Merck Manual

Please confirm that you are not located inside the Russian Federation


Radiographic Contrast Agents


Mehmet Kocak

, MD, Rush University Medical Center

Last full review/revision Jun 2019| Content last modified Jul 2019
Click here for the Professional Version
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version

During imaging tests, contrast agents may be used to distinguish one tissue or structure from its surroundings or to provide greater detail.

Contrast agents include

  • Radiopaque contrast agents (sometimes inaccurately called dyes): Substances that can be seen on x-rays

  • Paramagnetic contrast agents: Substances that are used in magnetic resonance imaging (MRI)

Radiopaque Contrast Agents

A radiopaque contrast agent absorbs x-rays and thus appears white on x-rays. It is typically used to show the following:

  • Blood vessels

  • The interior of the gastrointestinal, biliary, or urinary tract

  • Blood flow in organs

Usually, the contrast agent is injected into a vein (intravenous contrast), artery (angiography), taken by mouth (oral contrast), inserted through the anus (rectal contrast), or injected into a joint using a needle.

The contrast agent used depends on what type of test is done and which body part is being evaluated:

  • For blood vessels: Usually contrast agents that contain iodine (iodinated contrast agents)

  • For the gastrointestinal tract: Contrast agents that contain barium or iodine (gastrografin)

Before a test that uses a contrast agent, people may be asked to refrain from eating for several hours and from drinking for 1 hour. After the test, drinking extra fluids for the rest of the day is recommended.

When some contrast agents are injected, people may feel a warm sensation throughout the body. Other contrast agents may cause a cold sensation at the injection site. Contrast agents taken by mouth may have an unpleasant taste.

Side effects of radiopaque contrast agents

Generally, radiopaque contrast agents are very safe, particularly when given by mouth or into the rectum.

Side effects of injected iodinated contrast agents occur in a few people. They include

  • An allergic-type reaction

  • Kidney damage, especially when people have a preexisting kidney problem or when large amounts of contrast is used

Allergic-type contrast reactions

Allergic-type contrast reactions vary in severity:

  • Mild, such as nausea, flushing, or itching

  • Moderate, such as a rash, vomiting, or chills

  • Severe and life threatening (anaphylactoid), such as a swollen throat that interferes with breathing, wheezing, very low blood pressure, or an abnormal heart rate

At the first sign of a reaction, the contrast agent is stopped. Mild or moderate reactions are treated with the antihistamine diphenhydramine, given intravenously. Severe reactions may be treated with oxygen, fluids given intravenously, epinephrine, or other drugs, depending on the type of reaction.

Allergic-type contrast reactions are most likely to occur in people who have one of the following:

  • Have many other allergies

  • Have asthma

  • Have previously had allergic-type reactions after a contrast agent was used

If people have had several severe reactions to iodinated contrast agents, an imaging test that does not require this contrast agent should be done instead. If an iodinated contrast agent must be used, drugs (diphenhydramine and a corticosteroid) may be given before the test to prevent a reaction. People who previously have had a reaction to a contrast agent should tell their doctor before an imaging test is done.

Kidney damage

Kidney damage (contrast nephropathy) due to use of a iodinated contrast agent may occur in people with certain conditions:

  • Impaired kidney function

  • Dehydration

  • Age over 70

  • Diabetes

  • Heart failure

  • High blood pressure (hypertension)

  • Multiple myeloma

  • Use of drugs that can damage the kidneys

In over 99% of people, the kidney damage causes no symptoms and goes away within 1 week or so. Fewer than 1% have lasting damage, and only a very few of them require kidney dialysis.

If tests that require radiopaque contrast agents must be used in people at risk of kidney damage, people are given fluids intravenously before and after the agent is given. A low dose of the contrast agent is used if possible. People who have had impaired kidney function for a long time may be given acetylcysteine the day before and the day the contrast agent is given.

Paramagnetic Contrast Agents

Paramagnetic contrast agents change the magnetic properties of particles in a way that increases the contrast between different tissues, making MRI images clearer. These agents usually contain gadolinium.

Side effects of paramagnetic contrast agents

Usually, no side effects occur. However, in a few people who have severe kidney disease or who are undergoing dialysis, these agents may cause a life-threatening disorder called

  • Nephrogenic systemic fibrosis

In nephrogenic systemic fibrosis, the skin, connective tissue, and organs thicken. Red or dark patches may develop on the skin. The skin may feel tight, movement is difficult and limited, and organs may malfunction. This disorder is now very rare because doctors use gadolinium paramagnetic contrast agents in people with kidney problems only when necessary, and they use the lowest dose and safest agent possible. Doctors also consider using other imaging tests in people with severe kidney problems.

Drugs Mentioned In This Article

Generic Name Select Brand Names
No US brand name
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Also of Interest


View All
Cerebral Angiography: Catheter Insertion
Cerebral Angiography: Catheter Insertion
Angiography is used to obtain diagnostic information about the blood vessels that carry blood...
X-ray technology utilizes high-energy rays that can pass through certain body tissue and create...