(S)-Azelastine hydrochloride, with CAS number 153408-27-6, is the pharmacologically active (S)-enantiomer of the antihistamine drug azelastine. It is a white to off-white crystalline powder. The compound is a phthalazinone derivative with a complex structure featuring a chlorobenzyl group and a hexahydro-1-methyl-1H-azepine moiety. This compound is the active pharmaceutical ingredient (API) in various formulations, primarily used as a second-generation H1-receptor antagonist. Its main application is in the treatment of allergic conditions, particularly allergic rhinitis (both seasonal and perennial). It is administered via nasal spray or eye drops. Beyond its potent antihistaminic activity, (S)-azelastine hydrochloride also exhibits mast cell stabilizing and anti-inflammatory properties, contributing to its efficacy. As a single-enantiomer drug, it offers potential advantages in terms of potency and reduced side-effect profile compared to the racemic mixture. It is a key component in prescription medications and is subject to strict pharmaceutical quality control standards for API manufacturing.