Clarityn contains the active ingredient loratadine, which is a type of medicine called a non-sedating antihistamine. It works by preventing the actions of histamine.
Histamine is a substance produced by the body as part of its defense mechanisms. It is stored in cells called mast cells, in almost all tissues of the body. When the body reacts to a foreign substance (known as an allergen, e.g. flower pollen, dust mites), the mast cells stimulated by the allergen release their stores of histamine.