Krill Oil Side Effects: What to Know

Overview

Krill oil is generally well-tolerated, often better than fish oil for digestive side effects. The phospholipid form and smaller capsule size contribute to its favorable tolerability profile. However, as a shellfish-derived product, it carries allergy considerations.

Common Side Effects

Side effects are less common than with fish oil but may include:

• Fishy aftertaste — generally milder than fish oil
• Mild stomach upset
• Loose stools
• Heartburn — less common than with fish oil
• Belching — typically less “fishy” than fish oil

Why Krill Oil May Be Better Tolerated

• Phospholipid form: Mixes better with stomach contents
• Smaller capsules: Easier to swallow, less oil per capsule
• Antioxidant protection: Astaxanthin helps prevent rancidity
• Less fishy odor: Natural stability reduces off-flavors

Less Common Side Effects

• Nausea
• Bloating
• Diarrhea
• Headache
• Body odor changes

Serious Side Effects

Seek medical attention if experiencing:

• Allergic reaction symptoms:

  • Rash, hives, or itching
  • Swelling of face, lips, tongue, or throat
  • Difficulty breathing
  • Severe stomach pain
  • Dizziness or fainting

• Unusual bleeding or bruising

• Blood in stool or urine

Shellfish Allergy Warning

Critical consideration: Krill are crustaceans, similar to shrimp and crab.

• If you have a shellfish allergy, do not take krill oil
• Reactions can range from mild to severe (anaphylaxis)
• Even mild shellfish allergies warrant avoiding krill oil
• Choose fish oil or plant-based omega-3s (algae oil) instead

Blood Thinning Effects

Like fish oil, krill oil has mild anticoagulant properties:

• Standard doses: Minimal bleeding risk for most people
• With blood thinners: May enhance anticoagulant effects
• Before surgery: May need to stop 1–2 weeks prior
• Consult healthcare provider if on warfarin, aspirin, or other blood thinners

Drug Interactions

• Anticoagulants and antiplatelet drugs: May increase bleeding risk
• Blood pressure medications: May enhance hypotensive effects
• Other supplements: Combining with high-dose fish oil increases total omega-3 intake

Quality and Purity

Choose quality products to minimize risks:

• Third-party testing: Look for certifications (NSF, USP, IKOS)
• Sustainable sourcing: MSC certification indicates responsible harvesting
• Freshness: Check expiration dates; rancid oil causes more side effects

Sources

• National Institutes of Health, Office of Dietary Supplements. Omega-3 Fatty Acids: Fact Sheet for Health Professionals. Last updated February 2023. https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/. Accessed March 2026.
• National Institutes of Health, Office of Dietary Supplements. Omega-3 Fatty Acids: Fact Sheet for Consumers. Last updated February 2023. https://ods.od.nih.gov/factsheets/Omega3FattyAcids-Consumer/. Accessed March 2026.
• MedlinePlus. Fish oil. Health topic. https://medlineplus.gov/ency/article/002317.htm. Accessed March 2026.