Hi Pat,   Periactin (a brand of cyproheptadine) is an antihistamine that has been around for several decades. It was previously used for allergy symptoms but there are many better medications available now. Periactin is also used to treat migraine disorder and Cushing Disease (abnormal production of corticosteroid hormone).   Increased appetite and weight gain is a possible side effect of periactin. Sedation and fatigue are common side effects. As you know, a prescription is required for this medication.   My advice for you is to call your doctor and share with him/her your concerns about weight gain. There may be other alternatives for you. You may warrant consultation by a specialist depending on your underlying problem.   Good Luck!   J. Thompson, MD