As a nutritional supplement, fish oil has earned the characterization GRAS (generally regarded as safe) from the Food and Drug Administration (FDA). However, because of pollution in their watery habitats, many species of fish contain levels of toxicity that necessitate attention. Environmental and public health groups have raised awareness of poisons such as mercury, dioxins and PCBs found in common fish. Methylmercury is found in highest concentrations in predatory fish such as swordfish, shark and tuna [source: Environmental Defense Fund]. However, mercury accumulates only in the tissues of fish, not in fish oil, so fish oil supplements are considered non-toxic [source: Medline Plus].
High dosages or prolonged use of fish oil may lead to some side effects. Some activity in the gastrointestinal system is relatively common -- stomach upset, diarrhea and acid reflux/indigestion are regularly recorded. Fish oil takers commonly report a fishy taste in the mouth. Obviously, people allergic to fish should avoid taking fish oil [source: Medline Plus].
Some effects of fish oil may run contrary to certain health goals. There is no doubt that fish oil can lower triglyceride levels, while numerous reports also conclude that it has little to no effect on levels of low-density lipoprotein cholesterol (LDL, known as "bad" cholesterol). However, the Medline Plus database reports that LDL levels do increase by five to 10 percent in some people. Furthermore, information found on Medline suggests that fish oil supplements may either lead to an increase or decrease in blood sugar levels of diabetics, yet there is no noticeable impact on overall glucose control as recorded on the hemoglobin A1c test. [source: Medline Plus].
The action of fish oil on the blood may lead to an increased risk of bleeding, especially at high- dose levels. Those taking blood thinning medication (including herbs like gingko biloba as well as drugs like Coumadin) or NSAIDs should use caution and consult their physicians before adding fish oil to their diets [source: Medline Plus].