Iodine contrast is safe for the vast majority of people. In a large prospective study of nearly 6,000 patients receiving modern non-ionic contrast for CT scans, 4.3% experienced any adverse reaction, but the overwhelming majority of those were mild. Only 0.4% had a moderate reaction, and zero patients had a severe one. That said, certain pre-existing conditions can raise your risk, and knowing about them ahead of time makes the whole process safer.
What Most Reactions Look and Feel Like
The most common side effects are mild and short-lived. You might feel a warm, flushed sensation as the contrast is injected, a metallic taste in your mouth, or mild nausea. Some people develop a few hives or feel slightly itchy. These reactions typically pass within minutes and don’t require treatment.
Moderate reactions, things like widespread hives, significant swelling, or a drop in blood pressure, occur in roughly 1 in 250 injections. They’re uncomfortable but treatable on the spot. Truly severe, life-threatening anaphylactic reactions are exceptionally rare with modern contrast agents.
Delayed Skin Reactions
Not all reactions happen immediately. Delayed reactions can appear anywhere from one hour to seven days after the injection, with nearly half showing up on the same day and about 20% the following day. These are almost always skin-related: a rash, redness, or itching that develops well after you’ve left the imaging center. Published incidence ranges widely, from under 1% to as high as 14% in some study populations, depending on how closely patients were monitored afterward. Most delayed skin reactions are mild and resolve on their own.
Kidney Risk Is Real but Manageable
The contrast your kidneys filter out can, in rare cases, cause a temporary dip in kidney function. The risk is minimal if your kidneys are working normally. It becomes a meaningful concern when your estimated kidney filtration rate (eGFR) drops below 45, and a substantial risk factor below 30. For context, a healthy eGFR is above 90, so this applies to people who already have moderate to severe kidney disease.
If you’re in that higher-risk group, your care team will typically hydrate you with IV saline before and after the scan. Protocols vary, but a common approach for outpatients with reduced kidney function is 250 mL of saline infused before imaging and another 250 mL afterward. For patients whose kidney function is only mildly reduced (eGFR 45 to 60), simply drinking extra water before and after the scan is often sufficient. Staying well hydrated helps your kidneys flush the contrast out more quickly and reduces the chance of injury.
Metformin and Contrast
If you take metformin for diabetes, you may have been told to stop it before a contrast scan. Current guidelines from the American College of Radiology are more relaxed than they used to be. If your eGFR is 30 or above and your kidneys are functioning stably, there’s no need to stop metformin before or after receiving IV contrast. You also don’t need a follow-up blood test to check your kidneys.
The precaution still applies if your eGFR is below 30 or you have acute kidney problems. In those cases, metformin should be paused at the time of the procedure and held for 48 hours afterward, until kidney function is re-checked and confirmed normal. The concern isn’t that metformin interacts with the contrast directly. It’s that if the contrast were to impair already-vulnerable kidneys, metformin could accumulate to dangerous levels.
Thyroid Concerns
A single contrast injection delivers a large dose of iodine, sometimes hundreds of times more than your daily dietary intake. In most people, the thyroid handles this easily. But if you have an overactive thyroid, Graves’ disease, a multinodular goiter, or undiagnosed thyroid nodules, excess iodine can push the thyroid into overdrive. The reported incidence of contrast-induced hyperthyroidism ranges from 0.05% to 5%, depending on the population studied.
In extremely rare cases, this can escalate to thyroid storm, a dangerous spike in thyroid hormone levels. The onset can be delayed, developing anywhere from a few days to 16 weeks after the contrast injection. If you have a known thyroid condition, your doctor should be aware before you receive iodine contrast so the risk can be weighed or your thyroid monitored afterward.
The Shellfish Allergy Myth
If you’ve ever been asked whether you’re allergic to shellfish before a contrast scan, you’ve encountered one of the most persistent myths in medicine. The idea dates back to papers from the early 1970s, and it stuck. But the allergens in shellfish are proteins (primarily tropomyosin), not iodine. Iodine itself is an element your body needs and cannot be an allergen.
A survey published in the World Journal of Cardiology put it bluntly: there is no evidence that shellfish allergy increases the risk of a contrast reaction more than any other allergy. Having any allergy history (to pollen, medications, food) modestly raises your baseline risk of a contrast reaction. Shellfish specifically does not. Asking about it in pre-scan questionnaires only perpetuates the myth.
What If You’ve Had a Reaction Before
A prior reaction to iodine contrast is the single strongest risk factor for having another one. If you’ve reacted before, you’re not automatically disqualified from future contrast scans, but you’ll receive premedication to reduce the chance of a repeat event. The standard protocol is an oral steroid taken at 13 hours, 7 hours, and 1 hour before the procedure, combined with an antihistamine one hour before. This regimen significantly lowers the odds of a recurrence, though it doesn’t eliminate the risk entirely.
Contrast Leaking at the Injection Site
About 0.45% of power-injected contrast CT scans result in extravasation, where the contrast leaks out of the vein into surrounding tissue. In a large study, 79% of patients who experienced this had localized swelling, 24% had pain, and 8% had no symptoms at all. Most cases resolve with elevation and a cold compress.
Very rarely, leaked contrast can become trapped within a muscle compartment, which could lead to compartment syndrome. This is a surgical emergency, but it’s exceptionally uncommon. One tricky aspect: some patients don’t feel pain during or immediately after the leak. If you notice increasing tightness, numbness, or swelling in the arm used for injection after your scan, let the imaging staff know right away.
Breastfeeding After Contrast
Less than 0.04% of the injected contrast dose makes it into breast milk within the first 24 hours. Of that tiny amount, very little is absorbed through the infant’s gut. Based on this, the American College of Radiology considers it safe to continue breastfeeding normally after receiving contrast. If you’re still uncomfortable with the idea, you can pump and discard for 12 to 24 hours after the injection, but there’s no benefit to waiting longer than that.