You might be wondering why you have to avoid drinking or eating for several hours just before your CT scan. The reason for this is because doctors will need to use a special fluid called a contrast agent which will make certain parts of your intestines stand out in the scanner, making it easier for them to find the problem that is causing you to undergo a CT scan in the first place. If you eat or drink liquids near the time when you need to have the contrast agent put into your body, the contrast agent will be diluted and much less effective. Not following this rule could even cause you to have to take another CT scan.

In order to figure out how long in advance you will be asked to stop eating, ask your doctor exactly how the contrast agent will be put into your body.

1. By Mouth

If your contrast agent is going to be ingested, then there's a good chance that you will need to stop eating at least 12 hours before the procedure. Many doctors will try to schedule the CT scan very early in the day so that the period when you won't be eating will be while you are asleep. The reason why you can't eat this far in advance is because the contrast agent will be going down the same track as the food, increasing the chance that there will be disruption.

2. By Injection

If you have the contrast agent injected into your body, rather than ingested, then you will likely not have to fast for nearly as long. Many hospitals will ask that you fast between 6 and 10 hours, rather than the full 12 or more hours. This is because the contrast agent will likely not come into direct contact with the food because it is going into the body a different way. 

3. By Enema

A contrast agent is inserted into the body by enema if a doctor needs to look at anywhere around your rectum. Whether or not you will need to fast will be determined by your doctor and the exact timing of your injection. You might need to instead take laxatives to clear out that area and make it easier for the doctors to see any potential problems in the CT scan.

For more information, talk to your doctor. He or she will have specific instructions for you.