Omega-3 Side Effects Quick Reference: Common Reactions and When to Seek Care
A quick table of omega-3 side effects, from taste and stomach upset to bleeding and allergy, with the frequencies sources report and when to seek care.
Content last updated . This page has not been medically reviewed.
Urgent first
A serious allergic reaction needs emergency care: call 911 in the US. Prescription omega-3 labels list anaphylactic reaction and hives among reports after approval, and the icosapent ethyl label advises stopping and seeking medical attention if a reaction occurs in a person with fish or shellfish allergy. [1][2] Bleeding that will not stop, or a head injury while taking a blood thinner, also needs emergency care. Otherwise, a clinician or pharmacist can help with side effects; do not stop a prescribed medicine on your own.What the sources say
NCCIH describes the side effects of omega-3 supplements as usually mild, including unpleasant taste, bad breath, bad-smelling sweat and headache, and stomach symptoms such as heartburn, nausea and diarrhea. [3] NCCIH gives no percentages. The only frequencies we found are from prescription trials, so they apply to those products at those amounts and may differ for over-the-counter oils. Evidence label: established for mild stomach and taste effects; mixed for serious effects.Reference table
| Reaction | Reported frequency (as sources state) | When to seek care |
|---|---|---|
| Taste change, fishy aftertaste | Prescription EPA and DHA ethyl esters, pooled from 23 trials: taste perversion 4% vs under 1% on placebo; belching (eructation) 4% vs 1%. [1] Over-the-counter: no percentage in the sources we read. [3] | Usually not urgent; ask a pharmacist if it persists. See fish oil burps and aftertaste |
| Indigestion, heartburn | Dyspepsia 3% vs 2% on placebo (same prescription trials). [1] NCCIH lists heartburn. [3] | Ask a clinician if severe or with chest pain; call 911 for chest pain |
| Nausea, diarrhea | Listed by NCCIH without a percentage. [3] Constipation, vomiting and other digestive disorders were reported in prescription trials. [1] | Ask a clinician if it persists or if you cannot keep fluids down |
| Bad breath, body odor, headache | Listed by NCCIH without a percentage. [3] | Not urgent by themselves |
| Bleeding or bruising | Prescription EPA 4 g a day: bleeding events 12% vs 10% on placebo; serious bleeding 3% vs 2%; more bleeding with aspirin, clopidogrel or warfarin. [2] VITAL (1 g a day of marine omega-3, 25,871 adults) reported no excess risk of bleeding or serious adverse events. [4] | Tell your prescriber if on a blood thinner. Emergency care for heavy or non-stopping bleeding |
| Irregular heartbeat (atrial fibrillation) | Meta-analysis of 7 trials: hazard ratio 1.25 (95% CI 1.07 to 1.46). [5] Prescription EPA: hospitalization for atrial fibrillation or flutter 3.1% vs 2.1% on placebo. [6] | Palpitations, dizziness or breathlessness: seek medical advice; call 911 if severe |
| Joint or muscle pain, swelling, constipation, gout | Prescription EPA in a cardiovascular trial: musculoskeletal pain, peripheral edema, constipation, gout (3% or more, at least 1% above placebo). [2] | Ask your prescriber |
| Raised liver enzymes (ALT, AST), rash, itching | Reported in prescription trials of EPA and DHA ethyl esters; the label advises monitoring in liver impairment. [1] | Ask your prescriber; liver tests are clinician-led |
| Higher LDL cholesterol | The label says LDL-C increased in some patients. [1] | Lipid tests through your clinician |
| Allergic reaction (hives, anaphylaxis) | Reported after approval, frequency unknown. [1] | Call 911 in the US for a serious reaction |
Limits and uncertainty
Prescription trial percentages describe a few products at about 4 g a day and in specific patients. Proper evidence not found: we searched PubMed for pooled side-effect rates of over-the-counter fish oil capsules in the general population and found no source with usable percentages beyond the VITAL trial result. Reports after approval cannot show how common a reaction is or prove cause. For product quality questions see oxidation and rancidity.Safety and who should ask
Ask a clinician or pharmacist before using omega-3 if you take blood thinners, have liver disease, a heart rhythm problem, fish or shellfish allergy, are pregnant, or are giving it to a child. To report a problem, see Side effects, allergic reactions and reporting. The safety overview links to the detail pages.Frequently asked questions
What are the most common omega-3 side effects?
Unpleasant taste, bad breath, bad-smelling sweat, headache, heartburn, nausea and diarrhea. [3]
How common is the fishy aftertaste?
In pooled prescription trials, taste perversion was 4% vs under 1% on placebo and belching 4% vs 1%. Over-the-counter rates were not found. [1]
Can omega-3 cause bleeding?
A prescription EPA trial reported more bleeding than placebo; VITAL found no excess at 1 g a day. [2][4]
Does omega-3 cause diarrhea?
NCCIH lists diarrhea as a possible side effect, without a percentage. [3]
When should I stop and call someone?
Do not stop a prescribed medicine on your own. For a serious allergic reaction call 911 in the US. For other worries, call your clinician or pharmacist.
References
- FDA prescribing information for omega-3-acid ethyl esters capsules, via openFDA drug label API (label effective 2023-10-06). Read 2026-10-23.
- FDA prescribing information for icosapent ethyl capsules, via openFDA drug label API (label effective 2026-03-23). Read 2026-10-23.
- NCCIH, Omega-3 Supplements: What You Need To Know. Read 2026-10-23.
- Manson JE et al., Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer (VITAL). N Engl J Med 2019. Read 2026-10-23.
- Gencer B et al., Effect of Long-Term Marine ω-3 Fatty Acids Supplementation on the Risk of Atrial Fibrillation in Randomized Controlled Trials of Cardiovascular Outcomes. Circulation 2021. Read 2026-10-23.
- Bhatt DL et al., Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia (REDUCE-IT). N Engl J Med 2019. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.