Merck Manual

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Fish Oil

By

Laura Shane-McWhorter

, PharmD, University of Utah College of Pharmacy

Last full review/revision Jul 2020| Content last modified Jul 2020
Click here for the Professional Version
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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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. Other nonfish sources of omega-3 fatty acids are walnuts and flaxseed oil.

Medicinal claims

Fish oil is used for the prevention and treatment of coronary artery disease. Previous evidence showed that fish oil probably decreases heart attacks and death due to abnormal heart rhythms in people with coronary artery disease. Fish oil also appeared to decrease blood levels of triglycerides, a risk factor for coronary artery disease. Higher doses of fish oil may also decrease risk of coronary artery disease and heart failure without decreasing blood pressure. The most recent evidence shows that taking fish oil for 1 to 7 years decreased levels of triglycerides, but probably did not have much of an effect on heart attacks and death due to abnormal heart rhythms or in strokes in people with coronary artery disease. Fish oil may help 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.

Possible side effects

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.

Possible drug interactions

Fish oil should not be taken by people on antihypertensive drugs as it may lower blood pressure more than desired. Fish oil may increase the anticoagulant effect of warfarin, therefore people taking warfarin should avoid fish oil.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Drugs Mentioned In This Article

Generic Name Select Brand Names
NEORAL, SANDIMMUNE
COUMADIN
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
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