•The H1 histamine receptor plays an important role in allergic response and is widely distributed throughout the peripheral nervous system, particularly the smooth muscle, where its activation causes vasoconstriction. Activation of the H1 receptor also causes blood vessel dilation, increased vessel permeability, stimulation of sensory nerves in the airways and bronchoconstriction. In addition, activation of this receptor promotes the chemotaxis of eosinophils, which can lead to nasal congestion, sneezing and rhinorrhea. Once activated in the cerebral cortex, the H1 receptor can inhibit potassium channels in neuronal cell membranes, depolarizing the neurons and increasing neuronal excitation.