Nil per os

Nil per os (alternatively nihil/non/nulla per os) (npo or NPO) is a medical instruction meaning to withhold oral food and fluids from a patient for various reasons. It is a Latin phrase whose usual English expansion is nothing by mouth (most literally, "nothing through the mouth"). Variants include nil by mouth (NBM).

Typical reason for NPO instructions are the prevention of aspiration pneumonia, e.g. in those who will undergo general anesthetic, 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 warrants NPO instructions for a period.

When patients are placed on NPO orders prior to surgical general anesthesia, physicians would usually add the exception that patients are allowed a very small drink of water to take with their usual medication. This is the only exception to a patient's pre-surgery NPO status. Otherwise, if a patient accidentally ingested some food or water, the surgery would usually be canceled or postponed for at least 8 hours.

While NPO because of illness or in preparation for procedure, insulin drip offers best control for patients with diabetes mellitus. In such a case, the insulin dose would be determined by initially providing 1 g of insulin for every 5 g of dextrose provided through intravenous fluids.

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