Well, the worst thing that it can do to you is kill you.
Or it can damage your heart, cause delusions or brain damage, make you wet yourself (lose bladder control) even hours after taking it, cause nightmares, bedtime sweats that are so bad you look like you dumped gallons of water on your sheets, make you throw up violently, cause dizziness, diahreha, and just only a whole lot of other worse things.
But go ahead and take the stolen medicine for soem hopes of a fake high. I'm sure your parents and friends will get a good laugh out of you wetting your pants in public, or at the worst you'll just break their hearts and end up dying like some idiot child who took medicine that wasn't prescribed to him by a doctor.
P.S. Most anti-depressants also lower your sex drive, and make it so your penis won't get hard. Just consider it another joke for your gym friends to laugh at.
P.S.S. Xanax is made to be started on a very low dose, and the REAL patient has to build up to taking higher doses. This is for their own safety and health.. but hopefully you don't have a high dose

just think of it as a surprise if you do decide to take it (which by the way is illegal).