Atarax can make you gain weight. It's not uncommon with antihistamines in general actually.
Why is atarax making me gain weight?
The thing with Atarax and weight gain is that it isn't so much the active ingredients of the drug. It's actually the other side effects that can lead to a likelihood of weight gain.
For example, lack of sleep, constipation, and low blood pressure are all listed as possible side effects - and these can trigger other conditions such as weight gain.
If you're having problems with atarax making you gain weight, I'd suggest trying Benadryl as I switched to that and am in better shape, and a much healthier weight.