Antihistamines agents compete with histamines for H1 receptor sites. Most agents also have anticholinergic properties. Cyproheptadine also blocks the serotonin effects which lead to increased appetite. Antihistamine includes astemizole, azatadine, cetirizine, chlorpheniramine, clemastine, cyproheptadine, desloratidine, dimenhydrinate, diphenhydramine, fexofenadine, hydroxyzine, levocetirizine, loratidine, meclizine, and promethazine. Uses, dosage, brands, and side effects of all these drugs are also covered in this chapter. Side effects of antihistamine are drowsiness, headache, thickening of respiratory secretions, urinary retention, GI discomfort, dry mouth, and blurred vision.