Antihistamines and Allergy Drugs

Key points

• • Histamine is released from mast cells in allergy and acts on H1 receptors to cause itching, swelling, redness and narrowed airways.
• • H1-antihistamines block these receptors and relieve hay fever, hives and itching.
• • The older H1-antihistamines such as chlorphenamine cross into the brain and cause drowsiness, while newer ones such as cetirizine cause little.
• • H2-antihistamines block histamine's action on the stomach to reduce acid, and are used in acid-related disease.
• • Antihistamines do not replace adrenaline in a severe allergic reaction (anaphylaxis), which needs adrenaline.