Does alliance's atarax (hydroxyzine) actually cause weight gain as one of the side effects? If so why? And after how long of using it?

Yes it causes wt gai. Yes atarax(hydroxyzine) can cause weight gain and it starts in first month and as follows <1 month 27.7% 1-6 months 27.7% 6-12 months 9.09% 1-2 years0.00% 2-5 years 9.09% 5-10 years27.27% >10 years 0.00.