How Mizolastine works ? Mizolastine is an antihistaminic medication. It treats allergy symptoms such as itching, swelling, and rashes by blocking the effects of a chemical messenger (histamine) in the body. Common side effects of Mizolastine Headache, Dizziness, Increased appetite, Weakness, Nausea, Dryness in mouth, Abdominal pain, Diarrhea. Mizolastine is used in the treatment of allergic conditions.