Fish oil may be extracted directly or concentrated and put in capsule form. Active ingredients are omega-3 fatty acids (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]). Western diets typically are low in omega-3 fatty acids.
Fish oil is used for the prevention and treatment of atherosclerotic cardiovascular disease (see Atherosclerosis). Strong scientific evidence suggests that the fatty acids in fish oil reduce the risk of heart attack and death caused by abnormal heart rhythms in people who have coronary artery disease and are taking traditional drugs. These fatty acids also reduce triglycerides and slightly lower blood pressure. Fish oil helps prevent toxicity to the kidneys caused by the drug cyclosporine. Fish oil supplements are also used to treat rheumatoid arthritis. However, scientific evidence supporting any benefit is inconclusive. For infants, intake of omega-3 fatty acids must be adequate to help the brain develop. Thus, breastfeeding mothers must consume sufficient amounts of omega-3 fatty acids.
Fishy-tasting belching, acne exacerbation, nausea, and diarrhea may occur. A few studies suggest that too much fish oil can cause bleeding, but others do not show a relationship. Although some fish contain excess amounts of mercury, laboratory testing does not consistently show excess mercury in fish oil supplements. Even so, based on documented side effects, pregnant or breastfeeding women should not take omega-3 fatty acid supplements extracted from fish and should limit eating certain types and amounts of fish because of the potential risk of mercury contamination.