Fish and Krill Oil Side Effects

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The human body cannot synthesise Omega-3 three fatty acids, (essential for good health) and must acquire them from an outside source. There are three kinds of Omega-3s known as Alpha-Linolenic Acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). ALA is found in plant sources like hemp seeds, flax seeds walnuts but it does not offer the same health benefits as EPA and DHA. EPA and DHA are typically present in cold-water fish such as salmon, cod, mackerel and tuna in addition to phytoplankton and in smaller quantities in grass-fed beef, free-range chicken and their eggs. Fish oil is the relatively cheaper and more common source of EPA and DHA.

Krill are tiny crustaceans that roam the world’s oceans feeding on phytoplankton and forming the bottom of the food chain for whales, seabirds and fish. The oil derived from these creatures, krill oil contains the same fatty acids as fish oil and hence offers exactly the same health benefits; except these fatty acids are available in a slightly different form. The EPA and DHA molecules in krill have a phosphate group attached at the end of the fatty acid. This difference allows the acids in krill oil be absorbed into the body faster, hence requiring a lower dose.

To keep negative effects at bay, no more than three grams of fish oil a day is recommended by the U.S. Food and Drug Administration.1 Considering that fish oil is a medicine, side effects are not unheard of, although rare as long as one consumes them within the recommended doses.2

Excessive amounts of fish oil/krill oil could cause problems like stomach or bowel bleeding. This is because the Omega-3s cut down on the sticking properties of blood platelets. This is beneficial in cases where blood clots are present, as it minimizes dangerous clots and promotes healthy blood circulation. However, in healthy individuals it promotes bruising and bleeding, and it takes blood longer to clot.3

A number of other minor issues can arise when starting on a regiment of fish oil supplements. Some people experience diarrhoea when they start taking fish oil supplements. This is partially due to the fact that the body is not used to the additional oils and it needs time to adapt. Starting slowly and building up to the maximum recommended by your doctor is the best way to deal with this issue. If it persists, consult your doctor. Fishy burps are another common negative side effect. Taking the supplement immediately after a main meal or freezing the capsules before using them should help to resolve this problem.

Despite the fact that fish oil is intended to improve health, it is after all “oil” that is essentially composed of triglycerides. Some people find that it increases their cholesterol levels by a small degree.4 If the overall cholesterol is inside the recommended safe range, then it is not a problem. If you already have high cholesterol, then consult your doctor before taking the supplement.


  2. Turner MB. Safety of fish oil and omega-3 fatty acids. Medicines Safety Update Therapeutic Goods Administration 2010.
  3. Bays HE. Safety considerations with Omega-3 Fatty Acids Therapy. Am J Cardiol 2007;99:S35-S43.
  4. Weintraub H. Update on marine omega-3 fatty acids: Management of dyslipidemia and current omega-3 treatment options. Atheroscler 2013;230:381-9.

By Andy

Hi, my name is Dr. Andy Williams and I am a biologist with a keen interest in diet and nutrition. This site was set up to help me explore the research, facts and fiction about Krill Oil. Please feel free to contact me if you have any comments, questions or suggestions.

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