What would happen if i took 750 mg of hydroxyzine hcl?

You would be sleepy: Hydroxyzine HCl when marketed as Atarax was studied in patients with chronic urticaria at gradually increasing doses up to 100 mg every 6 hours. I've used it at single night doses of 400 mg in severe urticaria. Hydroxyzine Pamoate(Vistaril) was used as a mild antidepressant at higher doses. I would not recommend that you use 750 mg intentionally since there are better less sedating antihistamines.