Antihistamine Toxicosis

Antihistamines are H1-receptor antagonists widely used in human and veterinary medicine to provide symptomatic relief of allergic signs caused by histamine release, including pruritus and anaphylactic reactions. Antihistamines are also commonly used as sedatives and antiemetics. 1 Antihistamines can be divided into first and second-generation (also called nonsedating) agents (Table 1). First-generation antihistamines are small lipophilic molecules, so they may cause adverse effects because of their cholinergic activity and their ability to cross the blood-brain barrier. Second-generation antihistamines are more lipophobic than first-generation antihistamines and are thought to lack central nervous system (CNS) and cholinergic effects when given at therapeutic doses. 2 Antihistamines are often found in combination with other ingredients in many over-the-counter cold, sinus, and allergy medications (Table 2); liquid products sometimes contain large amounts of alcohol as well.