Can i take zyrtec and claritin together? or can i take zyrtec and benadryl (diphenhydramine) to boost the zyrtec? will taking mucinex dm along with zyrtec help?

See your doctor: If your allergies are not controlled with one med alone, stepping up to a different med like flonase, nasonex, etc., will likely help you more than taking two antihistamines. Zyrtec and Claritin will not help much at all if you have a cold and not allergies, though Benadryl (diphenhydramine) can dry up secretions somewhat. You can add Mucinex if you also have a cough. Talk with your doctor about your symptoms.