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Fish Oil Side Effects: What to Know

Last reviewed: December 2025

Overview

Fish oil supplements are generally well-tolerated. Most side effects are mild and related to the digestive system. Serious side effects are uncommon at standard doses but become more relevant at high therapeutic doses.

Common Side Effects

These effects are frequently reported but usually mild:

  • Fishy aftertaste or burps — the most common complaint
  • Bad breath (fish odor)
  • Heartburn or acid reflux
  • Nausea
  • Diarrhea or loose stools
  • Bloating

Tips to Reduce Common Side Effects

  • Take with food (especially fat-containing meals)
  • Use enteric-coated capsules (release in intestines, not stomach)
  • Refrigerate or freeze capsules
  • Split doses throughout the day
  • Try a different brand or formulation

Less Common Side Effects

  • Stomach pain
  • Changes in taste
  • Oily skin or increased sweating
  • Headache

Serious Side Effects

Seek medical attention if experiencing:

  • Unusual bleeding or bruising
  • Blood in stool or urine
  • Signs of allergic reaction:
    • Rash or hives
    • Swelling of face, lips, or throat
    • Difficulty breathing

Blood Thinning Effects

Fish oil has mild antiplatelet effects:

  • At standard doses (1–3 g/day): Minimal bleeding risk for most people
  • At high doses (4+ g/day): May increase bleeding time
  • With anticoagulants: Discuss with healthcare provider; monitoring may be needed
  • Before surgery: May need to stop 1–2 weeks prior

Drug Interactions

  • Blood thinners (warfarin, aspirin, clopidogrel): May increase bleeding risk
  • Blood pressure medications: May enhance blood pressure-lowering effects
  • Orlistat (weight loss medication): May reduce omega-3 absorption
  • Vitamin E: Often combined; generally safe but adds to antioxidant load

Quality and Purity Concerns

Low-quality fish oil may contain:

  • Oxidized (rancid) oils — can cause more GI upset and reduce benefits
  • Heavy metals (mercury) — reputable brands test for purity
  • PCBs and dioxins — should be minimized through purification

Choose products with third-party testing (USP, NSF, IFOS certification).

Allergies

  • Fish allergy: Those with fish allergy may or may not react to fish oil; consult allergist
  • Shellfish allergy: Fish oil is generally safe (derived from fish, not shellfish)

Sources

  • National Institutes of Health Office of Dietary Supplements — Omega-3 Fatty Acids
  • Mayo Clinic — Fish Oil Side Effects
  • FDA — FDA Announces Qualified Health Claims for Omega-3 Fatty Acids
Last reviewed: December 2025