Ketotifen is an antihistamine and mast cell stabilizer that works by blocking histamine, a substance that causes allergic reactions. Ketotifen is a potent antagonist of H1 histamine receptors, which contributes to its anti-allergic activity. Ketotifen is used to prevent itching of the eye due to allergic conjunctivitis. It can also be used as an add-on treatment during the build-up phase of oral immunotherapy to avoid mild allergic reactions.