Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Serious and sometimes fatal infections may occur during treatment with methimazole. Stop using  methimazole and call your doctor right away if you have signs of infection such as:
sudden weakness or ill feeling, fever, chills, sore throat, cold or flu symptoms;
painful mouth sores, pain when swallowing, red or swollen gums; or
pale skin, easy bruising, unusual bleeding.
Call your doctor at once if you have:
swollen glands in your neck or jaw; or
liver problems--nausea, upper stomach pain, itching, tiredness, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).
Common side effects may include:
nausea, vomiting, upset stomach;
headache, dizziness, drowsiness;
numbness or tingly feeling;
rash, itching, skin discoloration;
muscle or joint pain;
hair loss; or
decreased sense of taste.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Methimazole side effects (more detail)
