Antazoline is an anti histamine of the ethylenediamine class, which are selective H1-antagonist. Effects mediated by H1 receptors include the contraction of smooth muscle and the dilation and increased permeability of the capillaries. Tetryzoline is a sympathomimetic with alpha adrenergic activity. Its vasoconstrictive effect reduces redness and oedema in allergic conjunctivitis. Antihistamines, which act by blocking the H1 histamine receptor, are highly effective in providing relief of itching but are not very active in relieving the associated redness. The use of products combining an anti-histamine and a vasoconstrictor is well established in the symptomatic relief of allergic eye disease.