A nutrient in the vitamin B complex that the body needs in small amounts to function and stay healthy. Vitamin B2 helps make red blood cells, helps some enzymes work properly, and keeps skin, nails, and hair healthy. It is found in milk, eggs, malted barley, organ meats, yeast, and leafy vegetables. Vitamin B2 is water-soluble (can dissolve in water) and must be taken in every day. Not enough vitamin B2 can cause anemia (a low number of red blood cells), mouth sores, and skin problems. Amounts of vitamin B2 may be higher in the blood of patients with some types of cancer. Also called riboflavin.