethargy
- Skin reactions, especially itching, hair loss, or lesions around the face, head, and neck
- Head or facial itching or swelling
- Weight loss
- Diarrhea or loose stool
- Behavioral changes
- Elevated liver enzymes, elevated kidney values (BUN), and anemia
The manufacturer’s product information also lists thrombocytopenia (low platelets) among reported effects. The sources do not give incidence percentages for these common effects, so the likelihood for any one cat cannot be stated from them.
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Less frequent but serious reactions
- Liver disease (hepatopathy)
- Immune-mediated anemia
- Thrombocytopenia, which can be linked with a possible bleeding tendency because methimazole has anti-vitamin K activity
- Agranulocytosis, meaning severely low neutrophils, a type of white blood cell
Because these reactions can be serious, bloodwork is the early-warning system. A cat that stops eating, becomes lethargic, bruises or bleeds easily, or shows jaundice needs prompt veterinary attention.
If your cat seems ill while on Felimazole
- Contact your veterinarian promptly. Do not change, skip, or split a dose on your own.
- Describe the signs you have seen, when they began, and the dose and schedule your cat has been receiving.
- Expect that your veterinarian may recommend stopping treatment and running hematology and biochemistry tests. The label says the drug should be stopped and appropriate tests done if a cat becomes ill while receiving it.
- Follow the veterinarian’s instructions for restarting, changing, or switching treatment. Any new plan comes from the veterinarian after reviewing the results.
Who should not receive Felimazole, and interactions
The U.S. label says Felimazole should not be used in cats with hypersensitivity (allergy) to methimazole, carbimazole, or polyethylene glycol. It also advises against use in cats with:
- Primary liver disease or renal failure
- Autoimmune disease
- Hematological disorders, such as anemia, neutropenia, lymphopenia, or thrombocytopenia
- Coagulopathies (clotting disorders)
- Pregnancy or lactation in queens
The manufacturer’s summary also warns against use in pregnant or lactating cats and in cats with renal, hepatic, or hematological disorders. These are label contraindications that a veterinarian must assess for each cat.
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The label also notes possible drug interactions. Anticoagulants may be potentiated, meaning their effect may increase. Concurrent phenobarbital may reduce methimazole’s effectiveness. Doses of some other medicines may need adjustment once the cat becomes euthyroid (thyroid levels back in the normal range). Give your veterinarian a complete list of medications, supplements, and health conditions before treatment starts.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Handling Felimazole safely at home
Felimazole is intended for the cat, but people in the household can be exposed through the tablets, the cat’s litter, feces, urine, or vomit. The label and FDA guidance give the following precautions:
- Wash hands with soap and water after giving the medicine.
- Do not break or crush tablets. The label advises protective gloves when handling broken or moistened tablets.
- Wear disposable protective gloves when cleaning litter or contacting the cat’s bodily fluids, and when handling the medicine. Gloves supplement handwashing; they do not replace it.
- Pregnant people, people who may become pregnant, and nursing mothers should take extra care, because methimazole can harm a developing fetus and is present in breast milk.
- If someone accidentally swallows the medicine, seek medical advice immediately and show the product label to the clinician.
Handling guidance is in the FDA overview of hyperthyroidism treatment in cats, which also summarizes the two approved drug options.
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Other treatment paths and the generic version
FDA notes that oral methimazole may be used lifelong, or used to stabilize a cat before another treatment such as radioactive iodine therapy or surgery to remove the overactive thyroid gland. The sources establish that these paths exist but do not rank them for an individual cat. The table below lists the comparison points to discuss with your veterinarian.
| Question to ask | Ongoing methimazole (Felimazole or generic) | Radioactive iodine or surgery |
|---|---|---|
| Is treatment ongoing? | Yes; FDA notes it may be lifelong | Intended to address the overactive thyroid tissue; the sources do not describe follow-up needs in detail |
| Monitoring required | Scheduled bloodwork and kidney checks, as above | Not stated in the sources reviewed; ask the veterinarian |
| Form of treatment | Prescription oral tablet | Radioactive iodine therapy or surgical removal of the overactive gland |
| Concurrent kidney disease | Requires careful monitoring; lowering thyroid hormone may reveal kidney disease | Not stated in the sources reviewed; ask the veterinarian |
A generic option now exists. As of FDA’s July 22, 2025 announcement, the first FDA-approved generic methimazole coated tablets for feline hyperthyroidism had been approved, after evaluation for bioequivalence to Felimazole. The generic is also prescription-only and requires the same veterinary monitoring. Felimazole is therefore not the only FDA-approved methimazole tablet for cats. The announcement is at FDA: Approval of first generic methimazole coated tablets, July 22, 2025.
What the field study showed
In the U.S. effectiveness field study reported in the Dechra prescribing information (2021), 64 of 105 evaluable cases (61%) met the treatment-success criteria. Success required both a TT4 of 4.0 mcg/dL or lower and clinical improvement documented by the investigator. This is a result for a study population, not a prediction for an individual cat, and it does not indicate how a particular cat’s kidneys, blood counts, or liver will respond.
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