Contrast Reaction Symptoms — What to Watch For After Your CT Scan

Mild reactions are the most common type. You may notice warmth or flushing as the contrast is injected, a metallic taste, brief nausea, or mild itching. These usually pass within minutes on their own. Even mild symptoms should be reported to the technician — any symptom the team does not know about is a symptom they cannot monitor.

Go to the nearest emergency department if you develop hives spreading across a wide area of your body, swelling of the face or throat, difficulty breathing, chest tightness, severe dizziness, or loss of consciousness. These can be signs of a severe allergic reaction. Do not wait to see if they settle. Call for help or have someone take you — do not drive yourself.

Most reactions develop within the first few minutes of injection. Acute reactions — the type most likely to be serious — almost always begin during this early window. This is why the imaging centre asks you to stay for observation before you leave. A smaller number of delayed reactions, which are typically milder, can appear several hours to a couple of days later.

Yes. A delayed reaction typically appears as a skin rash, redness, or hives, and it can develop up to several days after the scan. Delayed reactions are almost always milder than acute ones. If you notice any new skin change or other symptom in the days following a contrast scan, contact the imaging centre or your oncology team — do not assume it is unrelated to the scan.

No. Warmth spreading through the body, a metallic taste, and a brief feeling that you have passed urine — you have not — are common and expected sensations caused by contrast moving through the bloodstream. They pass within a minute or two and are not signs of a reaction. It is still worth mentioning them to the technician, but they are not a cause for concern.

People who have had a contrast reaction before are at higher risk of reacting again in a future scan. People with a history of severe allergies or asthma may also face a slightly higher risk. Your scan form asks about both. Tell the team about any previous reaction, any allergies — food or drug — and all the medicines you currently take, every time you have a contrast scan.

If you develop hives, swelling, difficulty breathing, or any symptom that concerns you after leaving, go to the nearest emergency department or call for emergency help straight away. Tell the treating team you had a contrast injection and when. If symptoms are mild — a rash or mild itching — contact the imaging centre or your oncology team the same day. Do not drive yourself if you feel unwell.

Not necessarily. A mild previous reaction does not automatically rule out contrast in future scans. Your oncologist and radiologist will weigh the benefit of the imaging against the risk of another reaction. For a previous moderate or severe reaction, the team may recommend additional preparation before the next scan, or may consider a different imaging approach. Always tell every scan team about your history — every time, even years later.