Is there a good allergy medication similar to xyzal (levocetirizine) that doesn't cause weight gain?

Many: All of the antihistamines are similar, but different ones may work better for different people. Clarinex (desloratadine) is the only other second-generation antihistamine still available by prescription; cetirizine, fexofenadine and loratidine are all available over the counter. It is unusual for antihistamines themselves to cause weight gain, though - you should make sure that nothing else is going on.