Clarinase contains a combination of two medicines loratadine(an antihistamine) and pseudoephedrine (a decongestant). Antihistamines help reduce allergic symptoms by preventingt he effects of a substance called histamine. Histamine is produced by the body in response to foreign substances which the body is allergic to. Decongestants produce narrowing of blood vessels. This leads to clearing of the stuffy or blocked nose.