What are the side effects of using antihistamines?

Few. There are no long term adverse effects from using antihistamines. Short term sides effects are typically from older antihistamines such as Benadryl (diphenhydramine) that can cause drowsiness and delayed reaction time. New generation antihistamines such as claritin, clarinex, zyrtec, xyzal and Allegra are generally very well tolerated. No dependence and few if any drug interactions.