Antihistamine / alpha-1 agonist combinations are used to treat allergies, cold symptoms, and nasal congestion. These medications work by combining an antihistamine, which blocks the effects of histamine to reduce allergy symptoms, with an alpha-1 agonist, which narrows blood vessels in and around the nose to decrease nasal congestion. You can find these medications in different forms, including oral tablets and syrups. Examples include diphenhydramine / phenylephrine (Benadryl Plus Congestion) and fexofenadine / pseduoephedrine (Allegra-D). These options allow you to choose the form that best fits your needs.