Cod liver oil is an oil extracted from cod livers. Prior to the fortification of milk with vitamins A and D, it was commonly given to children as a nutritional supplement. As well as being an excellent source of vitamin A and vitamin D, it is also a good source of omega-3 fatty acids (EPA and DHA). Depending on the quality of the oil, the flavor and aroma range from a mild sardine-like flavor, to an intense and obnoxious odor of rotten fish and rancid oil. High quality cod liver oil is "a pale-yellow, thin, oily liquid, having a peculiar, slightly fishy, but not rancid odor, and a bland, slightly fishy taste." It has recently become popular to flavor cod liver oil with citrus or mint essence to make it more palatable.
Cod liver oil is made by cooking cod livers with steam and then pressing/decanting the cooked livers to extract the oil. By contrast, fish oils are extracted from the cooked whole body tissues of fatty fish during the manufacture of fish meal. Cod liver oil and fish oil are similar but have a somewhat different composition: fish oil has a much lower content of vitamins A and D compared to liver oils.
This may pose a problem in that one may need to exceed the Recommended Dietary Allowance (RDA) of vitamins A and D in order to obtain therapeutic amounts of EPA and DHA from cod liver oil. Because the body naturally produces vitamin D when exposed to sunlight, a common way to benefit from both oils while avoiding a vitamin D overdose is to take cod liver oil during the winter, and fish oil during the summer. On the other hand, the RDA of vitamin D is considered by many to be strongly understated.