Krill oil is a popular omega-3 supplement derived from Antarctic krill, valued for its content of EPA and DHA fatty acids. Whilst many people tolerate krill oil well, understanding potential krill oil side effects is essential for safe use. Common reactions include gastrointestinal disturbances such as nausea and fishy aftertaste, whilst more serious concerns involve bleeding risks and allergic reactions, particularly in those with shellfish allergies. This article examines the side effects of krill oil, identifies who should avoid it, and provides practical guidance on minimising adverse reactions whilst using this marine-derived supplement.

What Is Krill Oil and How Does It Work?

Krill oil is a marine-derived dietary supplement extracted from Antarctic krill (Euphausia superba), small crustaceans that form a crucial part of the ocean food chain. It has gained popularity as an alternative to traditional fish oil supplements, primarily due to its content of omega-3 polyunsaturated fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Unlike fish oil, krill oil also contains the antioxidant astaxanthin, which gives it a distinctive reddish colour.

The omega-3 fatty acids in krill oil are predominantly bound to phospholipids rather than triglycerides, the form found in most fish oils. Some research suggests this structural difference may affect how the body processes these fatty acids, though evidence for superior bioavailability remains limited. Once absorbed, EPA and DHA may influence inflammatory processes by affecting eicosanoid production and lipid metabolism.

Proposed mechanisms of action include:

• Potential reduction of certain inflammatory mediators

• Possible effects on blood lipid levels, primarily triglycerides

• Potential contribution to cardiovascular health

• Possible support for neurological function

It's important to understand that krill oil is classified as a food supplement in the UK, not a licensed medicine. The evidence supporting its clinical benefits remains mixed, with some studies suggesting modest effects whilst others show inconclusive results. Current NICE guidance (NG238) does not recommend routine use of omega-3 supplements for cardiovascular disease prevention.

Krill oil supplements should not be used as a substitute for prescribed lipid-lowering medications or prescription-only omega-3 products such as icosapent ethyl. As with any supplement, understanding potential side effects is essential for informed decision-making.

Common Side Effects of Krill Oil

Most individuals tolerate krill oil well when taken at recommended doses, typically ranging from 500 mg to 2,000 mg of total krill oil daily (providing approximately 100-300 mg of combined EPA and DHA). However, as with any supplement containing omega-3 fatty acids, certain mild and self-limiting side effects may occur, particularly during the initial weeks of use.

Gastrointestinal disturbances are amongst the most frequently reported side effects. Users may experience:

• Nausea or mild stomach discomfort

• Diarrhoea or loose stools

• Indigestion or bloating

• A fishy aftertaste or burping

These symptoms typically arise because omega-3 supplements can affect digestive processes. Taking krill oil with meals, particularly those containing some fat, may help reduce gastrointestinal upset and improve tolerability.

Other possible side effects include:

• Unpleasant breath or body odour with a fishy quality

• Changes in bowel habits

• Mild abdominal discomfort

Some users may experience headaches or skin reactions, though these appear to be less common. Most side effects are generally transient and do not require medical intervention. If gastrointestinal symptoms persist beyond two weeks or become troublesome, it may be worth reducing the dose or discontinuing use temporarily.

It is important not to exceed the recommended dose stated on the product label. Individual responses vary considerably, and what proves tolerable for one person may cause discomfort in another. Keeping a symptom diary during the first month of supplementation can help identify patterns and inform discussions with healthcare professionals.

If you experience persistent vomiting, severe abdominal pain, or black/tarry stools, stop taking the supplement and seek medical advice promptly.

Serious Side Effects and When to Seek Medical Advice

Whilst serious adverse effects from krill oil are uncommon, certain potentially significant reactions warrant medical attention. One area of concern relates to krill oil's possible effects on bleeding, as omega-3 fatty acids may have mild antiplatelet properties.

Bleeding-related signs to watch for:

• Unexplained bruising or petechiae (small red or purple spots on the skin)

• Prolonged bleeding from minor cuts or dental procedures

• Nosebleeds that are difficult to control

• Blood in urine or stools

• Unusually heavy menstrual bleeding

These effects are rare with krill oil alone at recommended doses in healthy individuals. However, if you take anticoagulant or antiplatelet medications such as warfarin, aspirin, or clopidogrel, discuss krill oil use with your GP or pharmacist before starting. If you're on warfarin, continue your usual INR monitoring schedule and inform your anticoagulation clinic about any new supplements.

Allergic reactions represent another serious concern, especially in individuals with shellfish allergies. Symptoms requiring urgent medical assessment include:

• Facial swelling (angioedema), particularly of the lips, tongue, or throat

• Difficulty breathing or wheezing

• Widespread urticaria (hives) or severe skin rash

• Dizziness, rapid pulse, or signs of anaphylaxis

Although krill oil proteins differ from those in shellfish such as prawns or crab, cross-reactivity can occur. Anyone with a known shellfish allergy should avoid krill oil.

When to seek medical help:

• Call 999 or go to A&E immediately for severe allergic reactions (anaphylaxis) or uncontrolled bleeding

• Contact your GP or NHS 111 for unexplained bruising, persistent side effects, or concerning symptoms

• Seek urgent medical advice for severe or persistent abdominal pain

• Contact your GP if you notice yellowing of skin or eyes (jaundice)

If you suspect krill oil has caused an adverse reaction, you can report it through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Who Should Avoid Krill Oil?

Certain groups face elevated risks from krill oil supplementation and should either avoid it entirely or use it only under healthcare professional guidance.

Individuals with shellfish allergies should avoid krill oil supplements. Krill are crustaceans, and whilst the refining process removes most proteins, trace amounts may remain. Even refined krill oil has triggered allergic reactions in susceptible individuals. Check product allergen labelling carefully, as krill (crustaceans) is one of the 14 major allergens that must be declared under UK food labelling regulations.

Patients on anticoagulant or antiplatelet therapy require particular caution. This includes those taking:

• Warfarin or other vitamin K antagonists

• Direct oral anticoagulants (DOACs) such as apixaban, rivaroxaban, or dabigatran

• Antiplatelet agents including aspirin, clopidogrel, or dipyridamole

• Non-steroidal anti-inflammatory drugs (NSAIDs) used regularly

Whilst not absolutely contraindicated, krill oil should only be used in these patients following discussion with their GP or pharmacist. If you're taking multiple medications, a medication review can help identify potential interactions.

Other groups who should exercise caution or avoid krill oil include:

• Individuals scheduled for surgery (inform your surgical and anaesthetic team about all supplements and follow their advice on whether and when to stop)

• People with bleeding disorders such as haemophilia or von Willebrand disease

• Pregnant or breastfeeding women (discuss with your midwife or GP before taking any supplements)

• Those with fish or seafood allergies (potential for cross-reactivity)

• Individuals with pancreatic disorders or severe liver disease

Children under 18 should not take krill oil supplements without paediatric advice, as safety and appropriate dosing have not been established in this age group. Anyone with multiple health conditions or taking several medications should consult their GP or a pharmacist before starting krill oil to assess potential drug-supplement interactions and individual risk factors.

How to Minimise Side Effects When Taking Krill Oil

Adopting evidence-based strategies can help reduce the likelihood and severity of side effects whilst taking krill oil supplements. These practical measures align with general principles of supplement safety.

Dosing and administration strategies:

• Start with a lower dose (250–500 mg daily) and gradually increase over 2–4 weeks to allow your body to adjust

• Take with meals, preferably those containing some dietary fat, to enhance absorption and reduce gastrointestinal upset

• Divide the daily dose if taking higher amounts (e.g., 500 mg twice daily rather than 1,000 mg once daily)

• Choose high-quality products with clear labelling of EPA and DHA content

• Store properly in a cool, dark place to prevent oxidation, which can increase fishy taste

Refrigerating krill oil capsules may help minimise fishy aftertaste and burping. Some users find that taking the supplement at bedtime reduces awareness of any gastrointestinal effects.

Monitoring and communication:

Keep a symptom diary during the first month, noting any side effects, their severity, and timing in relation to dosing. This information proves valuable when discussing supplementation with healthcare professionals. Inform your GP and pharmacist about krill oil use, particularly before starting new medications or prior to medical procedures.

Product selection considerations:

Look for krill oil supplements that display:

• Clear labelling of EPA and DHA content

• Evidence of sustainable sourcing (such as Marine Stewardship Council certification)

• Appropriate manufacturing standards

• Allergen information (particularly important for those with other food sensitivities)

When to reassess use:

If side effects persist beyond four weeks despite dose adjustments and optimal timing, consider discontinuing krill oil and exploring alternatives such as algae-based omega-3 supplements, which may be better tolerated. Regular review with your GP is advisable if using krill oil long-term, particularly for those with cardiovascular conditions or taking multiple medications.

Long-term safety data for krill oil are limited, so it's prudent to use the lowest effective dose and periodically reassess whether continued supplementation is necessary. If you experience any concerning symptoms, stop taking the supplement and seek medical advice.

Frequently Asked Questions