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Fasting

You have most likely already been asked at some point not to take medication directly after eating or to fast before having a particular examination. “Fasting” involves nothing more than keeping the stomach empty from a certain time on. But when should you start fasting? And what about liquids?

Eating

Medications: Taking medication on an empty stomach does not usually mean that a meal has to be skipped. The medication is instead taken before breakfast, lunch or dinner. What is understood by “meal” is food that – most often regularly – is eaten at a certain time of day, for example breakfast. Of course medicine can be taken by washing it down with a little water. Waiting for at least 30 minutes before beginning to eat is usually enough.

On the other hand, there are medications, such as certain types of painkillers, that are better tolerated when something has been eaten before they are taken. This will be described in the package insert.
Other treatments and examinations: Someone who has to go to an examination on an empty stomach or is about to have surgery, for example, usually has their last meal the evening before and skips breakfast. You can still have a small snack like pretzel sticks or an apple after the last full meal, but during the last 6 to 8 hours at least you should not eat anything at all. If the procedure will involve the stomach or bowel, you might need to fast for a longer time. You also should not smoke before having surgery.

Drinking

Medications: Medications are often taken with a glass of water, even though you otherwise fast beforehand. Taking certain drugs together with milk or fruit juice can reduce their effect.
Other treatments and examinations: When you should begin not drinking anything depends on the particular treatment or examination being done. While you usually do not have any carbonated drinks or dairy products after the last full meal, a small amount of clear water 2 to 3 hours before the treatment or examination often is not a problem.

Which treatments or examinations require fasting?

There are many different reasons why it is sometimes necessary to fast; for instance, so that medication works as it is intended to, or because the stomach has to be empty so that it can be examined. Especially for the following treatments and examinations you may need to do without eating and/or drinking for a certain time:

Taking medication
Blood tests
Examinations using contrast agents, such as computed tomography
Some ultrasound examinations of the abdominal region
Challenge tests done on suspicion of food intolerances
Endoscopy of the stomach, for example
Surgery performed under general anesthetic

Because it depends on the exact treatment or examination when exactly you should not eat or drink, it is important to discuss this with your doctor. You can also find more information about whether you need to fast in the medication’s package insert or in the information sheets for examinations and surgery. If you do not understand this information, you can get advice from your doctor or pharmacist.

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)