With the development of medical technology, more and more tests require the use of contrast agents, such as coronary angiography, CT-enhanced angiography, peripheral angiography, cerebral angiography, and so on. What are the contrast agents used in these examinations? Are contrast agents harmful to the human body? What should I pay attention to after using contrast agents? What kind of people should not use them? Contrast agent, also called contrast agent, is a chemical reagent injected into the body to enhance the image observation effect. The contrast agent has a higher (or lower) density under X-ray, and where the contrast agent fills the body, it can be fully developed. Some human tissue structures that cannot be distinguished on X-ray, such as blood vessels, can be clearly visualized only after the application of contrast. Contrast media can increase the difference between normal and abnormal tissues, allowing doctors to detect and identify some early, small lesions and distinguish the degree of benign and malignant lesions. This is why contrast is so important in the examination. The most commonly used contrast agent in cardiology is iodine contrast agent, which is not particularly harmful for the majority of patients. No special attention is generally needed after the application of contrast agents. Iodine contrast agent is excreted via the kidneys and digestive tract about 24 hours after injection. To speed up the excretion of the contrast agent, it is usually sufficient to drink plenty of water after the examination. However, a small number of patients may also cause serious adverse reactions. For example, common adverse reactions include nausea, vomiting and urticaria. Serious reactions include hypotensive vagal response, moderate and severe bronchospasm, laryngeal (tracheal) edema, seizures, severe hypotension, and anaphylaxis. Serious adverse reactions, although low in incidence, can be life-threatening. Some other people also develop contrast nephropathy from contrast and need to be monitored closely. If you are going to use contrast media and have a history of the following medical conditions, you should declare this to your doctor, who may discontinue the test or give you special attention, as appropriate: a history of allergy or hypersensitivity to contrast media; severe liver or kidney disease or severe cardiovascular disease (heart failure, infarction, etc.); abnormal thyroid disease; severe diabetes mellitus with a comorbid diabetic syndrome; and people who have recently have used high doses of contrast media; have recently used biguanides (e.g., metformin); have used drugs that are nephrotoxic; have malignancy, hypotension, or malnutrition. Although the hazards of contrast agents may seem serious, the incidence is not considered high, and early detection of contrast side effects after imaging is particularly important. Contrast agent adverse reactions are acute (within 1h after injection), late onset (between 1h and 1 week after injection) and late onset adverse reactions (after 1 week after injection) according to how quickly they occur. If a patient has the following symptoms after the examination, please seek medical attention and inform the doctor that the contrast examination has been used. Such as fever, nausea, vomiting, joint pain, and generalized pain, especially when there is itching in the throat, difficulty in breathing or a persistent burning sensation at the injection site, edema, pain and failure to return to normal skin color.