Omega-3 fatty acids.

Three names, different biology

ALA is an essential fatty acid: the body cannot make it. EPA and DHA are found in some cell membranes and can be obtained from seafood or algae. Fish oil, krill oil, cod-liver oil and algal oil differ in composition and amount. “Omega-3” on the front does not tell you which fats or how much are present. See storing omega-3 products.

The weight of fish oil is not the same as the weight of EPA plus DHA. The US National Institutes of Health gives an example: 1,000 mg of fish oil containing 180 mg EPA and 120 mg DHA provides 300 mg EPA plus DHA, not 1,000 mg. Products vary. Check whether amounts are per capsule or serving and how many capsules make a serving. This is an example, not a suggested intake or proof that a product reproduces a trial result. See how to read a label.

Cod-liver oil can also contain vitamins A and D, which matter when combining supplements. The UK NHS advises against cod-liver oil in pregnancy because excess preformed vitamin A can harm development. Ask a midwife or clinician about the exact product; see vitamin A and pregnancy.29101

What evidence supports

Eating fish and seafood is a dietary pattern with its own nutritional context. Supplement trials ask more specific questions: effects on triglycerides, cardiovascular events, pregnancy outcomes or inflammation can differ. A result in people at high cardiovascular risk should not automatically be applied to everyone.2

Fish-oil supplements in a general population

In the VITAL trial, 25,871 US adults were randomly assigned a marine omega-3 supplement or placebo. Vitamin D was studied separately. Over a median 5.3 years, the main combined outcome—heart attack, stroke or cardiovascular death—occurred in 386 people assigned omega-3 and 419 assigned placebo. The difference was not statistically significant.

A separate secondary analysis found fewer heart attacks. Individual outcomes need interpretation alongside all outcomes tested. This trial cannot determine the effect of every supplement, dose or population.3

A prescription medicine is a different question

REDUCE-IT tested icosapent ethyl, a prescription medicine derived from EPA, in 8,179 people taking statins. They had elevated triglycerides and either cardiovascular disease or diabetes plus other risk factors. Over a median 4.9 years, a combined cardiovascular outcome occurred in 17.2% assigned the medicine and 22.0% assigned placebo.

Hospitalisation for atrial fibrillation or flutter was more frequent with the medicine: 3.1% versus 2.1%. Serious bleeding was reported in 2.7% versus 2.1%. The manufacturer, Amarin Pharma, funded the trial, and its mineral-oil placebo has raised interpretation questions. These findings do not establish the same benefits or risks for ordinary fish-oil supplements. Discuss prescribed medicines and individual risks with a clinician.4

Why does the comparison oil matter?

A placebo should ideally have little effect on the outcome being studied. REDUCE-IT used mineral oil. A 2022 analysis of blood samples found several markers associated with inflammation and cardiovascular risk rose over time in the mineral-oil group while changing little in the icosapent ethyl group. For example, median LDL cholesterol was about 11% above its starting level after a year in the mineral-oil group. A marker associated with risk is not itself a heart attack or stroke.

There was no third, inactive comparison group. The analysis cannot establish why markers changed or how much, if any, of the difference in clinical events was caused by the oil. Researchers said the implications for the clinical results were uncertain. This was an analysis of REDUCE-IT samples, not a separate randomised comparison. Amarin funded the original trial, and an Amarin employee coauthored the analysis. It does not prove the medicine has no benefit or that supplements have the same benefit.84

Why did another high-dose trial find a different result?

The 2020 STRENGTH trial compared a different EPA-and-DHA formulation with corn oil in 13,078 people taking statins. Participants had high cardiovascular risk, high triglycerides and low HDL cholesterol. The main combined outcome—including cardiovascular death, heart attack, stroke and certain heart procedures or hospitalisations—occurred in 12.0% assigned omega-3 and 12.2% assigned corn oil. The difference was not statistically significant. The trial stopped early after an interim review indicated a low likelihood of showing benefit. AstraZeneca, the developer, funded it.

STRENGTH and REDUCE-IT used different products and comparison oils; their participants and designs also differed. Neither directly compared the two products, so the results alone cannot establish why findings differed. Neither establishes the effect of every shop-bought fish-oil product.64

Can omega-3 affect heart rhythm?

Atrial fibrillation is an irregular heart rhythm that can increase the risk of stroke. In the VITAL Rhythm study, 25,119 US adults without prior cardiovascular disease, cancer or atrial fibrillation were included in an analysis of an EPA-and-DHA supplement versus placebo. Over a median 5.3 years, newly diagnosed atrial fibrillation was recorded in 3.7% assigned the supplement and 3.4% assigned placebo; the difference was not statistically significant. This does not prove that risk is zero or establish the risk of every product or dose. The higher-dose prescription medicine in REDUCE-IT, discussed above, was associated with more hospitalisations for atrial fibrillation or flutter in a different, higher-risk population. Ask a clinician or pharmacist about a specific product, particularly if you have a heart-rhythm condition or take medicines.

In the separate STRENGTH trial described above, researchers reported new-onset atrial fibrillation in 144 of 6,539 people assigned a high-dose EPA–DHA formulation (2.2%) and 86 of 6,539 assigned corn oil (1.3%). This was a prespecified additional outcome, not the trial’s main cardiovascular endpoint. The trial studied a particular formulation in people at high cardiovascular risk; it does not determine the effect of every fish-oil supplement or explain by itself why VITAL Rhythm found a different result.

Palpitations have many possible causes. Seek emergency care if they do not stop, or if they occur with chest pain, shortness of breath, feeling faint or fainting. If those symptoms have stopped, seek urgent medical advice. Do not assume a supplement caused the symptoms or wait to see if they recur.5467

Harms and quality questions

Omega-3 supplements can cause usually mild effects such as an unpleasant taste or breath, heartburn, nausea or diarrhoea. The US FDA recommends no more than 5 g a day of EPA and DHA combined from supplements; this is not a target or a guarantee of safety for an individual. The heart-rhythm evidence above is relevant to high doses. Products differ in oxidation, contaminants, amount and added ingredients. High-dose products can interact with medicines or raise bleeding concerns in some circumstances. If you take warfarin or another anticoagulant, discuss the exact supplement and any planned change with the clinician managing it. NIH notes that high-dose fish oil may affect clotting measurements, although most studies it reviewed did not find a significant effect on warfarin control.29

Prescription omega-3 medicines have their own monitoring instructions. Do not stop prescribed medicines on your own. This page does not recommend a brand, dose or substitute for clinical care.