“Reading medical journals and following the mass media, it’s easy to get the idea that fish oil is something any sensible person should use. It’s rare to see anything suggesting that it could be dangerous.
Fish oil has been used for hundreds of years as varnish or for fuel in lamps, and the fatty fish have been used as fertilizer and animal feed, and later the hydrogenated solid form of the oil, which is more stable, has been used in Europe as a food substitute for people. When whale hunting was reduced around 1950, fish oil was substituted for whale oil in margarine production. Like the seed oils, such as linseed oil, the fish oils were mostly replaced by petroleum derivatives in the paint industry after the 1960s.
Although by 1980 many animal diseases were known to be caused by eating oily fish, and the unsaturated oils were known to accelerate the formation of the “age pigment,” lipofuscin, many “beneficial effects” of dietary fish oil started appearing in research journals around that time, and the mass media, responding to the industry’s public relations campaign, began ignoring studies that showed harmful effects from eating fish oil.” Raymond Peat, PhD, The Great Fish Oil Experiment
“Unsaturated Fats Are Essentially Involved In Heart Damage: The toxicity of unsaturated oils for the heart is well established, [24, 25, 26] though not well known by the public.
In 1962, it was found that unsaturated fatty acids are directly toxic to mitochondria.  Since stress increases the amount of free fatty acids circulating in the blood (as well as lipid peroxides), and since lack of oxygen increases the intracellular concentration of free fatty acids, stored unsaturated fats would seem to represent a special danger to the stressed organism. Meerson and his colleagues  have demonstrated that stress liberates even local tissue fats in the heart during stress, and that systematic drug treatment, including antioxidants, can stop the enlargement of stress-induced infarctions. Recently, it was found that the cardiac necrosis caused by unsaturated fats (linolenic acid, in particular) could be prevented by a cocoa butter supplement.  The author suggests that this is evidence for the “essentiality” of saturated fats, but points out that animals normally can produce enough saturated fat from dietary carbohydrate or protein, to prevent cardiac necrosis, unless the diet provides too much unsaturated fat. A certain proportion of saturated fat appears to be necessary for stability of the mitochondria. Several other recent studies show that the “essential” fatty acids decrease the P/O ratio, or the phosphorylation efficiency,  the amount of usable energy produced by cellular respiration.” Raymond Peat, PhD, Oils in Context
“Acrolein and hydroxynonenal (HNE) are lipid peroxides or breakdown products from consumption of polyunsaturated fatty acids (PUFA). Isoprostanes and neuroprostanes are inflammatory prostaglandin-like mediators (eicosanoids) formed from omega-3 PUFA fish oil (DHA/EPA). These substances are accurate markers of lipid peroxidation/oxidative stress and are implicated in a variety of brain degenerative conditions.” Rob Turner, PUFA, Fish Oil, and Alzheimers
“Pretend you’re a fish for a moment that doesn’t eat as it should and just loves to eat butter, a saturated fat (which by the way was derived from a warm blooded animal). Om nom nom. Pretty quickly the fish realizes that this idea isn’t a good one as his tissues harden in the cold water (think butter in the refrigerator), and he is no longer able to swim. Humans have greater adaptability than do cold-water fish, but the biochemistry of the situation isn’t any better when we as warm-blooded omnivores eat or supplement with the highly unsaturated oils from fish. Fish oils remain liquid even at cold temperatures and prevent fish from freezing in very cold water. In an oxygen rich and warm environment like your body, however, the structure of these fats is not stable and breaks down easily into health deteriorating substances. The biochemistry of fish oils and our body does not match. End of story. We have lost context.” Rob Turner, Fish Oil Toxicity
“…he has published four papers in peer-reviewed scientific journals, two of them devoted to another major culprit he has singled out as responsible for atherosclerosis, or the hardening of the arteries: an excess of polyunsaturated vegetable oils like soybean, corn and sunflower — exactly the types of fats Americans have been urged to consume for the past several decades… …Dr. Kummerow contends that the high temperatures used in commercial frying cause inherently unstable polyunsaturated oils to oxidize, and that these oxidized fatty acids become a destructive part of LDL particles. Even when not oxidized by frying, soybean and corn oils can oxidize inside the body…” A Lifelong Fight Against Trans Fats, Fred A. Kummerow
“Levels of oxysterols and OxLDL increase primarily as a result of three diet or lifestyle factors: the consumption of oxysterols from commercially fried foods such as fried chicken, fish, and french fries; oxidation of cholesterol in vivo driven by consumption of excess polyunsaturated fatty acids from vegetable oils; and cigarette smoking. Along with the consumption of trans fatty acids from partially hydrogenated vegetable oil, these diet and lifestyle factors likely underlie the persistent national burden of heart disease.” Fred A. Kummerow, Interaction between sphingomyelin and oxysterols contributes to atherosclerosis and sudden death