Omega-3 side effects: what the trials show, by dose, and who should be careful
Omega-3 side effects are mostly mild: fishy burps, nausea or loose stools. The serious ones depend on dose: in 8 large RCTs with 83,112 people, atrial fibrillation rose about 12% at up to 1 g a day and 51% above 1 g, mainly in people with heart disease or at high risk, while bleeding did not rise in 11 trials with 120,643 people (rate ratio 1.09). Omega-3 does not cause weight gain in 11 RCTs with 617 people. For keto shoppers avoiding fish, a fish-free algal oil fits.
Passionate about writing and a graduate in clinical dietetics. She is particularly interested in phytotherapy and the effects of ketogenic nutrition on cognitive brain function.
What are the side effects of omega-3? The side effects of omega-3 are digestive complaints such as fishy burps, nausea and loose stools, a dose-related rise in atrial fibrillation, a small LDL rise at 2 to 6 g a day, and, with cod liver oil, extra vitamin A. In 8 large RCTs with 83,112 people, atrial fibrillation rose with dose, with RR 1.51 above 1 g a day, mainly in people with heart disease or at high risk [5]. EFSA found EPA plus DHA at 2 to 6 g a day raises LDL by about 3%, and cod liver oil contains vitamins A and D [2], [1]. Bleeding in 11 trials with 120,643 people and weight gain in 11 RCTs with 617 people are not supported by trials [6], [8]. EFSA saw no safety concerns for omega-3 supplements up to 5 g EPA plus DHA a day in adults [2].
Side effect
What the trials show
Dose
Population
Digestive (fishy burps, nausea, loose stools)
common, usually mild
not quantified here
users of fish oil supplements
Atrial fibrillation
RR 1.12
up to 1 g a day
8 RCTs, 83 112 people, mainly heart disease or high risk
Atrial fibrillation
RR 1.51
above 1 g a day
as above
Bleeding
rate ratio 1.09, not significant
trial doses
11 trials, 120 643 people
LDL cholesterol
about 3% rise
2 to 6 g EPA plus DHA a day
EFSA assessment
Body weight
0.00 kg difference
trial doses
11 RCTs, 617 people
Vitamin A excess (cod liver oil only)
upper limit 3 000 µg RE a day
per day, all sources
adults
Taking other supplements as well?
Vitamins in drops and capsules are sold as separate products, so each label shows its own daily amount. See keto vitamins in the BeKeto shop.
Does omega-3 cause digestive side effects? Omega-3 can cause digestive side effects such as fishy burps, a fishy aftertaste, heartburn, nausea and loose stools, which are the most common complaints and are usually mild. Take omega-3 with a meal containing fat [4]. If you use a larger daily amount, splitting it across meals may be easier on your stomach. If you get loose stools, lower the dose and see whether they settle. Fishy burps come from omega-3 fish oil itself; fish-free oils cannot taste of fish. That does not mean fish-free oils have no digestive effects, only that the fishy aftertaste comes from fish oil.
Did you know? Chemical form changes short-term absorption: re-esterified triglycerides 124%, free fatty acids 91% and ethyl esters 73% of natural fish oil; differences largely disappear with long-term use.
Does omega-3 increase bleeding risk?
Does omega-3 increase bleeding risk? Omega-3 does not increase bleeding risk in trials, although it reduces platelet aggregation: across 11 trials with 120,643 people, bleeding was not significantly higher, with a rate ratio of 1.09. [6] That is a population result, not a guarantee for one person. If you take anticoagulants such as warfarin, or you are due to have surgery, ask your GP before starting an omega-3 supplement [6]. The practical point is simple: do not make changes yourself before an operation or while taking blood-thinning medicines. Take the product packaging and dose details to your appointment, so your GP can judge your own risk.
Can omega-3 cause atrial fibrillation?
Can omega-3 cause atrial fibrillation? Omega-3 supplements can raise the risk of atrial fibrillation, and the risk grows with dose: in 8 large RCTs with 83,112 people, it rose by about 12% at up to 1 g a day (RR 1.12) and about 51% above 1 g a day (RR 1.51), mainly in people with heart disease or at high risk. [5] If you are comparing labels, check how much omega-3 per day the serving actually provides. Higher omega-3 blood levels from diet were not linked to more AF in 17 cohorts with 54,799 people [5]. In September 2023, the EMA’s PRAC added atrial fibrillation as a common side effect of omega-3-acid ethyl ester medicines in patients with cardiovascular disease or risk factors, and the UK MHRA said the same in January 2024 [5].
Does omega-3 cause weight gain?
Does omega-3 cause weight gain? Omega-3 does not cause weight gain; across 11 RCTs with 617 people, the difference in body weight was 0.00 kg. [8] A teaspoon of oil adds calories, because it is oil, but the meta-analysis shows no body-weight change [8]. Omega-3 does not raise blood sugar. Meta-analyses show no effect or a small improvement in fasting glucose and insulin resistance, no effect on HbA1c, with high heterogeneity (I² about 90%) [7]. That does not mean omega-3 helps with weight loss either. In the evidence reviewed here, body weight does not move up or down because of omega-3 supplementation [8].
Does omega-3 raise LDL cholesterol?
Does omega-3 raise LDL cholesterol? Omega-3 can raise LDL cholesterol slightly at high doses: EFSA found EPA plus DHA at 2 to 6 g a day raises LDL by about 3%, while EPA up to 4 g a day does not significantly affect LDL. [2] In meta-analyses of RCTs of omega-3 alone, DHA at 2 g a day or more raised LDL by about 0.19 mmol/L versus placebo, while EPA did not [3]. This is why the dose and the type of omega-3 matter. It is not accurate to say simply that omega-3 lowers cholesterol, and it is not accurate to say omega-3 is bad for cholesterol.
Did you know? At 2 g a day or more, DHA also raised HDL in meta-analyses of omega-3 alone.
Which omega-3 suits a fish allergy, and how does BeKeto MEGA Omega compare?
Which omega-3 suits a fish allergy, and how does BeKeto MEGA Omega compare? For adults with a fish allergy, BeKeto MEGA Omega comes out best because it is made from Schizochytrium microalgae, is fish free and is free from allergens requiring declaration. [10] The same microalgae source also makes it a vegan omega-3 oil. One 5 ml teaspoon of lemon-flavoured MEGA Omega 3 vegan algae oil gives 570 mg DHA and 185 mg EPA, 755 mg in total, which falls within the “up to 1 g a day” band in the AF analysis, so do not exceed the stated serving [10], [5]. If you are browsing keto supplements, keep this omega-3 oil’s card wording in view: “This product is intended for adults. Not for use by children or by women who are pregnant or breastfeeding. If you take anticoagulant medication or are due to have surgery, consult your doctor before use.” [10]
Avoiding fish?
MEGA Omega 3 Vegan is fish free and contains no allergens requiring declaration. It is a lemon-flavoured algae oil with 570 mg DHA and 185 mg EPA in 5 ml.
This product is intended for adults. Not for use by children or by women who are pregnant or breastfeeding. If you take anticoagulant medication or are due to have surgery, consult your doctor before use.
Frequently asked questions (FAQ)
Who should not take omega-3 supplements?
Who should not take omega-3 supplements? Omega-3 supplements are not suitable for people allergic to their source, such as fish or shellfish. Ask your GP first if you take anticoagulants or antiplatelet drugs, are due to have surgery or have heart rhythm problems, since AF rose above 1 g a day mainly in people with heart disease or at high risk [5], [6]. Pregnancy or trying for a baby: avoid cod liver oil, because fish liver oils are high in vitamin A (retinol) [9].
Is it safe to take omega-3 every day?
Is it safe to take omega-3 every day? EFSA did not set an upper limit for EPA and DHA, but supplemental intakes of EPA and DHA combined up to 5 g a day do not raise safety concerns for adults. [2] Stick to the manufacturer’s serving, because AF risk rose with doses above 1 g a day, mainly in people with heart disease or at high risk [5].
What should you not mix omega-3 with?
What should you not mix omega-3 with? Do not combine omega-3 with anticoagulants or antiplatelet drugs without asking your GP; trials show no excess bleeding overall, with a rate ratio of 1.09, but individual risk differs. [6] Avoid stacking cod liver oil with other sources of vitamin A: the adult upper limit from all sources is 3 000 µg RE a day [1].
Are omega-3 side effects different for women?
Are omega-3 side effects different for women? Omega-3 side effects are not different for women in the trials cited here, but pregnancy changes the advice. The NHS says to avoid fish liver oil supplements such as cod liver oil when pregnant or trying for a baby, because they are high in vitamin A (retinol) [9].
Is cod liver oil riskier than other omega-3 oils?
Is cod liver oil riskier than other omega-3 oils? Cod liver oil carries an extra risk because it contains vitamins A and D: EFSA’s adult upper limits are 3 000 µg RE vitamin A and 100 µg vitamin D a day. [1] Fish oil from whole fish and algal oil contain no vitamins A and D [1].
Discover more in the BeKeto keto shop
Besides supplements, the shop has keto vitamins, snacks and bread.
A graduate in clinical dietetics whose interests begin, not end, with the word diet. She has written specialist content on nutrition. She is fascinated by contemporary food culture, phytotherapy and the effects of the ketogenic diet on cognitive brain function. She promotes diet therapy and the nutritional treatment of disease.
To provide the best experiences, we use technologies like cookies to store and/or access device information. Consenting to these technologies will allow us to process data such as browsing behaviour or unique IDs on this site. Not consenting or withdrawing consent, may adversely affect certain features and functions.
Functional
Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.Przechowywanie techniczne lub dostęp, który jest używany wyłącznie do anonimowych celów statystycznych. Bez wezwania do sądu, dobrowolnego podporządkowania się dostawcy usług internetowych lub dodatkowych zapisów od strony trzeciej, informacje przechowywane lub pobierane wyłącznie w tym celu zwykle nie mogą być wykorzystywane do identyfikacji użytkownika.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
United KingdomCurrent regionDelivery within the United KingdomCurrency: Pound Sterling (£)
GermanyDelivery within Germany and AustriaCurrency: Euro (€)
FranceDelivery within France and BelgiumCurrency: Euro (€)
ItalyDelivery within ItalyCurrency: Euro (€)
PolandDelivery within Poland and EuropeCurrency: Zloty (PLN)
Czech RepublicDelivery within the Czech Republic and SlovakiaCurrency: Czech Koruna (Kč)
SpainDelivery within SpainCurrency: Euro (€)
