You will not gain weight on methimazole, the weight gain is due to weight regain after hyperthyroidism is under control. Methimazole is a thyroid disease medication that can treat hyperthyroidism. Taking methimazole can cause skin itching and rash, leukopenia, hepatitis, gastrointestinal reactions, headache and arthralgia. These are side effects of methimazole and will not cause patients to gain weight. If weight gain occurs, the weight will decrease naturally after stopping the drug, and even if it doesn’t, you can lose weight on your own, by exercising.