Antihistamines are drugs that block the effects of histamine at its receptors, with H1 antagonists used primarily for allergic conditions and H2 antagonists for gastric acid suppression. Histamine is a biogenic amine involved in allergic inflammation, gastric acid secretion, and neurotransmission, and the development of selective receptor antagonists has yielded clinically important therapeutic agents.

What Are Antihistamines?

Histamine exerts its effects through four receptor subtypes, of which H1 and H2 are the most clinically relevant. H1 receptors mediate allergic and inflammatory responses including vasodilation, bronchoconstriction, and pruritus. H2 receptors are predominantly located on gastric parietal cells, where they stimulate acid secretion. Antihistamines are classified by their receptor specificity, which determines their therapeutic applications.

Mechanism of Action

H1 antihistamines are inverse agonists at the H1 receptor, stabilizing it in its inactive conformation and reducing the effects of histamine released from mast cells and basophils during allergic reactions. They effectively relieve sneezing, itching, rhinorrhea, and urticaria but are less effective for bronchoconstriction, which involves additional mediators.

First-generation H1 antihistamines such as diphenhydramine and chlorphenamine are lipophilic and cross the blood-brain barrier, causing sedation and cognitive impairment through central H1 receptor blockade. They also possess anticholinergic properties that can cause dry mouth, blurred vision, and urinary retention. Second-generation H1 antihistamines such as loratadine, cetirizine, and fexofenadine are less lipophilic and have limited central nervous system penetration, producing minimal sedation at therapeutic doses. Some second-generation agents are substrates for P-glycoprotein transport, which further limits brain entry.

H2 receptor antagonists such as ranitidine, famotidine, and cimetidine competitively block H2 receptors on gastric parietal cells, reducing basal and stimulated gastric acid secretion. Cimetidine is notorious for drug interactions through inhibition of CYP450 enzymes, a property less prominent with other H2 antagonists.

Therapeutic Uses

H1 antihistamines are used for allergic rhinitis, conjunctivitis, urticaria, and insect bite reactions, and as adjunctive therapy for anaphylaxis. First-generation agents are effective for motion sickness and insomnia due to their sedative and antiemetic properties. H2 antagonists are used for gastroesophageal reflux disease, peptic ulcer disease, and stress ulcer prophylaxis, though they have been largely replaced by proton pump inhibitors for most indications.

Adverse Effects

First-generation H1 antihistamines cause sedation, cognitive and psychomotor impairment that can affect driving and occupational performance. Anticholinergic effects include dry mouth, blurred vision, constipation, and urinary hesitancy. Second-generation H1 antihistamines are generally well tolerated, with headache and fatigue being the most common complaints. H2 antagonists are well tolerated, though cimetidine can cause gynecomastia and impotence with prolonged use.

Contraindications

H1 antihistamines should be used cautiously in patients with narrow-angle glaucoma, benign prostatic hyperplasia, and urinary retention due to anticholinergic effects. Sedating antihistamines should not be used when alertness is required. H2 antagonists require dose adjustment in renal impairment.

Conclusion

Antihistamines remain important therapeutic agents for allergic disorders and gastric acid suppression. The evolution from sedating first-generation H1 antihistamines to non-sedating second-generation agents has improved tolerability and expanded their utility for long-term management of allergic conditions.