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

Ulven SM, et al. Metabolic effects of krill oil. Lipids Health Dis. 2011
American College of Allergy, Asthma & Immunology — Shellfish Allergy
National Institutes of Health — Omega-3 Fatty Acids