What you should avoid before a CT scan depends on the type of scan you’re getting, whether contrast dye is involved, and what part of the body is being imaged. The most common mistakes are eating too close to your appointment, forgetting to mention medications you take, wearing metal clothing or jewelry, and not drinking enough water when contrast is planned. Here’s what to watch for so your scan goes smoothly and produces clear images.
Don’t Eat Too Close to Your Scan
If your CT scan involves contrast dye (the liquid injected into a vein or swallowed before the scan), you’ll typically need to stop eating for a period beforehand. For most contrast-enhanced scans, fasting for about four hours is standard. Abdominal and gastrointestinal CT scans may require four to eight hours without food, since a full stomach can interfere with how well the images turn out and increase nausea risk from the contrast.
If your scan doesn’t involve contrast, fasting usually isn’t required. Your imaging center should tell you when you schedule the appointment, but if they don’t, call and ask. The worst outcome of eating when you shouldn’t have is a rescheduled scan.
Don’t Skip Water (Unless Told Otherwise)
This one surprises people: for contrast-enhanced scans, staying well hydrated actually protects your kidneys. The contrast dye is filtered through your kidneys, and dehydration makes it harder for them to clear it safely. Research protocols for patients receiving IV contrast call for drinking about 500 mL (roughly two cups) of water in the one to two hours before the scan. If you have heart failure, that amount is typically cut in half.
“Fasting” before a CT scan generally means no food, not necessarily no water. Clear liquids are usually fine unless your facility tells you otherwise. Showing up dehydrated is one of the more common and avoidable preparation mistakes.
Don’t Forget to Mention Your Medications
If you take metformin for diabetes, this is especially important. Metformin needs to be stopped at or before the time of a contrast-enhanced CT scan and withheld for 48 hours afterward. The combination of metformin and contrast dye can, in rare cases, lead to a dangerous buildup of lactic acid, particularly if your kidneys are already under stress. Your doctor will check your kidney function before restarting the medication.
Blood thinners, blood pressure medications, and other prescriptions generally don’t need to be stopped, but your imaging team needs to know about them. Bring a list of everything you take, including supplements. Some facilities will ask about kidney-related lab work (specifically a measure called eGFR) before giving contrast. If your kidney filtration rate falls below 60, the risk of contrast-related kidney injury rises, and additional precautions are needed.
Don’t Ignore a Previous Contrast Allergy
If you’ve ever had a reaction to contrast dye, even a mild one like hives or itching, tell your doctor well before your scan date. Patients with known contrast allergies typically receive a combination of a steroid and an antihistamine before the procedure, and these medications need at least four hours to take effect. If the imaging team doesn’t know about your allergy history until you arrive, your scan will likely be delayed or rescheduled.
Allergies to shellfish, by the way, do not indicate a contrast allergy. That’s a persistent myth. But any prior reaction to iodinated contrast dye is worth reporting.
Don’t Wear Metal Clothing or Jewelry
Metal creates bright streaks and distortions on CT images that can obscure exactly what your doctor needs to see. Zippers, underwire bras, belt buckles, necklaces, earrings, and even some athletic wear with metallic threads can all cause problems. Hair clips and piercings in the area being scanned are common culprits too.
Wear comfortable, loose-fitting clothes without any metal components, and you may avoid having to change into a hospital gown. If your scan is focused on your head or neck, remove earrings, necklaces, and any facial piercings before you arrive. For a chest or abdominal scan, a plain cotton t-shirt and elastic-waist pants work well.
Don’t Move During the Scan
A CT scan is fast, often just a few minutes, but staying still during that window matters more than most people realize. Movement of even 3 millimeters or more during imaging can make the resulting pictures unreadable, creating streaks and blurred edges that force a repeat scan. That means additional radiation exposure and more time on the table. Movements smaller than 3 mm generally don’t cause significant problems, so you don’t need to be perfectly frozen, but avoid shifting your body, adjusting your arms, or swallowing repeatedly if your head or neck is being scanned.
If you’re anxious about enclosed spaces or holding still, let your team know ahead of time. CT scanners are much more open than MRI machines, and scans are far shorter, but anxiety can still cause restlessness. For children who may not be able to hold still, sedation is sometimes used. Pediatric fasting rules for sedation are stricter: two hours without clear liquids, four hours without breast milk, six hours without formula or light meals, and eight hours without fatty or fried foods.
Don’t Drink Oral Contrast Incorrectly
Some abdominal and pelvic CT scans require you to drink an oral contrast solution (usually barium sulfate) before arriving. If your facility gives you a bottle to drink at home, the timing matters. A typical protocol is to begin drinking the contrast 90 minutes before your appointment, consuming about one-third of the bottle every 15 minutes and saving the final third to bring with you. Do not eat or drink anything else for three hours before the scan.
The contrast coats your digestive tract so it shows up clearly on the images. Drinking it too early, too late, or not finishing it can reduce image quality and may mean repeating the prep or the scan entirely. It doesn’t need to be refrigerated, but most people find it more tolerable cold.
Pregnancy and Breastfeeding Considerations
If there’s any chance you’re pregnant, tell your care team before the scan. CT uses ionizing radiation, and while the dose from a standard scan is generally well below levels associated with fetal harm, ultrasound or MRI are preferred when they can answer the same clinical question. A CT scan isn’t automatically ruled out during pregnancy, but your doctor should weigh whether it’s the best option.
If you’re breastfeeding and receiving IV contrast, there’s good news: you do not need to pump and dump or interrupt breastfeeding afterward. The iodinated contrast used in CT scans is excreted in extremely small amounts into breast milk and is not considered harmful to a nursing infant.