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.
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.