If you’ve ever prepared for an operation involving anesthesia, then you’re probably familiar with the phrase “NPO after midnight.” Nil per os in latin, or nothing by mouth, is a policy that asks patients to refrain from eating or drinking anything in the eight to 12 hours leading up to their operations. Doctors usually have a reason for the advice they give, but many inquisitive patients wonder why they have to starve themselves before going under the knife.

Although general anesthesia is essential to controlling pain by rendering a patient unconscious during a procedure, there are risks involved with this medication that anesthesiologists have worked to reduce. They ban food or drink after midnight to prevent pulmonary aspiration, the inhalation of food, liquids, solids, or vomit into the lungs or airways rather than through the esophagus to the stomach. This can lead to blocked airways and subsequent pneumonia.

For decades, fasting guidelines have been given to anyone preparing for an operation, but more recent evidence has shown this may be an overly cautious practice. In fact, around 50 percent of anesthesiologists say they no longer require NPO after midnight for elective surgery. However, parents are still advised to stop giving their children solid food at midnight the evening before surgery. Clear liquids, such as water, apple juice, sports drinks, Pedialyte, and JELL-O, are safe to drink until four hours before a procedure.