High levels of the oils in blood samples were linked with a 71 per cent increased risk of developing an aggressive and dangerous form of prostate cancer, according to the research. That study, if I recall correctly, mentioned concern about men eating fish more than a certain number of times a week having a 54% increased risk of developing prostate cancer.
Fish oil is effective in reducing inflammation in the blood and tissues. Regular consumption of fish oil supplements, tablets, pills, and capsules is helpful to those who suffer from chronic inflammatory diseases. Fish oil is effective in treating gastrointestinal disorders, Celiac disease, short bowel syndrome and inflammatory bowel disease (IBD) including Crohn’s Disease and ulcerative colitis, which are both typical disorders of the intestine. Patients suffering from Crohn’s disease often find it difficult to absorb vitamins, fats, and essential supplements. Fish oil supplements are an effective diet for such patients.
Several studies confirmed the benefit of omega-3 supplementation during pregnancy in terms of proper development of the brain and retina. Of the 2 most important long-chain omega-3 fatty acids, EPA and DHA, DHA is the more important for proper cell membrane function and is vital to the development of the fetal brain and retina (17). During the third trimester, vast amounts of DHA accumulate in fetal tissue (20). The 2 most infiltrated fetal areas include the retina and brain, which may correlate with normal eyesight and brain function (19). A study by Judge et al. (20) found that children whose mothers had taken DHA supplementation during pregnancy (n = 29) had significantly better problem-solving skills at 9 mo old (P = 0.017) than those whose mothers had not taken DHA supplementation during pregnancy (n = 15). Another study provided a cognitive assessment of children 2.5 y after maternal EPA+DHA supplementation during pregnancy from 20 wk of gestation until delivery (n = 33) compared with children in a placebo group (n = 39). Children in the EPA + DHA–supplemented group attained significantly higher scores for eye and hand coordination [mean score, 114 (SD 10.2] than those in the placebo group [mean score, 108 (SD 11.3)] (P = 0.021, adjusted P = 0.008) (19).
Agency for Healthcare Research and Quality. Effects of Omega-3 Fatty Acids on Lipids and Glycemic Control in Type II Diabetes and the Metabolic Syndrome and on Inflammatory Bowel Disease, Rheumatoid Arthritis, Renal Disease, Systemic Lupus Erythematosus, and Osteoporosis. AHRQ Publication No. 04-E012-1; 2004. Available at: https://archive.ahrq.gov/downloads/pub/evidence/pdf/o3lipid/o3lipid.pdf. (Accessed February 7, 2017).
Smithers, L. G., Collins, C. T., Simmonds, L. A., Gibson, R. A., McPhee, A., and Makrides, M. Feeding preterm infants milk with a higher dose of docosahexaenoic acid than that used in current practice does not influence language or behavior in early childhood: a follow-up study of a randomized controlled trial. Am J Clin Nutr 2010;91(3):628-634. View abstract.

Most U.S. adults fail to consume adequate amounts of foods rich in EPA and DHA on a regular basis (at least 8 ounces of fatty fish per week is recommended), and probably consume too many omega-6 fats in comparison (soybean oil, canola oil, cottonseed oil, etc.). This omega-3:omega-6 imbalance can have a negative effect on inflammation patterns and may also be implicated as a contributing factor to other processes related to cellular metabolism, hormone signaling, and even weight regulation.
Those foods provide enormous amounts of other nutrients that are good for you. nSo it is way better to eat those foods than to take fish oil. With that said, some people find it very difficult to get vitamin A or vitamin D, and particularly for vitamin A, cod liver oil may be a very important source of that vitamin. Cod liver oil is a form of fish oil that happens to be high in the fat-soluble vitamins. Vitamin A is best found in liver. It’s better in my opinion to eat liver once a week, but there are a lot of people out there who are not going to eat liver once a week. So if you are using cod liver oil to get the vitamins that you can’t get from food—and I should point out that vitamin A can also be derived from plant foods, but many people genetically or for other reasons don’t derive it very well from plant foods.
One meta-analysis concluded that omega−3 fatty acid supplementation demonstrated a modest effect for improving ADHD symptoms.[39] A Cochrane review of PUFA (not necessarily omega−3) supplementation found "there is little evidence that PUFA supplementation provides any benefit for the symptoms of ADHD in children and adolescents",[40] while a different review found "insufficient evidence to draw any conclusion about the use of PUFAs for children with specific learning disorders".[41] Another review concluded that the evidence is inconclusive for the use of omega−3 fatty acids in behavior and non-neurodegenerative neuropsychiatric disorders such as ADHD and depression.[42]
Krill oil is joining the toolkit for fighting arthritis, thanks to its exceptional anti-inflammatory properties resulting from its phospholipid form of omega-3s. A study in mice with experimental arthritis showed that krill oil supplements reduced arthritis scores and markedly diminished joint swelling. When examined under a microscope, the animals’ joints were remarkably free of inflammatory infiltrates of immune system cells.85
1. Omega-3 benefits your heart health. An Italian study (GISSI)5 of 11,324 heart attack survivors found that patients supplementing with fish oils markedly reduced their risk of another heart attack, stroke, or death. In a separate study, 6 American medical researchers reported that men who consumed fish once or more every week had a 50 percent lower risk of dying from a sudden cardiac event than do men who eat fish less than once a month.
Irving, G. F., Freund-Levi, Y., Eriksdotter-Jonhagen, M., Basun, H., Brismar, K., Hjorth, E., Palmblad, J., Vessby, B., Vedin, I., Wahlund, L. O., and Cederholm, T. Omega-3 fatty acid supplementation effects on weight and appetite in patients with Alzheimer's disease: the omega-3 Alzheimer's disease study. J Am Geriatr Soc 2009;57(1):11-17. View abstract.
Although there was significant heterogeneity among the included studies (Cochran Q, 178.820; df, 18; I2, 89.934%; P < .001), the sensitivity test suggested that the main significant results of the meta-analysis would not change after removal of any of the included studies. However, through direct inspection of the forest plot, we detected the potential influence of some outliers, such as the studies by Sohrabi et al56 and Yehuda et al.61 These 2 studies evaluated anxiety symptoms with a visual analog scale of anxiety and test anxiety severity, which are seldom used in psychiatric research and lack a definite report to prove their equivalent sensitivity and specificity to some other frequently used anxiety rating scales, such as depression, anxiety, and stress scales or the Hamilton anxiety rating scale. Therefore, these studies might have affected the interpretation of the current meta-analysis.
AAKG β-hydroxy β-methylbutyrate Carnitine Chondroitin sulfate Cod liver oil Copper gluconate Creatine/Creatine supplements Dietary fiber Echinacea Elemental calcium Ephedra Fish oil Folic acid Ginseng Glucosamine Glutamine Grape seed extract Guarana Iron supplements Japanese Honeysuckle Krill oil Lingzhi Linseed oil Lipoic acid Milk thistle Melatonin Red yeast rice Royal jelly Saw palmetto Spirulina St John's wort Taurine Wheatgrass Wolfberry Yohimbine Zinc gluconate
Fish oils seem to help reduce some fat levels in the blood. These fats are called triglycerides. Birth control pills might decrease the effectiveness of fish oils by reducing these fat levels in the blood.Some birth control pills include ethinyl estradiol and levonorgestrel (Triphasil), ethinyl estradiol and norethindrone (Ortho-Novum 1/35, Ortho-Novum 7/7/7), and others.
One reason omega-3 fatty acids may be so beneficial to this many aspects of health could be that they help decrease system-wide inflammation. (49, 50, 51, 52, 53) Inflammation is at the root of most diseases and is related to the development of nearly every major illness, so by eating a nutrient-dense, anti-inflammatory diet, you give your body its best chance to fight disease like it was designed to do.
Cochrane lead author, Dr. Lee Hooper from the University of East Anglia, UK said: “We can be confident in the findings of this review which go against the popular belief that long-chain omega 3 supplements protect the heart. This large systematic review included information from many thousands of people over long periods.  Despite all this information, we don’t see protective effects.
Omega-3 fatty acids have been found to play a role in atherosclerosis and peripheral arterial disease (PAD). It is thought that both EPA and DHA improve plaque stability, decrease endothelial activation, and improve vascular permeability, thereby decreasing the chance of experiencing a cardiovascular event (41). It was found that EPA supplementation is associated with significantly higher amounts of EPA in the carotid plaque than placebo (P < 0.0001), which may lead to decreased plaque inflammation and increased stability (42). PAD, a manifestation of atherosclerosis, is characterized by buildup of plaque in the arteries of the leg and can eventually lead to complete blockage of the arteries. EPA+DHA supplementation has been shown to improve endothelial function in patients with PAD by decreasing plasma levels of soluble thrombomodulin from a median value of 33.0 μg/L to 17.0 μg/L (P = 0.04) and improve brachial artery flow–mediated dilation from 6.7% to 10.0% (P = 0.02) (43). Patients who had PAD and were supplemented with EPA experienced a significantly lower major coronary event HR than those who did not take EPA (HR: 0.44; 95% CI: 0.19–0.97; P = 0.041) (44).
Flaxseed (or linseed) (Linum usitatissimum) and its oil are perhaps the most widely available botanical source of the omega−3 fatty acid ALA. Flaxseed oil consists of approximately 55% ALA, which makes it six times richer than most fish oils in omega−3 fatty acids.[126] A portion of this is converted by the body to EPA and DHA, though the actual converted percentage may differ between men and women.[127]
Preventing re-blockage of blood vessels after angioplasty, a procedure to open a closed blood vessel. Research suggests that fish oil decreases the rate of blood vessel re-blockage by up to 45% when given for at least 3 weeks before an angioplasty and continued for one month thereafter. But, when given for 2 weeks or less before angioplasty, it doesn't seem to have any effect.