/reference/drugs/antihistamines

Antihistamines

Diphenhydramine, famotidine, cetirizine

Diphenhydramine

First-generation H1 receptor antagonist

Competitive H1 antagonist with significant anticholinergic + sedating activity from CNS penetration. Crosses BBB freely.

Famotidine

Pepcid

H2 receptor antagonist

Reversible competitive antagonist at parietal cell H2 receptors → reduces basal + stimulated gastric acid secretion. Raises gastric pH within 30 min.

Cetirizine

Zyrtec

Second-generation H1 receptor antagonist (peripheral)

Selective peripheral H1 antagonist. Carboxylic acid structure → poor BBB penetration → minimal sedation compared with first-generation antihistamines like diphenhydramine.

Education only — not a substitute for current package inserts or clinical judgment. Always confirm doses.