Fish oil is oil derived from the tissues of oily fish. Fish oils contain the omega−3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), precursors of certain eicosanoids that are known to reduce inflammation in the body and improve hypertriglyceridemia. There has been a great deal of controversy in the 21st century about the role of fish oil in cardiovascular disease, with recent meta-analyses reaching different conclusions about its potential impact.

The fish used as sources do not actually produce omega−3 fatty acids. Instead, the fish accumulate the acids by consuming either microalgae or prey fish that have accumulated omega−3 fatty acids. Fatty predatory fish, like sharks, swordfish, tilefish, and albacore tuna, may be high in omega−3 fatty acids, but due to their position at the top of the food chain, these species may also accumulate toxic substances through biomagnification. For this reason, the United States Environmental Protection Agency recommends limiting consumption (especially for women of childbearing age) of certain (predatory) fish species (e.g., albacore tuna, shark, king mackerel, tilefish and swordfish) due to high levels of the toxic contaminant mercury. Dioxins, like PCBs and chlordane, as well as other chlorinated cyclodiene insecticides are also present. Fish oil is used in aquaculture feed, in particular for feeding farmed salmon.

Marine and freshwater fish oil vary in contents of arachidonic acid, EPA and DHA. The various species range from lean to fatty, and their oil content in the tissues has been shown to vary from 0.7% to 15.5%. They also differ in their effects on organ lipids. Studies have revealed that there is no relation between either 1) total fish intake or 2) estimated omega−3 fatty acid intake from all fish and serum omega−3 fatty acid concentrations. Only fatty fish intake, particularly salmonid, and estimated EPA + DHA intake from fatty fish has been observed to be significantly associated with increase in serum EPA + DHA.

The United States Food and Drug Administration (FDA) has approved four fish oil-based prescription drugs for the management of hypertriglyceridemia, namely Lovaza, Omtryg (both omega-3-acid ethyl esters), Vascepa (ethyl eicosapentaenoic acid), and Epanova (omega-3-carboxylic acids). None of these drugs are actually fish oil; they are all derivatives of acids found in fish oil.

Often marketed and sold for consumption as part of the diet or in dietary supplements in contemporary societies, fish oils also have found roles in external use, as emollients or as general ointments as well as in body art, or for alleged insulation against cold temperatures.

Fishmeal and fish oil are the principal sources of omega-3 long-chain polyunsaturated fatty acids (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]) in animal feed.

The most widely available dietary source of EPA and DHA is cold-water oily fish, such as salmon, herring, mackerel, anchovies, and sardines. Oils from these fish have a profile of around seven times as much omega−3 oils as omega−6 oils. Other oily fish, such as tuna, also contain omega−3 in somewhat lesser amounts. Although fish is a dietary source of omega−3 oils, fish do not synthesize them; they obtain them from the algae (microalgae in particular) or plankton in their diets.

EPA and DHA are available as dietary supplements most commonly as fish oil capsules, softgels, and gummies,¹ krill oil, and less commonly as algae oil.² Generally, salmon oil has more DHA than EPA while other fish oils such as herring, mackerel, anchovies, sardines, and pollock have more EPA than DHA. (See Supplements section below.)