Merck Manual

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Fish Oil


Laura Shane-McWhorter

, PharmD, University of Utah College of Pharmacy

Reviewed/Revised Jan 2022 | Modified Sep 2022

What is fish oil?

Fish oil is derived from the tissues of oily fish, such as mackerel, herring, tuna, and salmon. Fish oil supplements contain oil that is extracted directly or concentrated and put in capsule form.

The active ingredients in fish oil are omega-3 fatty acids (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]). Some plant-based foods, such as flaxseeds, chia seeds, walnuts, and canola oil, contain alpha-linolenic acid, which the body can convert to DHA and EPA. Also, yeast strains that can naturally produce substantial amounts of these oils have been genetically engineered.

Western diets typically are low in omega-3 fatty acids.

What claims are made about fish oil?

Does fish oil work?

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 Heart Failure (HF) Heart failure is a disorder in which the heart is unable to keep up with the demands of the body, leading to reduced blood flow, back-up (congestion) of blood in the veins and lungs, and/or... read more Heart Failure (HF) without decreasing blood pressure. Recent studies showed 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. The 2019 American Heart Association science advisory described limitations of using nonprescription (over-the-counter) fish oil supplements and advised that only prescription products approved by the U.S. Food and Drug Administration be used for people with high triglyceride levels.

Scientific evidence supporting any benefit for people with rheumatoid arthritis is inconclusive.

Controlling risk factors for atherosclerosis Atherosclerosis Atherosclerosis is a condition in which patchy deposits of fatty material (atheromas or atherosclerotic plaques) develop in the walls of medium-sized and large arteries, leading to reduced or... read more Atherosclerosis (such as high blood pressure) and regularly eating more foods that have omega-3 fatty acids and more dark green leafy vegetables may help slow the progression of age-related macular degeneration. However, taking supplements of omega-3 fatty acids does not slow the progression of the disorder.

Some recent studies in animals suggest that omega-3 fatty acids may stop cancers from growing or slow their growth. However, these findings have not been replicated in humans.

Some evidence suggests that fish oil, as supplements or in food, helps relieve symptoms of dry eyes, but the evidence is inconsistent.

Observational evidence shows that individuals who take fish oil supplements may have less depression. However, any such effects have not been proven conclusively and are likely to be small.

What are the possible side effects of fish oil?

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. Taking very high doses of fish oil may increase the risk of developing atrial fibrillation Atrial Fibrillation and Atrial Flutter Atrial fibrillation and atrial flutter are very fast electrical discharge patterns that make the atria (upper chambers of the heart) contract very rapidly, with some of the electrical impulses... read more , an abnormal heart rhythm.

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.

What drug interactions occur with fish oil?

Fish oil should be taken cautiously by people on antihypertensive (blood pressure–lowering) drugs because it may lower blood pressure more than desired.

Fish oil may increase the anticoagulant effect of warfarin, therefore people taking warfarin should check with their doctor about taking fish oil.


Fish oil may decrease rates of heart attack and deaths caused by abnormal heart rhythms and decrease blood levels of triglycerides, a risk factor for coronary artery disease. To maximize the chance of these benefits, prescription formulations should be used. Fish oil may help relieve dry eye symptoms, but evidence for this is inconsistent. Fish oil is relatively safe, but people taking drugs to treat high blood pressure or the anticoagulant warfarin should be very cautious when taking fish oil supplements.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Lovaza, Omacor, Triklo
Cequa, Gengraf , Neoral, Restasis, Sandimmune, SangCya, Verkazia, Vevye
Coumadin, Jantoven
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