Ketones have a dark side: ketoacidosis. And it does NOT only happen in diabetes. Ketoacidosis is a serious and life-threatening medical condition wherein ketones accumulate to such high levels that they overwhelm the body’s natural buffering capacity and sink the pH of the blood to dangerous and possibly fatal levels. Ketoacidosis is most often associated with poorly controlled diabetes. Contrary to many popular claims, however, it is not limited to diabetes. Alcohol abuse with malnourishment, fasting during pregnancy or lactation, and in rare cases low-carbohydrate diets can be causes of ketoacidosis. This lesson covers the science behind ketoacidosis and reviews several case reports to illustrate what it looks like in practice.