The label says, word for word:
Felimazole Coated Tablets (methimazole tablets) are indicated for the treatment of hyperthyroidism in cats.
Do not use in cats with primary liver disease or renal failure. Do not use in cats with hematological disorders (such as anemia, neutropenia, lymphopenia, or thrombocytopenia) or coagulopathies. Do not use in pregnant or lactating queens.
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.
Wear protective gloves to prevent direct contact with litter, feces, urine, or vomit of treated cats, and broken or moistened tablets. 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 tablets, litter or bodily fluids of treated cats.
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.
Methimazole has anti-vitamin K activity and may induce bleeding diathesis without evidence of thrombocytopenia.
Every approved medicine has a field study behind it, and the label prints what happened:
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.
Approved by FDA under NADA # 141-292; original approval May 27, 2009 (FOI summary; sponsor Dechra, Ltd.). Label revised November 2021. The label gives no percentages for the US field study. Additional contraindications on the label, verbatim: "Do not use in cats with hypersensitivity to methimazole, carbimazole or the excipient, polyethylene glycol." and "Do not use in cats with autoimmune disease." Other label precautions: "Use of Felimazole Coated Tablets in cats with renal dysfunction should be carefully evaluated."
The emergency page sorts the signs by how long you have, and the symptom checker asks what you are seeing. Reactions to a medicine can also be reported to the FDA yourself: how to report a problem.
These answers come from the approved label on DailyMed, read on 2 October 2026. Where the label does not say, the answer says so, and the vet or the pharmacist is the one to ask.
The approved use is quoted at the top of this page. It is a coated tablet for cats with an overactive thyroid gland, which the label calls hyperthyroidism.
The label gives no time for when a cat's signs should ease. It says the level of the drug in the blood peaks within 1 to 1.5 hours of a tablet. It has the vet check the thyroid hormone level in the blood after 3 weeks of treatment and adjust the dose from the result. In the US field study on the label, the investigators judged 91.8% of the cats to be clinically improved on day 21 and 87.6% on day 42.
The label sets a starting dose, and it has the vet adjust the dose after 3 weeks from the blood tests and the cat's response. It does not say whether to give the tablets with food. Its pharmacology section says they are absorbed better on an empty stomach, so ask the vet which is right for your cat. The label says not to break or crush the tablets, and to wash your hands with soap and water after giving one. It also says to wear protective gloves when you handle the litter, feces, urine or vomit of a treated cat, or a broken or damp tablet, and to wash your hands after contact with a treated cat's litter. Women who are pregnant or may become pregnant, and nursing mothers, should wear gloves when handling the tablets, the litter or the body fluids of a treated cat. Blood tests come with this medicine: the label describes a blood count, a chemistry panel and a thyroid hormone level before treatment starts, after 3 weeks, after 6 weeks and then every 3 months. The blood count and the chemistry panel are explained on the page on blood tests.
The label does not say what to do about a missed dose. Call the clinic or the pharmacy before you give the next one.
The studies and the warnings are quoted above. The label tells the vet that if a cat becomes ill while on Felimazole, the drug should be stopped and blood tests done. So if your cat seems ill, call the vet straight away, before the next tablet. The signs of illness the label lists are not eating, vomiting, itching or swelling of the head or face, depression or lethargy, weight loss, anemia, sores on the skin, diarrhea, fever and swollen lymph nodes.
Store it at room temperature, 77°F; the label allows 59° to 86°F. Keep the container tightly closed to protect the tablets from moisture. Keep it out of the reach of children.
The FDA says the best way to get rid of most unused or expired medicines is a take-back option: a drug take-back location, which can be at a pharmacy or a police station, or a prepaid mail-back envelope. If you cannot use one and the medicine is not on the FDA's flush list, the FDA says to mix it, uncrushed, with something unappealing such as dirt, cat litter or used coffee grounds and to seal the mixture in a plastic bag before throwing it away. Flushing is only for medicines on that list, such as opioids. Do not give leftovers to another pet.
Write the dose, the times and the stop date here, then print this page for the fridge.
Dose and times: _____________________________________________
Stop date: _____________________________________________