My wife found out the hard way that she's allergic to high-contrast dye today. She went in for an X-ray of her bladder, and wound up with the ER team taking care of her. She said that she started sneezing right after she got the dye, and then had problems with her throat. After a shot of anti-histamine, her blood pressure still wasn't going down, and the doc called for the ER team to respond. They elevated her feet, and after a few minutes more, the drugs did their job, and she started to return to normal, so she was dicharged after a few hours of observation. She's okay now, is at home resting, after taking the rest of the day off work. They hit her with benadryl to stop the reaction, and that's why she's so drowsy. I wasn't there when it happened, it was only supposed to be a simple X-ray after all, but she said she thought she was going to die for a bit there.

For future reference, if you're having a similar procedure done:

Allergy to iodinated contrast (dye) may take the form of hives, skin rash, itching, running nose, sneezing, bronchospasm, wheezing, laryngeal edema, pulmonary edema, as well as more severe life threatening reactions. The incidence of allergic reaction has decreased considerably with the advent on the newer non-ionic contrast agents.

If there is a pre-existing allergy to contrast material (dye), the patient may be pre-treated with Benadryl and Prednisone and then given non-ionic IV contrast.