Clemastine fumarate is the fumarate salt of clemastine, a first-generation ethanolamine H1-antihistamine. It is a white to off-white crystalline powder with the molecular formula C25H30ClNO5. The compound is a potent and selective antagonist of the histamine H1 receptor. Its primary application is as an active pharmaceutical ingredient (API) in the treatment of allergic conditions, including allergic rhinitis, urticaria (hives), and other allergic skin reactions. It functions by competitively inhibiting histamine at the H1-receptor sites, thereby alleviating symptoms such as sneezing, itching, watery eyes, and runny nose. Due to its ability to cross the blood-brain barrier, it also possesses significant sedative and anticholinergic side effects, categorizing it as a sedating antihistamine. Clemastine fumarate is formulated into various dosage forms, most commonly oral tablets and syrups. It is available as a prescription or over-the-counter medication in many regions. Important safety considerations include its potential to cause drowsiness, impair mental alertness, and interact with central nervous system depressants like alcohol.