Can a non drowsy antihistamine that had no side effects that I've noticed hurt you if u take 2 pills in 24hrs instead of 2?

No. Which antihistamine? Why using it? Sometimes, for chronic urticaria, cetirizine [Zyrtec] is dosed at 3 a day . The official dose of these is 1 a day. Allegra and claritin are also sometimes used at double or triple dose.It is best if you consult a Dr before you take doses higher than usual.