Contrast in a CT scan refers to a substance taken by mouth or injected into an intravenous (IV) line that causes the particular organ or tissue under study to be seen more clearly. This substance, also known as contrast material, helps distinguish or "contrast" selected areas of the body from surrounding tissue, which improves the visibility of specific organs, blood vessels, or tissues. The contrast material blocks X-rays and appears white on images, which can help emphasize blood vessels, intestines, or other structures. It can be given to the patient by mouth or through an injection into a vein in the arm to help certain body parts stand out on the images. The contrast material can be iodine-based or barium-sulfate compounds, and it is used to temporarily change the way X-rays or other imaging tools interact with the body. While contrast materials are generally safe, adverse reactions ranging from mild to severe can occur, but severe reactions are very uncommon. If a patient needs contrast material for a CT examination, it can be given by mouth (oral contrast) or through an intravenous (IV) line. The use of contrast material in CT scans can vary depending on the specific type of examination, such as a double contrast study that may require the patient to drink a contrast media before the exam begins in addition to the IV contrast.