What Are the Side Effects of Omega-3, 6 and 9? Dose by Dose

The side effects of omega-3, 6 and 9 supplements depend on which oil is in the capsule and how much you take. For EPA and DHA from fish, krill or algal oil, the trials record a fishy aftertaste, burps, loose stools and nausea, mostly at the 2 to 4 g a day used in heart trials, and a rise in atrial fibrillation that is clear above 1 g a day and small at or below it. For a flaxseed-oil 3-6-9 those findings do not apply, because the capsule contains neither EPA nor DHA. Bleeding did not increase even in surgical patients given 8 to 10 g before surgery, blood sugar did not change across 83 trials, and the vitamin A warning belongs to cod liver oil alone.^{1, 2, 3, 4}

• Common and harmless: fishy taste, burps, nausea, loose stools; about 25% against 15% at 4 g a day, worse with oxidised oil
• Dose-related: new atrial fibrillation, about 50% more likely above 1 g of EPA and DHA a day, about 12% at 1 g or less
• Ask first: anticoagulant or antiplatelet medicines, a heart rhythm problem with doses above 1 g, pregnancy

Sources checked on 29 October 2026.

Omega 3 side effects: what the trials recorded

The largest safety analysis pooled 21 randomised trials of prescription-strength EPA and DHA in 24,460 people and found no serious adverse event attributable to the oil, but more fishy taste, belching, nausea and skin itching, and small shifts in some blood tests, including fasting glucose.⁵ At 4 g a day in STRENGTH, 24.7% reported a digestive complaint against 14.7% on corn oil; at 840 mg a day for five years in VITAL, no excess of bleeding or serious adverse events.^{6, 7}

What are the side effects early on when taking omega-3?

A fishy taste and the occasional fishy burp, sometimes mild nausea or looser stools, and for a few an itchy rash.⁵ Oxidised fish oil tastes rancid, and a survey of 171 shop-bought omega-3 products found half over the voluntary oxidation limits, and a strongly fishy capsule is telling you about the batch.¹¹ A meal helps; when to take omega 3-6-9 covers timing.

What are the side effects of taking too much omega-3?

Above about 1 g a day, the effect that matters is on heart rhythm. Across seven large trials in 81,210 people, those given omega-3 were about 25% more likely to develop atrial fibrillation than those on placebo, and the risk tracked the dose: about 50% higher in trials using more than 1 g a day, about 12% higher at 1 g or less, from a starting rate of about 3.6% over about five years.² In the VITAL Rhythm trial, 840 mg a day for five years produced no significant rise.⁸ Prescription doses belong with a cardiologist. EFSA concluded that supplemental EPA and DHA up to 5 g a day raise no safety concern for adults: a ceiling, not a target.¹² How much omega 3-6-9 you need a day has the dose marks.

Fish oil makes platelets slightly less sticky, yet across 52 studies it did not increase bleeding or transfusions during or after surgery, and 1,516 cardiac-surgery patients given 8 to 10 g over the 2 to 5 days before surgery had no more major bleeding and needed fewer transfusions.^{1, 3} The anticoagulant evidence is on is there any danger when taking omega-3.

Do the omega-6 and omega-9 in a 3-6-9 capsule add side effects?

None that trials have found at the amounts people eat or take. The omega-6 is linoleic acid, the main fat in sunflower, rapeseed and soya oils, nuts and seeds, and across 15 randomised trials raising linoleic acid intake did not increase any inflammatory marker measured.¹³ No UK or European advisory body sets an omega-6 to omega-3 ratio, and every register claim built on one was rejected.¹⁴ The omega-9 is oleic acid, most of olive oil; the body makes it, so no deficiency or excess syndrome exists, and it does not "build up": a two-capsule serving is under half a teaspoon of oil, digested and used or stored like any other fat. What are the dangers of omega-6 and what are the dangers of omega-9 have the detail.

Which "side effects" does the evidence not support?

Too much vitamin A is a cod liver oil matter: fish liver oils are high in retinol, which is why the NHS advises avoiding them in pregnancy and keeping total vitamin A under 1.5 mg a day.¹⁰ Prostate cancer traces to a single 2013 analysis; a meta-analysis of 21 studies found no association, and VITAL's five years at 840 mg made no difference to cancer rates.^{7, 15} Raised blood sugar: across 83 trials omega-3 changed neither HbA1c nor the rate of new diabetes, with a hint of worse readings only above 4.4 g a day.⁴ Why is fish oil no longer recommended covers the headline.

Who should avoid taking omega 3-6-9 supplements?

A flaxseed-oil 3-6-9 contains no EPA or DHA, so those findings rule nobody out; the questions that remain apply to any supplement.

• Anyone on warfarin, a DOAC or an antiplatelet medicine. The pooled data show no increase in clinically significant bleeding, and the NHS tells people on warfarin to let their doctor know about any supplements they take; tell your GP or anticoagulant clinic and keep the daily dose the same.^{1, 16}
• Before an operation. Tell your surgeon; the evidence does not support stopping weeks ahead.^{1, 3}
• Pregnancy and breastfeeding. The NHS caution is about fish liver oil and its vitamin A, which flaxseed oil does not contain.¹⁰
• Allergy. Flaxseed allergy is rare but recorded;¹⁷ fish and shellfish allergies concern fish and krill oils, not flaxseed or algal oil.
• Heart rhythm problems. Ask your cardiologist before taking more than 1 g of EPA and DHA a day.²

A vegan omega 3-6-9 capsule is flaxseed oil: ALA with linoleic acid (omega-6) and oleic acid (omega-9), often with vitamin E, which contributes to the protection of cells from oxidative stress, and no EPA or DHA. How ALA, EPA and DHA differ is in the Wellgard omega-3 guide.

What we don't know

The pooled safety data come from prescription doses of 2 to 4 g; at label doses the large trials report "no excess" rather than counted rates.^{5, 7} Whether the atrial fibrillation signal exists at 1 g or less is unresolved: the pooled estimate was 12% higher; the one large supplement-dose trial found no significant difference.^{2, 8} No trial has recorded the side effects of a flaxseed-oil 3-6-9 capsule.

Frequently asked questions

What are the signs you are taking too much omega-3?

Fishy burps, loose stools and nausea, mainly at 2 g a day and above. The atrial fibrillation signal has no warning sign, which is why doses above 1 g belong with a doctor.^{2, 6}

Does omega-3 raise blood sugar?

No: across 83 trials omega-3 changed neither HbA1c nor the rate of new diabetes.⁴

Is it safe to take omega-3 every day?

At label doses, yes: 840 mg a day for five years produced no excess of serious adverse events in VITAL, and EFSA's safety ceiling is 5 g.^{7, 12}

Sources

1. Begtrup KM, et al. No impact of fish oil supplements on bleeding risk. Danish Medical Journal, 2017. Systematic review, 52 publications; funding not stated on PubMed. PubMed 28552094.
2. Gencer B, et al. Marine omega-3 supplementation and the risk of atrial fibrillation. Circulation, 2021. Meta-analysis, 7 trials, 81,210 people, 4.9 years; publicly funded (NIH). PubMed 34612056.
3. Akintoye E, et al. Fish oil and perioperative bleeding. Circulation: Cardiovascular Quality and Outcomes, 2018. Randomised trial, 1,516 cardiac-surgery patients; funding not stated on PubMed. PubMed 30571332.
4. Brown TJ, et al. Polyunsaturated fat and type 2 diabetes. BMJ, 2019. Meta-analysis, 83 trials of 24 weeks or longer; publicly funded (WHO). PubMed 31434641.
5. Chang CH, et al. Safety and tolerability of prescription omega-3 fatty acids. Prostaglandins, Leukotrienes and Essential Fatty Acids, 2018. Meta-analysis, 21 trials, 24,460 people; funding not stated on PubMed. PubMed 29482765.
6. Nicholls SJ, et al. High-dose omega-3 fatty acids vs corn oil and major adverse cardiovascular events (STRENGTH). JAMA, 2020. Randomised trial, 13,078 statin-treated patients, stopped early; manufacturer-funded. PubMed 33190147.
7. Manson JE, et al. Marine n-3 fatty acids and prevention of cardiovascular disease and cancer (VITAL). New England Journal of Medicine, 2019. Randomised trial, 25,871 adults, median 5.3 years; publicly funded (NIH). PubMed 30415637.
8. Albert CM, et al. Marine omega-3, vitamin D and incident atrial fibrillation (VITAL Rhythm). JAMA, 2021. Randomised trial, 25,119 adults, median 5.3 years; publicly funded (NIH). PubMed 33724323.
9. Bhatt DL, et al. Cardiovascular risk reduction with icosapent ethyl for hypertriglyceridemia (REDUCE-IT). New England Journal of Medicine, 2019. Randomised trial, 8,179 statin-treated patients, median 4.9 years; manufacturer-funded. PubMed 30415628.
10. NHS. Fish and shellfish. Page last reviewed 26 June 2026. nhs.uk.
11. Jackowski SA, et al. Oxidation levels of over-the-counter omega-3 supplements. Journal of Nutritional Science, 2015. Product survey, 171 supplements; funding not stated on PubMed. PubMed 26688721.
12. European Food Safety Authority. EFSA assesses safety of long-chain omega-3 fatty acids. Published 27 July 2012. efsa.europa.eu.
13. Johnson GH, Fritsche K. Dietary linoleic acid and markers of inflammation. Journal of the Academy of Nutrition and Dietetics, 2012. Systematic review, 15 randomised trials; funding not stated on PubMed. PubMed 22889633.
14. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Page last updated 19 May 2026. gov.uk.
15. Alexander DD, et al. Long-chain omega-3 fatty acids and prostate cancer. Nutrition and Cancer, 2015. Meta-analysis, 21 prospective studies, 461,140 people; funding not stated on PubMed. PubMed 25826711.
16. NHS. Warfarin. Page last reviewed 24 February 2026. nhs.uk.
17. Parrinello G, Villalta D. Flaxseed allergy: insights from a case series. European Annals of Allergy and Clinical Immunology, 2026. Case series, 3 patients; funding not stated on PubMed. PubMed 41823680.

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This article is general information, not medical advice. If you take an anticoagulant or antiplatelet medicine, have a heart rhythm disorder, are pregnant or have surgery planned, speak to your GP or clinic first.