Fasting

Sometimes it is important to fast before having a particular examination or not to take medication directly after eating. “Fasting” means keeping your stomach empty from a certain time on. But when should you start fasting? And what about liquids?

Eating

  • 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.
  • 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. 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.

Drinking

  • 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 glass of clear water 2 to 3 hours before the treatment or examination often is not a problem.
  • Medications: Medications are often taken with a glass of water, even though you otherwise fast beforehand. If possible, you should always take medications with water, since taking certain drugs together with milk or fruit juice can reduce their effect.

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:

  • Blood tests
  • Examinations using contrast agents, such as
  • 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
  • Taking medication

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

Kasper DL, Fauci AS, Hauser SL, Longo DL, Jameson JL, Loscalzo J. Harrison's Principles of Internal Medicine. New York: McGraw-Hill; 2015.

Pschyrembel. Klinisches Wörterbuch. Berlin: De Gruyter; 2017.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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Updated on October 22, 2020
Next planned update: 2023

Authors/Publishers:

Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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