Lactic acid is an organic acid with a molecular formula of CH3CH(OH)COOH. It is white in the solid state and is miscible with water. When in the dissolved state, it forms a colorless solution. Lactic acid is produced by the body when cells break down carbohydrates for energy, and it is also produced by muscle tissue and red blood cells. Most people associate lactic acid with muscle soreness after a tough workout, but it can also be produced by any tissue in the body when it uses more oxygen than usual. The liver and kidneys filter lactic acid out of the blood and break it down into glucose (blood sugar) .

Lactic acid is used as a synthetic intermediate in many organic synthesis industries and in various biochemical industries. It is also used as a food preservative, curing agent, and flavoring agent, and is an ingredient in processed foods. Lactic acid is approved for use as a food additive in the EU, United States, Australia, and New Zealand.

A lactic acid test measures the level of lactic acid, also known as lactate, in the blood. This test is most often done to diagnose lactic acidosis. Normal results range from 4.5 to 19.8 milligrams per deciliter (mg/dL) .