Nil per os

Nothing by mouth is a medical instruction meaning to withhold food and fluids from a person for various reasons. It is also known as nil per os (npo or NPO), a Latin phrase whose English translation is most literally, "nothing through the mouth". Variants include nil by mouth (NBM), nihil/non/nulla per os, or complete bowel rest. A liquid only diet may also be referred to as bowel rest.

NPO is one of the abbreviations that is not used in AMA style; "nothing by mouth" is spelled out instead.

The typical reason for NPO instructions is the prevention of aspiration pneumonia, e.g. in those who will undergo general anesthesia, or those with weak swallowing musculature, or in case of gastrointestinal bleeding, gastrointestinal blockage, or acute pancreatitis. Alcohol overdoses that result in vomiting or severe external bleeding also warrant NPO instructions for a period.

Pre-surgery NPO orders are typically between 6 and 12 hours prior to surgery, through recovery suite discharge, but may be longer if long acting medications or oral post-meds were administered. It is not uncommon for the food NPO period to be longer than that for liquid, as the American Board of Anesthesiology advises against liquid NPO periods greater than eight hours. The NPO periods for illness tend to be much longer, although exceptions are made for small scheduled amounts of water consumption if an IV drip is not in use. With sufficient IV fluids, NPO periods of several days have been utilized successfully in non-diabetic patients (although short NPO periods in diabetics are possible with IV fluids, insulin, and dextrose. Extended periods (greater than 12 hours) are still contraindicated.

The American Board of Anesthesiology recommends that patients should not eat solid food for at least 8 hours prior to a procedure, and should not drink even clear liquids for at least 2 hours prior.

For extended periods without food or water, patients may be started on total parenteral nutrition (TPN).

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