Side effects tend to occur in the first three months of treatment. If you observe any of the effects listed below, contact your veterinarian immediately.
May see loss of appetite, vomiting, or lethargy. These usually occur within the first two weeks of treatment and may stop even with continuation of treatment.
Less common side effects include liver problems resulting in the above signs plus yellowing of the gums, skin, or eyes; itching of the face, resulting in scratching; or bleeding tendencies. Your veterinarian will determine if these effects necessitate stopping the medication and treating with surgery or radioactive iodine therapy.
Treatment with Methimazole may unmask hidden kidney disease.
Rarely, you may see the development of myasthenia gravis, a neuromuscular disease that severely weakens muscles and may cause difficulty swallowing. If your pet is unusually tired, has a fever (temperature over 103°F), or shows signs of bruising or bleeding, contact your veterinarian immediately.
If your pet experiences an allergic reaction to the medication, signs may include facial swelling, hives, scratching, and sudden onset of diarrhea, vomiting, shock, seizures, pale gums, cold limbs, or coma. If you observe any of these signs, contact your veterinarian immediately.