Xanax is a mild tranquilizer used to treat anxiety disorders and causes an over all relaxation to the body. Its intention is to relax the nervous system and relieve panic attacks and other symptoms of anxiety when they occur. One thing Xanax does not do is cause the pupils to dilate. Usually opiates and pain killers are notorious for causing the pupils to dilate.
Xanax has a completely different set of side effects, none of which cause the pupils to dilate. Some of the side effects are dizziness, withdrawal symptoms when stopping the medication, allergic reaction, changes in appetite, confusion, drowsiness and dry mouth. These are just a few of the side effects possible. Some side effects are more serious than others. As with any drug, you should notify your physician of any and all side effects.
Pupil dilation is caused by an increase in heart rate, among other reasons. This can be drug induced, eye-drop induced and light induced. Xanax has a calming effect. Therefore, the theory that Xanax dilates pupils is unfounded.