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Fasting for a set period of time before an operation, also known as “nothing by mouth” or NPO, is a requirement to prevent patients from aspirating stomach contents during surgery.1 When under anesthesia, patients’ gag and cough reflexes are suppressed, preventing them from vomiting normally. Additionally, some anesthetic agents also relax the muscle that prevents stomach contents from passing back through the esophagus. Under these conditions, stomach contents could end up in the patient’s airway, potentially causing serious complications. As a result, requiring patients to fast for a number of hours before a procedure is common practice. However, patients are not always required to fast before a medical procedure, as research has found evidence to support more moderate requirements compared to traditional guidelines.
A clear distinction between procedures that require fasting and those that do not is whether the patient receives anesthesia, and if so, what kind. The physiological effects that elevate the risk of aspiration are related to anesthetic agents that provide general anesthesia or sedation. For example, patients who only receive local anesthesia are typically not at risk of aspiration and do not need to fast before their procedure.
Fasting guidelines have evolved for procedures that always require patients to fast beforehand. The current NPO guidelines in large part derive from a study undertaken in 1946 by New York obstetrician Curis Lester Mendelson, in which, out of 44,000 pregnancies, Mendelson observed a total of 66 instances of chemical pneumonitis in women undergoing childbirth under anesthesia.2 Two of the patients died from the condition, which later came to be known as “Mendelson Syndrome” on account of his role in characterizing it. Following his work, the requirement for patients to enter surgery with a completely empty stomach, often without having had any food or drink since the midnight before the operation, became more formalized.3
Research conducted in more recent decades has provided a basis for more lenient fasting requirements when it comes to the consumption of certain items. Maltby et al. showed that in patients given 150 ml of water 2.5 hours before surgery, gastric volume was lower than in those who did not drink before surgery.4 This study, among others, motivated the American Society of Anesthesiologists to change their guidelines in 1998 to allow clear liquids up to 2 hours before surgery in low-risk patients undergoing certain kinds of operations.5 In contrast, a light meal requires a minimum fast of 6 hours and a heavy meal at least 8 hours.
More recent research has suggested that an even more permissive fasting policy could improve patient comfort without compromising on safety. In one study of over 76,000 patients in the Netherlands, those who adhered to a liberal fasting policy that permitted drinking clear fluids right up until surgery reported lower rates of preoperative thirst and postoperative nausea and vomiting with only slightly higher rates of regurgitation and vomiting (18 events vs. 24 and 1.7 vs. 2.4, respectively, per 10,000 cases).6
Finally, some studies suggest that fasting at large does not correlate with better patient outcomes. A review of clinical trials of preoperative fasting and aspiration events published earlier this year found that aspiration events were quite rare, occurring in only 4 of 801 patients (0.5%) in experimental groups and 7 of 990 patients (0.71%) in control groups—meaning there was no observed connection between fasting and reduced aspiration.7 The authors of the study conclude that “further study of more liberal fasting policies is unlikely to find an association” and that “preprocedural fasting might be replaced by bedside gastric ultrasound examination of gastric volume.” However, whether this conclusion will have an impact on professional guidelines is unclear.
Patients and doctors should discuss and decide on a fasting plan before surgery, as fasting remains very important in many cases. However, it appears that the conventional wisdom of always having “nothing by mouth after midnight” is starting to fade away.
References
1. Brady, M., Kinn, S. & Stuart, P. Preoperative fasting for adults to prevent perioperative complications. Cochrane Database Syst Rev CD004423 (2003) doi:10.1002/14651858.CD004423.
2. Salik, I. & Doherty, T. M. Mendelson Syndrome. in StatPearls (StatPearls Publishing, Treasure Island (FL), 2025).
3. Chon, T. et al. Perioperative Fasting and the Patient Experience. Cureus 9, (2017), DOI: 10.7759/cureus.1272
4. Maltby, J. R., Sutherland, A. D., Sale, J. P. & Shaffer, E. A. Preoperative oral fluids: is a five-hour fast justified prior to elective surgery? Anesth Analg 65, 1112–1116 (1986), https://journals.lww.com/anesthesia-analgesia/abstract/1986/11000/preoperative_oral_fluids__is_a_five_hour_fast.3.aspx
5. Black, M. K. et al. Things We Do for No ReasonTM: NPO After Midnight. J Hosp Med 16, 368–370 (2021), DOI: 10.12788/jhm.3537
6. Marsman, M. et al. Association of a Liberal Fasting Policy of Clear Fluids Before Surgery With Fasting Duration and Patient Well-being and Safety. JAMA Surg 158, 254–263 (2023), DOI: 10.1001/jamasurg.2022.5867
7. Lam, S., Cannesson, M. P., Osuna-Garcia, A. & Livingston, E. H. No association between preprocedural fasting and witnessed pulmonary aspiration: A systematic review and meta-analysis. Surgery 184, (2025), DOI: 10.1016/j.surg.2025.109483
