Contrast medium is a substance given by mouth, injection, rectum or stoma (a surgically-created opening on the surface of the abdomen) during an imaging procedure. It highlights certain areas of the body, making results more readable. Some people may experience an allergic reaction to contrast medium, so it's important to tell your healthcare provider if you have any allergies, particularly to iodine. An itching sensation or shortness of breath may mean an allergic reaction. Other potential reactions include flushing, nausea, headache or a salty taste in the mouth.