I am aware that the person who posted this question most likely already had their surgery - and I sincerely hope that the urologist didn't decide on taking him off the Xanax. But other people out there might have the same question and I am very happy to provide this information.
As a Specialist in Anesthesiology I see these things all too often. Unless the drug interferes with the surgery itself, it should not be withheld because of the damage withdrawal syndrome and rebound effects can cause! Surgeons often don't understand the implications such a decision can have on the individual patient. Because they often focus only on the aspects of the surgery they loose track of "the bigger picture".
Seriously, I cannot think of one single indication that Xanax should be withheld because of surgery. It will not interfere with the anesthetic, general or spinal. On the contrary - us anesthesiologists might actually cancel or postpone the surgery if the patient is in actute withdrawal. We always titrate an anesthetic to effect, so even if the patient needs a slightly higher dose for the induction of anesthesia, we will still be able to provide a quality anesthetic under highest safety standards.
If your surgeon insists on stopping a benzodiazepine before surgery, have him or her explain exactly why you should and ask to speak with the anesthesiologist as well.
Hope this helps!
Chris Bonney, MDhttp://www.feelgoodaftersurgery.com