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Ingredient guide

Fish oil side effects: what studies report

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Fish oil side effects are usually mild: a fishy taste, bad breath, heartburn, nausea, stomach upset, loose stools, headache and smelly sweat. FDA considers supplements with up to 5 g a day of EPA and DHA safe, but high doses can affect bleeding, so ask first if you take a blood thinner.1,2

Key facts

  • In trials of a prescription omega-3 at 4 g a day, burping, indigestion and taste changes were the most common complaints.3
  • Bleeding time might lengthen at 2 to 15 g a day of EPA and DHA.2
  • An irregular heart rhythm was more common with omega-3 supplements in large trials, mostly above 1 g a day.4
  • Omega-3 supplements have not been found to contain methyl mercury.2

The common ones

Side effects are usually mild: an unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhea, headache and smelly sweat.1,2,5

In trials of a prescription omega-3 medicine at 4 g a day, the most common side effects were burping, indigestion and a change in taste.3

These are mostly digestive. In the prescription product's studies the capsules were taken with meals, and the timing page explains why a meal with some fat also helps absorption.3,6

Bleeding and blood thinners

Doses of 2 to 15 g a day of EPA and DHA might lengthen bleeding time by making platelets less likely to clump.2

Your doctor may want to check on you

High doses of omega-3s may cause bleeding problems when taken with warfarin or other anticoagulants, and fish oil can have antiplatelet effects at high doses. Fish oil might also prolong clotting time, measured as INR, in people taking warfarin. The label of a prescription omega-3 medicine says people who also take an anticoagulant or an antiplatelet medicine should be monitored periodically.1,2,3

On the medicine's label

In the prescription product's own trials, longer bleeding times stayed within normal limits and did not cause clinically significant bleeding episodes. Its label also says that use with anticoagulants has not been studied thoroughly, which is why it advises monitoring.3

Heart rhythm

Two large trials found that 4 g a day for several years slightly raised the risk of atrial fibrillation, an irregular heart rhythm, in people with or at high risk of heart disease. Across seven large trials, the rise was larger above 1 g a day than at 1 g or less.2,4

In REDUCE-IT, 3.1% of people taking 4 g a day of a prescription EPA product were admitted to hospital for an irregular heart rhythm, against 2.1% on placebo. The label of a prescription omega-3 medicine warns of more frequent recurrences in people who already have atrial fibrillation or flutter, especially in the first months of treatment.3,7

Allergy, cholesterol and contaminants

It is not known whether people allergic to fish or shellfish are more likely to react to a prescription omega-3 medicine made from fish oil. Its label advises caution in people with these allergies.3

The label of a prescription omega-3 medicine says it may raise LDL cholesterol in some people and advises checking it during treatment.3

Omega-3 supplements have not been found to contain methyl mercury, which is removed during processing and purification.2

Cod liver oil is the exception among fish oils. It also supplies vitamins A and D, which can be harmful in excessive amounts, so check how much it adds to any other vitamins you take.5

How much is too much

No tolerable upper intake level has been set for EPA and DHA. FDA has concluded that supplements providing no more than 5 g a day of EPA and DHA are safe when used as recommended, and the European Food Safety Authority considers long-term intakes of up to about 5 g a day to be safe.1,2

The Institute of Medicine set no upper limit, but it noted that 900 mg of EPA plus 600 mg of DHA a day or more for several weeks might reduce immune function. The European Food Safety Authority later found that intakes up to about 5 g a day had not been shown to affect immune function or cause bleeding problems.2

Who should ask a doctor first

  • People allergic to fish or shellfish
  • Anyone taking warfarin, another anticoagulant or an antiplatelet medicine such as aspirin
  • People with atrial fibrillation, another heart rhythm problem or a bleeding disorder
  • Anyone prescribed omega-3 medicine for high triglycerides, since a supplement is not a substitute
  • Anyone pregnant, nursing or under 18

Questions people ask

Is it safe to take fish oil every day?

Daily use is how the long trials gave it: VITAL used 1 g a day for a median of 5.3 years. NIH describes side effects as usually mild, and FDA considers up to 5 g a day of EPA and DHA from supplements safe when used as recommended.2

Are the side effects different for women and men?

The sources on this page list the same side effects for everyone. If you are pregnant or nursing, ask your doctor before starting.1

Does fish oil raise cholesterol?

The label of a prescription omega-3 medicine taken at 4 g a day says it may raise LDL cholesterol in some people and advises checking it. The sources here do not report this at typical supplement amounts.3

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Sources

  1. Office of Dietary Supplements, National Institutes of Health. Omega-3 Fatty Acids: Fact Sheet for Consumers. Updated July 18, 2022. Opened 2026-10-07.
  2. National Institutes of Health, Office of Dietary Supplements. Omega-3 Fatty Acids: Fact Sheet for Health Professionals. Updated August 22, 2025. Opened 2026-10-07.
  3. Omega-3-acid ethyl esters capsules: prescribing information (Waylis Therapeutics). DailyMed, U.S. National Library of Medicine. Published March 11, 2026. Opened 2026-10-07.
  4. Gencer B, et al. Effect of Long-Term Marine Omega-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;144(25):1981-1990. Opened 2026-10-01.
  5. National Center for Complementary and Integrative Health, National Institutes of Health. Omega-3 Supplements: What You Need To Know. Last updated November 2024. Opened 2026-10-07.
  6. Lawson LD, Hughes BG. 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;156(2):960-963. Opened 2026-10-07.
  7. Bhatt DL, et al. Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia. N Engl J Med. 2019;380(1):11-22. Opened 2026-10-01.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.