Skip to content
Winnowleaf home

Fish Oil Supplements: What the Big Trials Show for Heart, Joints and Brain

Fish oil's clearest benefit is lower triglycerides, about 15% lower in pooled trials (grade A). In large trials of adults without heart disease, it did not clearly prevent heart attacks or strokes (grade A). In heart-outcome trials, atrial fibrillation (an irregular heartbeat) became more common, especially at more than 1 g a day (grade A). Prescription purified EPA is a drug tested in high-risk statin users and is not the same as shop-bought fish oil.

Published How we grade evidenceEditorial policy

Fish oil supplies two omega-3 fats, EPA and DHA. A typical 1,000 mg capsule holds about 180 mg of EPA and 120 mg of DHA, and no upper limit has been set. The FDA considers up to 5 g a day of EPA plus DHA from supplements safe when used as recommended, but in trials of people with or at high risk of heart disease, 4 g a day for several years slightly raised the risk of atrial fibrillation, and high doses may lengthen bleeding time (NIH ODS; Gencer 2021). Across heart trials, the risk of atrial fibrillation rose with dose, mostly above 1 g a day (Gencer 2021). If you have heart rhythm problems, take more than 1 g a day only with your clinician's advice; the same applies with heart disease, a bleeding disorder or a blood thinner. This guide covers adults.

The short version

  • Heart attacks and strokes, in adults without heart disease: no clear drop in two large trials. Grade A (Manson 2019; ASCEND 2018).
  • Triglycerides: about 15% lower, more at higher doses. Grade A (Abdelhamid 2020).
  • Atrial fibrillation (an irregular heartbeat): more common on omega-3 in heart trials, most above 1 g a day. Grade A for harm (Gencer 2021).
  • Prescription omega-3 drugs in high-risk statin users: purified EPA cut heart events in one trial; an EPA plus DHA drug did not in another. Grade B, mixed (Bhatt 2019; Nicholls 2020).
  • Memory and thinking: no protection against decline in older adults. Grade B for no effect, moderate certainty (Brainard 2020).
QuestionWhat trials foundSize of effectGrade
Heart events, adults without heart diseaseNo clear differenceHazard ratio 0.92 (VITAL)A
Death from any causeLittle or no differenceRisk ratio 0.97A
TriglyceridesLowerAbout 15%A
Blood pressure (systolic, adults, against placebo)Slightly lower in short trials; little or no change in trials of a year or moreAbout 2.6 mmHg at 2 to 3 g a dayB
Atrial fibrillation (heart-outcome trials)More commonHazard ratio 1.25; 1.49 above 1 g a dayA (harm)
Prescription omega-3 drugs, high-risk statin usersPurified EPA: fewer heart events; EPA plus DHA drug: no differenceHazard ratio 0.75 vs 0.99B, mixed
Krill vs fish oilBlood levels similar or slightly higher with krillBlood level onlyC

Is fish oil good for your heart?

Grade A: not for preventing a first heart attack or stroke. In VITAL, 25,871 US adults (men 50 and older, women 55 and older) took 1 g of omega-3 a day or placebo for a median of 5.3 years. Major heart events, a main outcome, were not clearly fewer (hazard ratio 0.92), and neither was cancer; heart attacks alone, a secondary outcome, were lower (hazard ratio 0.72) (Manson 2019). In ASCEND, 15,480 adults with diabetes but no heart disease took 1 g capsules or an olive oil placebo for 7.4 years: serious vascular events were 8.9% against 9.2% (ASCEND 2018).

A Cochrane review of 86 trials (162,796 people, with and without heart disease) found little or no effect on deaths from any cause (risk ratio 0.97, high certainty) or on heart and stroke events (0.96, high certainty). Deaths from heart and blood vessel disease also showed little or no difference (0.92, moderate certainty), though coronary heart disease deaths and events may fall slightly (low certainty) (Abdelhamid 2020). In heart trials, atrial fibrillation became more common (hazard ratio 1.25; 1.49 above 1 g a day) (Gencer 2021). Our fish oil page shows D for deaths from any cause, from 2 studies; this review supports A for little or no effect.

What about prescription EPA (REDUCE-IT and STRENGTH)?

Prescription omega-3 drugs, including purified EPA (icosapent ethyl), are FDA-approved medicines for very high triglycerides (Skulas-Ray 2019). Their trial results don't carry over to shop-bought fish oil.

Grade B, mixed. In REDUCE-IT, 8,179 people with heart disease, or diabetes plus other risks, whose triglycerides stayed high on a statin took 4 g of icosapent ethyl a day or placebo for a median of 4.9 years: the main outcome fell from 22.0% to 17.2% (hazard ratio 0.75). Hospital stays for atrial fibrillation rose (3.1% against 2.1%), and serious bleeding was 2.7% against 2.1%, not statistically clear (Bhatt 2019). In STRENGTH, 13,078 statin users at high risk with high triglycerides took 4 g a day of a different prescription omega-3 (a carboxylic acid form) or corn oil; the trial stopped early with no difference (hazard ratio 0.99) and more gut side effects (24.7% against 14.7%) (Nicholls 2020).

REDUCE-IT's placebo was mineral oil, and inflammation markers and some cholesterol-related markers rose in that group over the trial (Ridker 2022). A Danish cohort analysis suggested the different placebo oils may partly explain why the two trials disagree (Doi 2021), while a 2020 review of 80 studies that used mineral oil concluded it did not meaningfully change REDUCE-IT's conclusions (Olshansky 2020).

Does fish oil lower triglycerides and cholesterol?

Grade A for triglycerides. Across the Cochrane trials, omega-3 lowered triglycerides by about 15%, more at higher doses (high certainty), with little effect on other blood fats (Abdelhamid 2020). Pooling 90 trials (72,598 people), triglycerides fell almost in step with dose, especially in people with high blood fats taking more than 2 g a day (Wang 2023). Amounts above 1 g a day raised atrial fibrillation in heart trials; ask your clinician before taking more than 1 g a day, especially with heart disease or a blood thinner (Gencer 2021). In people with triglycerides of 500 mg/dL or more, prescription omega-3 medicines lowered triglycerides by 30% or more under medical care; EPA plus DHA forms raised LDL cholesterol, while EPA alone did not (Skulas-Ray 2019). Triglycerides this high need a doctor, not shop-bought fish oil. For LDL itself, see supplements for high cholesterol.

Does fish oil lower blood pressure?

Grade B: a small drop in adults, larger in people with high blood pressure. Pooling 71 mostly short trials (4,973 adults), EPA plus DHA at 2 to 3 g daily cut systolic pressure by about 2.6 mmHg against placebo, with bigger drops in people with high blood pressure or high blood fats and in older people (Zhang 2022). In the Cochrane trials, which lasted a year or more, blood pressure showed little or no change (Abdelhamid 2020). Amounts above 1 g a day raised atrial fibrillation in heart trials; ask your clinician before taking more than 1 g a day (Gencer 2021). See supplements for blood pressure.

Does fish oil cause atrial fibrillation?

Grade A: it raises the risk. In 7 heart-outcome trials (81,210 people, average age 65, followed about 5 years), omega-3 supplements raised atrial fibrillation (hazard ratio 1.25). The rise was smaller in trials of 1 g a day or less (1.12) and larger in trials of more than 1 g (1.49), and each extra gram added about 11% (Gencer 2021). If you have atrial fibrillation or palpitations, ask your doctor before taking fish oil; see our atrial fibrillation protocol. A Cochrane review found no clear change in heart rhythm problems overall (low certainty) (Abdelhamid 2020). Our fish oil page shows B for this harm; a meta-analysis of 7 heart trials found a dose-related rise overall, which supports A.

Does fish oil thin your blood?

Grade A: no overall rise in bleeding in large heart trials, but more with high-dose EPA. Pooling 11 trials (120,643 people), omega-3 did not raise bleeding overall (rate ratio 1.09), but these heart trials were not designed to test safety with blood thinners or antiplatelet drugs. Bleeding rose with the EPA dose, and high-dose purified EPA raised the relative risk by half, an absolute rise of 0.6% (Javaid 2024). High doses may lengthen bleeding time (NIH ODS). If you take an anticoagulant such as warfarin or a DOAC (apixaban, rivaroxaban, dabigatran or edoxaban), or an antiplatelet drug such as aspirin or clopidogrel, have a bleeding disorder, or have surgery or dental work coming up, ask your clinician before taking fish oil, especially at high doses. With warfarin, fish oil might raise the INR clotting test, so get it checked (NIH ODS). These trials don't show how fish oil behaves with anticoagulants such as apixaban; see supplements and blood thinners and can I take fish oil with Eliquis?, and ask your prescriber or pharmacist.

Does fish oil help joints, eyes, memory or mood?

  • Rheumatoid arthritis: grade C, little clear benefit. Across 23 placebo-controlled trials, pain and swollen or tender joints changed only slightly, with low to very low certainty (Gkiouras 2024). For knee and other joint pain, see supplements for joint pain.
  • Dry eye: grade B, no clear benefit. In 535 people with moderate to severe dry eye, 3,000 mg a day for a year did no better than an olive oil placebo (DREAM 2018). See does fish oil help dry eye?
  • Memory: grade B, no effect. Across 5 trials (33,296 adults), long-chain omega-3 did not lower new cognitive impairment (risk ratio 0.99), with moderate certainty (Brainard 2020). See supplements for memory and brain fog.
  • Treating depression: grade B, small benefit. In 26 double-blind trials of omega-3 for depression (2,160 people), symptoms eased against placebo (standardized mean difference -0.28), mainly with products that were at least 60% EPA (Liao 2019). If you take an antidepressant, see supplements and antidepressants and depression.
  • Preventing depression in adults 50+ without depression: grade B, no benefit. In 18,353 adults aged 50 and older without depression at the start, 1 g a day for a median 5.3 years slightly raised depression or depressive symptoms against placebo (hazard ratio 1.13), with no change in mood scores (Okereke 2021).
  • Pregnancy: see prenatal supplements and take what your midwife or obstetric team advises.

Is krill oil better than fish oil?

Grade C, blood levels only. In 24 healthy adults, krill oil raised blood omega-3 more than fish oil at the same 600 mg dose over 4 weeks (Ramprasath 2013). A 4-week trial matching dose and EPA and DHA content found similar blood levels with fish oil and krill oil (Yurko-Mauro 2015). There is no good evidence that krill oil works better for heart or joint health. Krill are crustaceans, so with a shellfish allergy, ask your clinician first; see our krill oil page.

When should you take fish oil?

Grade C: with a meal containing fat. In male volunteers, a high-fat meal raised EPA absorption from natural fish oil from 69% to 90%, and tripled absorption from the ethyl ester form, to about 60% (Lawson 1988).

Does fish oil go rancid or contain mercury?

  • Rancid (oxidized) oil: of 171 products bought in Canada, half of those that could be tested were over voluntary oxidation limits (Jackowski 2015), while all 20 unflavored products tested in France passed (Pasini 2022). These are lab tests of the oil, not tests of health effects; see how to choose a quality supplement.
  • Mercury: purification removes it (NIH ODS), and 5 products tested had none or negligible amounts (Foran 2003).
  • Cod liver oil also contains vitamin A and vitamin D (NIH ODS). Upper limit for adults: 3,000 mcg RAE (10,000 IU) of preformed vitamin A a day, because more can cause birth defects and liver damage; take more only with blood tests or your clinician's advice (NIH ODS; check your stack). Experts advise anyone who is or might be pregnant, or breastfeeding, not to take high-dose vitamin A supplements (NIH ODS); ask your midwife or obstetric team before taking cod liver oil.

What we'd skip

  • Fish oil to ward off a first heart attack or stroke: no clear benefit in large trials (Manson 2019; ASCEND 2018).
  • High doses on your own if you have atrial fibrillation or palpitations: risk rises with dose (Gencer 2021).
  • Fish oil for memory or dry eye: no benefit in the largest trials (Brainard 2020; DREAM 2018).

Safety and interactions

  • Side effects are usually mild: unpleasant taste, bad breath, heartburn, nausea, stomach upset, diarrhea, headache and fishy-smelling sweat (NIH ODS).
  • Atrial fibrillation: risk rises with dose (see above); ask your clinician before taking more than 1 g a day.
  • Bleeding and blood thinners: see above, and tell your surgeon or dentist what you take.
  • Statins: REDUCE-IT and STRENGTH gave omega-3 on top of statins (Bhatt 2019; Nicholls 2020); don't swap a statin for fish oil.
  • Antidepressants, clopidogrel and other medicines: ask your pharmacist. Don't stop a prescribed medicine on your own.
  • Stacking: cod liver oil plus a multivitamin or separate vitamin A or D can pass the upper limits; check your stack.

When to see a doctor

  • Emergency: chest pain, fainting, or a fast, irregular heartbeat with breathlessness or dizziness.
  • Soon: new palpitations or an irregular pulse on fish oil; unusual bruising, nosebleeds, blood in urine or black stools, especially on a blood thinner.
  • Before starting: atrial fibrillation or other heart rhythm problems, a bleeding disorder, an anticoagulant (such as warfarin or apixaban) or an antiplatelet drug (such as aspirin or clopidogrel), upcoming surgery or dental work, triglycerides of 500 mg/dL or more (which need medical care), or a fish or shellfish allergy.

Don't stop a prescribed medicine on your own; ask the prescriber or pharmacist. If you are pregnant or breastfeeding, ask your midwife or obstetric team. If you are under 18, ask a clinician.

FAQ

Is fish oil good for you? It lowers triglycerides (grade A), but in large trials of adults without heart disease it did not clearly prevent heart attacks or strokes (grade A), and in heart-outcome trials it raised the risk of atrial fibrillation, more so above 1 g a day.

Does fish oil thin your blood? Across 11 large trials (120,643 people), it did not raise bleeding overall; high-dose purified EPA raised it slightly. Those trials don't show how it behaves with anticoagulants such as apixaban. If you have a bleeding disorder, take an anticoagulant such as warfarin or apixaban or an antiplatelet drug such as aspirin or clopidogrel, or have surgery coming up, ask your prescriber first.

Is krill oil better than fish oil? Only blood levels have been compared, with mixed results (grade C); there is no good evidence that either works better for health.

What is the best time to take fish oil? With a meal that contains fat, which helped absorption in a small study (grade C).

Is prescription omega-3 the same as fish oil? No. Purified EPA cut heart events in one trial of high-risk statin users, but it is a prescription drug, and it raised hospital stays for atrial fibrillation (3.1% against 2.1%). A high-dose EPA plus DHA drug did not help in another trial.

Sources

  1. Manson JE, et al. Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer. N Engl J Med. 2019. PubMed 30415637 · doi:10.1056/NEJMoa1811403
  2. ASCEND Study Collaborative Group, et al. Effects of n-3 Fatty Acid Supplements in Diabetes Mellitus. N Engl J Med. 2018. PubMed 30146932 · doi:10.1056/NEJMoa1804989
  3. Abdelhamid AS, et al. Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2020. PubMed 32114706 · doi:10.1002/14651858.CD003177.pub5
  4. Gencer B, et al. Effect of Long-Term Marine ɷ-3 Fatty Acids Supplementation on the Risk of Atrial Fibrillation in Randomized Controlled Trials of Cardiovascular Outcomes: A Systematic Review and Meta-Analysis. Circulation. 2021. PubMed 34612056 · doi:10.1161/CIRCULATIONAHA.121.055654
  5. Bhatt DL, et al. Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia. N Engl J Med. 2019. PubMed 30415628 · doi:10.1056/NEJMoa1812792
  6. Nicholls SJ, et al. Effect of High-Dose Omega-3 Fatty Acids vs Corn Oil on Major Adverse Cardiovascular Events in Patients at High Cardiovascular Risk: The STRENGTH Randomized Clinical Trial. JAMA. 2020. PubMed 33190147 · doi:10.1001/jama.2020.22258
  7. Brainard JS, et al. Omega-3, Omega-6, and Polyunsaturated Fat for Cognition: Systematic Review and Meta-analysis of Randomized Trials. J Am Med Dir Assoc. 2020. PubMed 32305302 · doi:10.1016/j.jamda.2020.02.022
  8. Skulas-Ray AC, et al. Omega-3 Fatty Acids for the Management of Hypertriglyceridemia: A Science Advisory From the American Heart Association. Circulation. 2019. PubMed 31422671 · doi:10.1161/CIR.0000000000000709
  9. Ridker PM, et al. Effects of Randomized Treatment With Icosapent Ethyl and a Mineral Oil Comparator on Interleukin-1β, Interleukin-6, C-Reactive Protein, Oxidized Low-Density Lipoprotein Cholesterol, Homocysteine, Lipoprotein(a), and Lipoprotein-Associated Phospholipase A2: A REDUCE-IT Biomarker Substudy. Circulation. 2022. PubMed 35762321 · doi:10.1161/CIRCULATIONAHA.122.059410
  10. Doi T, et al. A possible explanation for the contrasting results of REDUCE-IT vs. STRENGTH: cohort study mimicking trial designs. Eur Heart J. 2021. PubMed 34455435 · doi:10.1093/eurheartj/ehab555
  11. Olshansky B, et al. Mineral oil: safety and use as placebo in REDUCE-IT and other clinical studies. Eur Heart J Suppl. 2020. PubMed 33061866 · doi:10.1093/eurheartj/suaa117
  12. Wang T, et al. Association Between Omega-3 Fatty Acid Intake and Dyslipidemia: A Continuous Dose-Response Meta-Analysis of Randomized Controlled Trials. J Am Heart Assoc. 2023. PubMed 37264945 · doi:10.1161/JAHA.123.029512
  13. Zhang X, et al. Omega-3 Polyunsaturated Fatty Acids Intake and Blood Pressure: A Dose-Response Meta-Analysis of Randomized Controlled Trials. J Am Heart Assoc. 2022. PubMed 35647665 · doi:10.1161/JAHA.121.025071
  14. Javaid M, et al. Bleeding Risk in Patients Receiving Omega-3 Polyunsaturated Fatty Acids: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. J Am Heart Assoc. 2024. PubMed 38742535 · doi:10.1161/JAHA.123.032390
  15. Gkiouras K, et al. Efficacy of n-3 fatty acid supplementation on rheumatoid arthritis' disease activity indicators: a systematic review and meta-analysis of randomized placebo-controlled trials. Crit Rev Food Sci Nutr. 2024. PubMed 35900212 · doi:10.1080/10408398.2022.2104210
  16. Dry Eye Assessment and Management Study Research Group, et al. n-3 Fatty Acid Supplementation for the Treatment of Dry Eye Disease. N Engl J Med. 2018. PubMed 29652551 · doi:10.1056/NEJMoa1709691
  17. Liao Y, et al. Efficacy of omega-3 PUFAs in depression: A meta-analysis. Transl Psychiatry. 2019. PubMed 31383846 · doi:10.1038/s41398-019-0515-5
  18. Okereke OI, et al. Effect of Long-term Supplementation With Marine Omega-3 Fatty Acids vs Placebo on Risk of Depression or Clinically Relevant Depressive Symptoms and on Change in Mood Scores: A Randomized Clinical Trial. JAMA. 2021. PubMed 34932079 · doi:10.1001/jama.2021.21187
  19. Ramprasath VR, et al. Enhanced increase of omega-3 index in healthy individuals with response to 4-week n-3 fatty acid supplementation from krill oil versus fish oil. Lipids Health Dis. 2013. PubMed 24304605 · doi:10.1186/1476-511X-12-178
  20. Yurko-Mauro K, et al. Similar eicosapentaenoic acid and docosahexaenoic acid plasma levels achieved with fish oil or krill oil in a randomized double-blind four-week bioavailability study. Lipids Health Dis. 2015. PubMed 26328782 · doi:10.1186/s12944-015-0109-z
  21. Lawson LD, et al. Absorption of eicosapentaenoic acid and docosahexaenoic acid from fish oil triacylglycerols or fish oil ethyl esters co-ingested with a high-fat meal. Biochem Biophys Res Commun. 1988. PubMed 2847723 · doi:10.1016/s0006-291x(88)80937-9
  22. Jackowski SA, et al. Oxidation levels of North American over-the-counter n-3 (omega-3) supplements and the influence of supplement formulation and delivery form on evaluating oxidative safety. J Nutr Sci. 2015. PubMed 26688721 · doi:10.1017/jns.2015.21
  23. Pasini F, et al. Assessment of Lipid Quality in Commercial Omega-3 Supplements Sold in the French Market. Biomolecules. 2022. PubMed 36291569 · doi:10.3390/biom12101361
  24. Foran SE, et al. Measurement of mercury levels in concentrated over-the-counter fish oil preparations: is fish oil healthier than fish? Arch Pathol Lab Med. 2003. PubMed 14632570 · doi:10.5858/2003-127-1603-MOMLIC