What are the chances of having a severe allergic reaction to the cat scan dye they use?

Low: Severe reactions to ct scan dyes are low unless you have a history of a reaction, are on certain medications that work through your kidneys or you are very dehydrated. It is always good to questions why you need the ct scan in the first place as it also exposes you to more radiation. Will the results of the scan change how you are treated?.. And that can outweigh the small risks of the procedure.