Claritin etc. (loratadine) is a second-generation long-acting antihistamine which is sometimes used to treat small animal allergy conditions.
Second generation antihistamines (SGA's) like Loratadine produce fewer negative side effects than the older, first generation antihistamines (FGA's) like diphenhydramine and brompheniramine. Loratadine blocks one type of receptor for histamine (the H1 receptor) and thus prevents activation of cells by histamine.
Loratadine is less likely to cause drowsiness as it does not enter the brain from the blood unlike other antihistamines. Note - products that contain loratadine and pseudoephedrine such as Claritin-D, should not be used in animals, because pseudoephedrine can be toxic.