Omega-3 Side Effects Quick Reference: Common Reactions and When to Seek Care

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

1. Bleeding or bruising

   Serious

   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]

   When to seek care: Tell your prescriber if on a blood thinner. Emergency care for heavy or non-stopping bleeding

2. Allergic reaction (hives, anaphylaxis)

   Serious

   Reported after approval, frequency unknown. [1]

   When to seek care: Call 911 in the US for a serious reaction

3. Joint or muscle pain, swelling, constipation, gout

   Moderate

   Prescription EPA in a cardiovascular trial: musculoskeletal pain, peripheral edema, constipation, gout (3% or more, at least 1% above placebo). [2]

   When to seek care: Ask your prescriber

4. Raised liver enzymes (ALT, AST), rash, itching

   Moderate

   Reported in prescription trials of EPA and DHA ethyl esters; the label advises monitoring in liver impairment. [1]

   When to seek care: Ask your prescriber; liver tests are clinician-led

5. Nausea, diarrhea

   Mild

   Listed by NCCIH without a percentage. [3] Constipation, vomiting and other digestive disorders were reported in prescription trials. [1]

   When to seek care: Ask a clinician if it persists or if you cannot keep fluids down

6. Bad breath, body odor, headache

   Mild

   Listed by NCCIH without a percentage. [3]

   When to seek care: Not urgent by themselves

7. Taste change, fishy aftertaste

   Unclear

   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]

   When to seek care: Usually not urgent; ask a pharmacist if it persists. See fish oil burps and aftertaste

8. Indigestion, heartburn

   Unclear

   Dyspepsia 3% vs 2% on placebo (same prescription trials). [1] NCCIH lists heartburn. [3]

   When to seek care: Ask a clinician if severe or with chest pain; call 911 for chest pain

9. Irregular heartbeat (atrial fibrillation)

   Unclear

   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]

   When to seek care: Palpitations, dizziness or breathlessness: seek medical advice; call 911 if severe

10. Higher LDL cholesterol

    Unclear

    The label says LDL-C increased in some patients. [1]

    When to seek care: Lipid tests through your clinician

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.