“What is the strongest and most effective antihistamine?”

I am sorry but there is no one answer to that. Every persons responds differently to antihistamines and what works for one may not work for another. I would suggest trying the OTC ones - Claritin, Zyrtec, Allegra, Zyrtec or Xyzal and see which works the best for you Good luck  Monika Korff, MDFACAAIoffice-410-730-6000www.allergyasthmamaryland.com