An antithyroid agent is any pharmaceutical that prevents the formation of thyroid hormones. They are commonly prescribed for patients with hyperthyroidism, including Graves' disease.

The drugs do have a number of side effects. One of the most dangerous is a marked reduction in some types or all types of white blood cells. Aplastic anemia is also fairly common. However, these usually resolve themselves if the drug is discontinued.

Antithyroid agent at Wikipedia