Emedastine is a histamine H receptor antagonist (K = 1.3 nM). It is selective for histamine H over H and H receptors (Ks = 49 and 12.43 µM, respectively), as well as α-, α-, and β-adrenergic and dopamine D1 and D receptors, and the serotonin (5-HT) receptor subtypes 5-HT and 5-HT at 10 µM. Emedastine inhibits histamine-induced phosphoinositide turnover and intracellular calcium mobilization in primary human conjunctival epithelial cells (HCECs; ICs = 1.6 and 2.9 nM, respectively). It also inhibits histamine-stimulated secretion of IL-6, IL-8, and GM-CSF by primary HCECs (ICs = 2.23, 3.42, and 1.50 nM, respectively). Ocular application of emedastine prior to histamine challenge inhibits vascular permeability in guinea pigs. Formulations containing emedastine have been used in the treatment of allergic conjunctivitis.