Fish oil is a form of fatty acid that is derived from the tissues of oily fish. It contains the omega-3 fatty acids, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These omega-3 fats are usually found in fish, animal products and phytoplankton. Fish oil is recommended as a source of these omega-3 fats as they are the cheapest and most common source of them.

Fish oil is readily absorbed as a dietary supplement. After one week of supplementation, increases in omega-3 fatty acids are detectable in the tissues. Tissue elevations in EPA and DHA persist for as long as one to two months after supplementation is discontinued. These fatty acids also prevent the blood from clotting easily.