Decongestants are commonly used in people to treat allergic rhinitis, but they are rarely used for this purpose in animals. The α-adrenergic agonist drugs cause local vasoconstriction in mucous membranes, which reduces swelling and edema. They are used topically as nasal decongestants in allergic and viral rhinitis, or systemically in combination with antihistamines as respiratory tract decongestants. Antihistamines are effective for treatment of allergic rhinitis in people when combined with the α-adrenergic agonist drugs, but their effectiveness in animals has not been demonstrated. The topical α-adrenergic agonist drugs act within minutes with few adverse effects, but extended use may cause rebound hyperemia and mucosal damage. Systemic administration can result in hypertension, cardiac stimulation, urinary retention, CNS stimulation, and mydriasis. Systemic administration of antihistamines often causes sedation.