cod-liver oil, yellowish oil obtained from the liver of the codfish. The oil is rich in vitamin A and vitamin D (calciferol). It was long used as a preventive and cure for rickets in Baltic and Scandinavian countries, where fish is a dietary staple. However, it was not until the 1920s that doctors in the U.S. finally recognized its therapeutic usefulness. More palatable synthetic vitamins have largely replaced cod-liver oil as dietary supplements, and almost all the milk sold in the United States and Europe now contains added vitamins A and D. See vitamin.