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EFAs and their sources are generally regarded as harmless, except when there might be a food allergy involved (such as an allergy to shellfish). One concern with fish and fish-oil supplements involves the accumulation of mercury, PCBs, and other environmental toxins. For information that can help you make safe fish choices, see the Environmental Working Group's "Fish List." For fish oil supplements, look for products that are certified to not contain high levels of environmental toxins.
Fish oil has a mild blood-thinning effect;2 in one case report, it increased the effect of the blood-thinning medication warfarin (Coumadin).3 Fish oil does not seem to cause bleeding problems when it is taken by itself4 or with aspirin.5 Nonetheless, people who are at risk of bleeding complications for any reason should consult a physician before taking fish oil.
Fish oil does not appear to raise blood sugar levels in people with diabetes.6 Nonetheless, if you have diabetes, you should not take any supplement except on the advice of a physician.
Fish oil may modestly increase weight and lower total cholesterol and HDL (“good”) cholesterol levels.7 It may also raise the level of LDL ("bad") cholesterol; however, this effect may be short-lived.8 If you decide to use cod liver oil as your fish oil supplement, make sure you do not exceed the safe maximum intake of vitamin A and vitamin D . These vitamins are fat soluble, which means that excess amounts tend to build up in your body, possibly reaching toxic levels. The official maximum daily intake of vitamin A is 3,000 mcg for pregnant women as well as other adults. Look at the bottle label to determine how much vitamin A you are receiving. (It is less likely that you will get enough vitamin D to produce toxic effects.)