Histamine is a biogenic amine present in many tissues that functions as a neurotransmitter and is involved in inflammatory and hypersensitivity reactions. It is synthesized from the amino acid histidine. Histamine acts through multiple receptor subtypes and is involved in various physiological processes like gastric acid secretion, smooth muscle contraction, and allergic responses. Antihistamines competitively inhibit histamine receptors, with first generation antihistamines having sedative effects and second generation ones having minimal side effects. They are used to treat allergic disorders, as antiemetics, and for gastric acid reduction with H2 blockers. Concerns have been raised about impurities in the H2 blocker ranitidine. While H3