Omega-3 fish oil: what the evidence says

Fish oil supplies two long-chain omega-3 fats, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). The body can make them from ALA, the plant omega-3 in flaxseed, chia seeds and walnuts, but the NIH notes the conversion is very limited, so oily fish such as salmon, mackerel, herring and sardines, and supplements, are the practical sources. In the UK, the NHS recommends at least 2 portions of fish a week, including 1 of oily fish. Large trials have now examined many of the uses fish oil is bought for, with more modest results than the packaging often suggests.

What the evidence shows

This review only counts human studies. Grades mean: strong (consistent results from several good-quality trials or meta-analyses), moderate (good trials exist, but effects are small, trials are few or results vary), weak (small or low-quality studies, mixed results) and insufficient (too little human research to judge). Where good evidence points to no benefit, or to harm, the table says so.

• Lowering blood triglycerides. Strong. A 2020 Cochrane review found long-chain omega-3s lowered triglycerides by about 15%. An American Heart Association advisory reported that 4 g a day of prescription EPA plus DHA lowered very high triglycerides (500 mg/dL or more) by 30% or more.
• Heart attacks, strokes and deaths (around 1 g a day). Strong, little or no benefit. The 2020 Cochrane review (86 trials, 162,796 people) found high-certainty evidence of little or no effect on deaths from any cause or on cardiovascular events, with low-certainty evidence of a slight reduction in coronary heart disease events. The VITAL trial (25,871 adults, 1 g a day) and the ASCEND trial (15,480 people with diabetes, 1 g a day) found no significant reduction in their main cardiovascular outcomes.
• High-dose prescription omega-3 in people at high heart risk. Moderate. REDUCE-IT (8,179 statin-treated patients) found 4 g a day of icosapent ethyl, a prescription form of EPA, cut major cardiovascular events by 25% compared with a mineral oil placebo. STRENGTH (13,078 patients) found 4 g a day of a prescription EPA and DHA product had no benefit compared with corn oil. The NIH notes that mineral oil is not a neutral placebo and may have made REDUCE-IT's benefit look larger. Neither product is an over-the-counter supplement.
• Atrial fibrillation (an irregular heart rhythm). Evidence of harm. A 2021 meta-analysis of seven trials (81,210 people) linked omega-3 supplements with a 25% higher risk of atrial fibrillation. The increase was 49% in trials using more than 1 g a day and 12% in trials using 1 g a day or less.
• Preterm birth (supplements in pregnancy). Strong. A 2018 Cochrane review (70 trials, 19,927 women) found high-quality evidence that omega-3s in pregnancy lowered the risk of birth before 37 weeks by 11% and before 34 weeks by 42%.
• Depression. Weak. According to the NIH, a 2015 Cochrane review found insufficient evidence to judge whether omega-3s help major depression in adults; a small effect on symptoms was not considered clinically meaningful.
• Memory and dementia. Moderate, no benefit. The NIH reports that systematic reviews show omega-3 supplements don't improve cognitive function in healthy older adults or in people with Alzheimer's disease. Possible small benefits in mild cognitive impairment still need confirming.
• Dry eye disease. Moderate, no benefit. The DREAM trial gave 535 people with moderate-to-severe dry eye 3,000 mg a day of EPA and DHA or an olive oil placebo for 12 months and found no significant difference in symptoms or signs.

What this means in practice

Large trials don't support standard fish oil capsules for heart protection in generally healthy people. The clearest effects are lower triglycerides and a lower risk of preterm birth in pregnancy. Because the atrial fibrillation signal grows with dose, more is not necessarily better.

Doses used in studies

These are amounts researchers gave participants. They aren't recommendations.

• VITAL and ASCEND: a 1 g capsule a day providing 460 mg EPA and 380 mg DHA (840 mg in total).
• REDUCE-IT: 4 g a day of icosapent ethyl (prescription EPA).
• STRENGTH: 4 g a day of a prescription EPA and DHA product.
• DREAM: 3,000 mg a day (2,000 mg EPA plus 1,000 mg DHA).
• Cochrane heart review: trials used anything from 0.5 g to more than 5 g a day.

For context, the European Food Safety Authority (EFSA) set an adequate intake of 250 mg of EPA plus DHA a day for adults, with an extra 100 to 200 mg of DHA in pregnancy. EFSA didn't set an upper limit, but concluded that supplemental intakes of up to about 5 g a day of EPA and DHA combined don't raise safety concerns for adults.

Safety, side effects and interactions

Side effects are usually mild. The NIH lists an unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhoea, headache and smelly sweat.

The main safety signal is atrial fibrillation. In heart trials it was more common at higher doses, and the NIH notes that two large trials of 4 g a day found a slightly higher risk in people with or at high risk of heart disease.

Bleeding and blood thinners. High doses of fish oil can affect platelets. The NIH says fish oil might raise INR (a blood-clotting measure) in people taking warfarin, although most research found 3 to 6 g a day did not significantly change anticoagulation; prescribing information for omega-3 medicines advises periodic INR monitoring with anticoagulants. The US National Center for Complementary and Integrative Health (NCCIH) advises anyone taking blood thinners or anti-inflammatory painkillers (NSAIDs) to talk to a health professional before taking omega-3 supplements.

Allergies, pregnancy and children. NCCIH also advises checking with a health professional first if you're allergic to fish or shellfish, are pregnant or breastfeeding, or are thinking of giving omega-3s to a child. Unless your GP advises otherwise, the NHS says not to take cod liver oil or other fish liver oil supplements when you're pregnant or trying for a baby, because they're high in vitamin A, which can harm a developing baby.

If you have chest pain, a racing or irregular heartbeat with breathlessness or dizziness, or bleeding that won't stop, call 999. For urgent advice that isn't an emergency, call NHS 111.

UK regulatory status

Fish oil supplements are sold in the UK as food supplements, under the Food Supplements (England) Regulations 2003 (as amended) and equivalent rules in Scotland, Wales and Northern Ireland. They don't need approval before sale, and local authorities enforce the rules. Labels may only use authorised health claims from the GB Nutrition and Health Claims Register, and each EPA/DHA claim comes with conditions:

• "EPA and DHA contribute to the normal function of the heart": the benefit is obtained with 250 mg of EPA and DHA a day.
• DHA claims for normal brain function and normal vision: the benefit is obtained with 250 mg of DHA a day.
• Normal blood triglyceride levels (2 g of EPA and DHA a day) and normal blood pressure (3 g a day): on supplements, these claims must also tell consumers not to exceed a supplemental daily intake of 5 g of EPA and DHA combined.

Prescription omega-3 medicines are a separate category. NICE has issued guidance on icosapent ethyl (Vazkepa) with statin therapy for reducing the risk of cardiovascular events in adults with raised triglycerides.

What to look for when buying

• EPA plus DHA, not "fish oil". According to the NIH, a typical 1,000 mg fish oil capsule contains about 180 mg EPA and 120 mg DHA, though amounts vary widely. Higher Nature's Omega 3 Fish Oil at Holland & Barrett, for example, is labelled "600mg EPA & DHA".
• Serving size. Check whether the EPA and DHA figure is per capsule or per two- or three-capsule serving.
• Mercury. Fish can contain methylmercury, but the NIH says omega-3 supplements haven't been found to contain it because it's removed during processing and purification.
• Fish oil, not fish liver oil. Cod liver oil also provides vitamins A and D, which count towards your total intake.
• Vegan options. Algal oil supplements usually provide around 100 to 300 mg of DHA, and some contain EPA. Plant foods such as flaxseed provide ALA, which the body converts to EPA and DHA only in small amounts (under 15%, per the NIH).
• Capsule material. Check the ingredients if you avoid certain animal products; many softgels are made from gelatine.

FAQ

Does fish oil protect your heart?

For most people taking around 1 g a day, large trials found little or no effect on heart attacks, strokes or deaths. Prescription icosapent ethyl is a separate medicine used for some high-risk patients, and that's a decision for your doctor.

How much oily fish should I eat?

The NHS says a healthy, balanced diet should include at least 2 portions of fish a week, including 1 of oily fish. A portion is around 140g. There are upper limits on oily fish for girls and women who could become pregnant.

Can I take fish oil in pregnancy?

Omega-3 supplements in pregnancy lowered the risk of preterm birth in trials, but don't take cod liver oil or any supplement containing vitamin A (retinol) when pregnant. Check with your midwife or GP first.

Can I take fish oil with blood thinners?

Ask your doctor or pharmacist first. Fish oil can affect clotting, and people on warfarin may need their INR monitored.

Does fish oil help dry eyes?

The largest trial, DREAM, found no benefit over placebo. Speak to an optometrist or GP about other options.

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