Lactic acidosis is one type of anion-gap metabolic acidosis. It occurs when aerobic tissue metabolism converts to anaerobic metabolism, which produces a significant amount of lactic acid.

Lactic acidosis occurs at the cellular level (mitochondria) when the oxygen delivery to the tissue is inadequate or the cells cannot utilize the oxygen. Many causes of lactic acidosis are recognized, with the most common being due to low oxygen delivery from shock (septic, hypovolemic, cardiogenic, neurogenic), severe anemia, and severe hypoxemia. It also can occur from exposure to drugs (e.g., metformin, antiretroviral therapy, acetaminophen, salicylates, cocaine, valproic acid, salfasalazine, isonicotinic acid hydrazide, fluorouracil), toxins, and sugar alcohols (sorbitol, xylitol, fructose).