Antihistamines relieve allergic symptoms by blocking the actions of histamine, a chemical released by the immune system in response to allergies or tissue injury.

They compete with histamines and block their binding at the histaminic H1-receptor sites, reducing inflammation and swelling of the local tissues.

Antihistamines can be classified into first-generation and second-generation agents.

First-generation drugs block histamines at central and peripheral H1 receptors and acetylcholine at neuronal and neuromuscular muscarinic receptors. They can also cross the blood-brain barrier into the central nervous system, leading to potential CNS depression and drowsiness.

In contrast, second-generation drugs selectively bind to peripheral H1 receptors. Because they are designed to have reduced CNS penetration, they result in less sedation.

Typically taken orally, antihistamines can also be administered via intravenous and intramuscular routes.

Although considered safe, certain antihistamines may lead to CNS side effects such as fatigue, dry mouth, and dizziness.