The contrast that they usually use is called Cystoconray or iothalmate meglumine. The exam is not typically painful at all only uncomfortable. Only about 1 percent of the dose of the contrast used is actually absorbed by the body the rest is voided out during urination. The amount absorbed is pretty negligible and it would be very unlikely that it would cause any damage.
There are currently no studies that I know of linking iothalmate meglumine to cancer. Just like any drug, it can have side effects, but usually it only causes problems if a patient ends up being allergic to it. You'd be more likely to get cancer from the radiation from the x-rays during the exam and the dose you get from that is minimal. You probably are exposed to more background radiation in your life than you are ever exposed to during medical imaging.