antihistamines

Antihistamines are medicines that block the effects of histamine, a chemical released during allergic reactions. Most antihistamines are antagonists of the histamine H1 receptor, and they help relieve common allergy symptoms such as sneezing, itching, runny nose, and conjunctival irritation. They may also reduce itching from urticaria and, in some formulations, provide antiemetic or anti-nausea effects.

There are two general generations of H1 antihistamines. First-generation agents, such as diphenhydramine, chlorpheniramine, and promethazine, cross the blood–brain barrier and commonly cause drowsiness and anticholinergic effects. Second-generation agents, including cetirizine, loratadine, fexofenadine, and desloratadine, are less sedating and generally have fewer central nervous system effects, partly due to lower lipophilicity and active efflux at the blood–brain barrier.

Antihistamines are used for allergic rhinitis, itchy eyes, skin urticaria, and other allergic dermatoses. Some formulations are also used for motion sickness, nausea, and sleep aid, particularly first-generation drugs. They can be taken orally or via topical, intranasal, or ophthalmic routes, depending on the product.

Side effects vary by generation. Drowsiness, dry mouth, dizziness, and blurred vision are common with first-generation agents. Anticholinergic effects may be problematic for older adults and people with glaucoma or urinary retention. Interactions with alcohol and other central nervous system depressants can amplify sedation. Safety in pregnancy and breastfeeding varies by drug; some are considered safer than others.

Antihistamines are not a substitute for epinephrine in severe allergic reactions and are generally not sufficient treatment for anaphylaxis or asthma control. They are often used as part of a broader allergy management plan.