As the name suggests, and in very simple terms, cod liver oil is made from the oil that is found in the livers of cod.
Cod livers have a very high oil content, in a similar way to nuts or seeds. In broad terms, cod liver oil is the oil in these livers, but in reality, many cod liver oils have different ingredients added to them. Cod liver can undergo various processing methods to release this oil, which can then be filtered off, and stored. The way in which the livers are processed and sourced will impact the quality of the oil, and what goes in them.
Some processing methods (like the common molecular distillation) remove all the vitamins, and so synthetic versions have to be added into the oil again. Some manufacturers also chemically alter the oil to make it easier to filter, use oil from cheaper fish, and add vegetable oil to carry synthetic vitamins (this is true for most cheap cod liver oils). In these instances, you can think of cod liver oil being made of chemically altered oil from various fish, synthetic vitamins, vegetable oil and a gelatin/ cellulose capsule. This actually sounds very different to what the name ‘cod liver oil’ suggests… It is like buying a beef burger which may or may not contain any beef but is bulked up with things that are a bit similar to beef.
Other cod liver oils, which are made very naturally are ‘made’ of different things. Fermented cod liver oil for example, doesn’t have synthetic vitamins added in, but does have small amounts of various micro-nutrients. The nutritional list for this product would be too long to add to this page, but in essence, it is made from real cod liver oil, and nothing is added to it. Other examples of pure cod liver oil are Dropi cod liver oil, which also doesn’t use anything other than cod liver oil to make cod liver oil.
To learn more about how the processing methods of cod liver oil impact what the final product is made of, have a read of this article, which compares various different products.