What should I know when choosing an OTC allergy medication?

Claritin (loratadine) The following work for most people. . . Non-sedating products over the counter that help with sneezing, itchy/watery eyes: claritin, zyrtec, allegra. Benadryl (diphenhydramine) works well, but can cause drowsiness. Any medication that says "d" (contains a decongestant) can raise blood pressure and should be avoided for most people. Try Mucinex plain, nasal saline, and Vicks vapor rub for congestion.
OTC antihistamine. Know that Claritin (loratadine) and zyrtec can cause some drowsiness, whereas Allegra does not cross blood brain barrier (no drowsy). Claritin (loratadine) is the weakest binding antihistamine with zyrtec being the strongest. The generics are similar in strength so brand name is unnecessary. Lastly, you really should see an allergist for testing since an otc antihistamine might not be best.