Side effects occur in less than 5% of all cases, but can be serious, such as a decreased white blood cell count (agranulocytosis); contact a veterinarian immediately if your pet display signs of a sore throat. Excessive bleeding can also occur as a result of a decrease in blood platelets. Very rarely, liver failure and its ensuing complications have been associated with Methimazole.
More common side effects are skin-related, including rash, itching, hives, abnormal hair loss, and skin pigmentation. Other side effects may include swelling, nausea, vomiting, heartburn, loss of taste, joint or muscle aches, drowsiness, dizziness, numbness, and headache. Do not use in pregnant or nursing animals.
Methimazole must be used only in cats. It should never be used on animals, which are prone to allergy to the drug. Extreme care should be taken while using this medication in animals with a known history of liver impairment, immune-system disease or blood disorders. Regular blood tests to monitor toxicity on blood cells and liver are suggested in cats, which are on methimazole for a long period.The medication should be administered life long as the symptoms might reappear once the medication is stopped. It is advisable to use methimazole for the short term management of the disease as it involves side effects. Surgical removal of the thyroid gland or radiation treatment would be more appropriate for a lasting solution. Pregnant and nursing cats should not be given this drug. Drug overdose might result in blood disorders or liver toxicity.
Do not take with theophylline (Theo-Dur), warfarin (Coumadin), digoxin (Lanoxin), a beta blocker such as atenolol (Tenormin), metoprolol (Lopressor), or propranolol (Inderal). Talk to your vet before starting any serious treatment plan.