Xanax is used to treat different types of anxiety. Most side effects subside after a while, and it important to give the medication a chance for at least a few weeks before switching to something else.
The most common side effects are drowsiness and light headedness, though some patients also report headaches, confusion and insomnia. Both weight gain and weight loss were listed as possible side effects as well, so I guess it all depends on your eating/exercising habits. You can read more about what to expect here.