Omega-3 side effects are generally mild but can affect tolerability and adherence to supplementation. Omega-3 fatty acids—including EPA, DHA, and ALA—are essential polyunsaturated fats obtained through diet or supplements, available both over the counter and as licensed medicines for specific conditions such as hypertriglyceridaemia. Whilst these supplements offer potential health benefits, they are not without adverse effects. Common issues include gastrointestinal discomfort, fishy aftertaste, and belching, whilst rarer but more serious concerns involve increased bleeding risk, allergic reactions, and potential cardiovascular effects. Understanding these side effects enables informed decision-making and helps patients and healthcare professionals implement strategies to maximise benefits whilst minimising risks.

What Are Omega-3 Supplements and Why Are They Used?

Omega-3 fatty acids are essential polyunsaturated fats that the body cannot synthesise and must therefore be obtained through diet or supplementation. The three main types are eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and alpha-linolenic acid (ALA). EPA and DHA are primarily found in oily fish such as salmon, mackerel, and sardines, whilst ALA is present in plant sources including flaxseeds, chia seeds, and walnuts.

Omega-3 products are available both as food supplements and as licensed medicines. The licensed medicines include omega-3-acid ethyl esters (typically prescribed at 2-4g daily for hypertriglyceridaemia) and icosapent ethyl. It's important to note that NICE does not recommend fish oil supplements for cardiovascular disease prevention. However, NICE has approved icosapent ethyl (TA805) specifically for use with statins in selected adults with raised triglycerides and cardiovascular disease.

The mechanism of action involves incorporation into cell membranes, where omega-3 fatty acids influence membrane fluidity and cellular signalling pathways. EPA and DHA serve as precursors to specialised pro-resolving mediators, which help regulate inflammatory responses. Additionally, these fatty acids can reduce the production of pro-inflammatory eicosanoids derived from omega-6 fatty acids.

Whilst omega-3 supplements are available over the counter in various formulations—including fish oil capsules, krill oil, and algal oil for vegetarians—it is important to recognise that they are not without potential adverse effects. Some prescription omega-3 products may increase LDL-cholesterol levels in certain patients. Understanding these side effects enables patients and healthcare professionals to make informed decisions about supplementation and to implement strategies that maximise benefits whilst minimising risks.

Common Side Effects of Omega-3 Supplements

The most frequently reported side effects of omega-3 supplementation are gastrointestinal in nature and generally mild. Fishy aftertaste and belching (eructation) are particularly common with fish oil preparations, affecting a significant proportion of users. This can be accompanied by nausea, especially when supplements are taken on an empty stomach or at higher doses. Some individuals also experience loose stools or diarrhoea, particularly during the initial weeks of supplementation or when doses exceed 3 grams daily.

Gastrointestinal discomfort, including bloating and indigestion (dyspepsia), may occur as the digestive system adapts to increased fat intake. These symptoms are typically transient and often resolve with continued use or dose adjustment. The quality and formulation of the supplement can influence the severity of these effects, with enteric-coated preparations sometimes better tolerated.

Another commonly reported effect is a fishy body odour or breath, which can be socially distressing for some users. This occurs as the body processes the fish oil components.

Some users notice changes in taste perception, particularly a lingering fishy or metallic taste that can affect appetite and food enjoyment. These sensory changes are generally dose-dependent and more pronounced with liquid formulations compared to capsules.

Prescription omega-3-acid ethyl esters may cause an increase in LDL-cholesterol levels in some patients, particularly those being treated for high triglycerides. This effect should be monitored by healthcare professionals.

It is worth noting that these common side effects rarely necessitate discontinuation of therapy. Most are manageable through simple interventions such as taking supplements with meals, choosing alternative formulations, or adjusting the timing of doses. Patients experiencing persistent or bothersome symptoms should discuss these with their GP or pharmacist to explore suitable alternatives.

Serious Side Effects and When to Seek Medical Advice

Whilst serious adverse effects from omega-3 supplements are uncommon, certain risks warrant clinical attention. Increased bleeding tendency represents a clinically significant concern, particularly at higher doses. Omega-3 fatty acids inhibit platelet aggregation and can prolong bleeding time, which may manifest as easy bruising, prolonged bleeding from minor cuts, nosebleeds, or blood in urine or stools. This risk is heightened when taken with anticoagulants or antiplatelet medications. Patients taking warfarin should have their INR monitored when starting or stopping omega-3 supplements. Seek immediate medical advice if you experience unexplained bleeding.

Allergic reactions, though rare, can occur, especially in individuals with fish or shellfish allergies. Symptoms may include skin rash, itching, facial swelling, difficulty breathing, or anaphylaxis in severe cases. If you experience signs of a severe allergic reaction, call 999 or 112 immediately and discontinue the supplement.

There is evidence suggesting that high doses of omega-3 supplements might be associated with atrial fibrillation in susceptible individuals, particularly those with existing cardiovascular disease. This has been noted in clinical trials of icosapent ethyl. Patients experiencing palpitations, irregular heartbeat, chest discomfort, or unexplained breathlessness should contact their GP promptly for cardiovascular assessment.

Vitamin toxicity can occur with cod liver oil supplements, which contain vitamins A and D alongside omega-3 fatty acids. Excessive vitamin A intake may cause headaches, dizziness, nausea, and in chronic cases, liver damage. This is particularly concerning during pregnancy. Symptoms of vitamin D toxicity include excessive thirst, frequent urination, and confusion.

Patients should seek medical advice if they experience:

• Persistent or severe gastrointestinal symptoms lasting beyond two weeks

• Signs of bleeding or unusual bruising

• Allergic symptoms or skin reactions

• New or worsening cardiovascular symptoms

• Symptoms suggestive of vitamin toxicity

Healthcare professionals should be informed of all supplements being taken, as this information is crucial for safe prescribing and monitoring, particularly before surgical procedures where bleeding risk is relevant. Suspected side effects can be reported via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or the Yellow Card app).

Who Should Avoid or Use Omega-3 With Caution?

Several patient groups require careful consideration before initiating omega-3 supplementation. Individuals taking anticoagulants such as warfarin, apixaban, or rivaroxaban, or antiplatelet agents including aspirin, clopidogrel, or dipyridamole, should exercise particular caution. The additive antiplatelet effects of omega-3 fatty acids may increase bleeding risk. Patients on warfarin should have their INR monitored when starting or stopping omega-3 supplements. Always inform your GP or anticoagulation clinic before starting omega-3 supplements.

People with known fish or shellfish allergies should avoid fish oil and krill oil supplements due to the risk of allergic reactions. Always check product labels carefully for allergens, including potential cross-contamination. Algal oil derived from marine algae represents a suitable alternative for these individuals, providing EPA and DHA without fish-derived proteins that trigger allergic responses.

Patients with diabetes should monitor their blood glucose levels as usual when starting omega-3 supplements, though most evidence suggests minimal impact on glycaemic control. Any significant changes should be discussed with the diabetes care team.

Individuals with liver disease require medical supervision when considering omega-3 supplements, particularly cod liver oil, due to concerns about vitamin A accumulation and potential hepatotoxicity.

Pregnant and breastfeeding women should seek medical advice before taking omega-3 supplements. Whilst omega-3 fatty acids are important for foetal development, certain fish oil preparations may contain contaminants such as mercury or polychlorinated biphenyls (PCBs). Cod liver oil and other supplements containing vitamin A should be avoided during pregnancy and when trying to conceive due to potential risks to the developing foetus. Pharmaceutical-grade, purified preparations without vitamin A are preferable during pregnancy.

Patients scheduled for surgery should inform their surgeon and anaesthetist about omega-3 supplementation and follow their individualised advice. Do not stop prescribed omega-3 medicines (such as icosapent ethyl) without medical advice.

How to Minimise Side Effects When Taking Omega-3

Several practical strategies can significantly reduce the likelihood and severity of omega-3-related side effects. Taking supplements with meals, particularly those containing other fats, enhances absorption and reduces gastrointestinal discomfort. The presence of food in the stomach slows gastric emptying and dilutes the concentration of fish oil, minimising nausea and reflux symptoms.

Starting with a lower dose and gradually increasing over 2–4 weeks allows the digestive system to adapt. Rather than immediately taking the full recommended dose, begin with one capsule daily and incrementally increase as tolerated. This titration approach is particularly beneficial for individuals prone to gastrointestinal sensitivity.

Choosing high-quality supplements from reputable brands that comply with UK and EU contaminant limits can make a substantial difference. Look for products that have undergone purification processes to remove contaminants and reduce fishy aftertaste. Always check labels for EPA and DHA content and for vitamin A content (particularly important to avoid during pregnancy or when trying to conceive).

Enteric-coated capsules are specifically designed to dissolve in the intestine rather than the stomach, significantly reducing fishy burps and reflux.

Some people find that freezing fish oil capsules before consumption may help minimise aftertaste and belching, though this is based on anecdotal experience rather than clinical evidence. The delayed release as the capsule thaws in the digestive tract may reduce immediate gastric effects. Alternatively, taking supplements at bedtime may allow any minor gastrointestinal effects to occur during sleep.

Dividing the daily dose into two or three smaller doses taken throughout the day, rather than a single large dose, can improve tolerability. This approach maintains steady blood levels whilst reducing the gastrointestinal burden at any one time.

For those unable to tolerate fish oil despite these measures, alternative formulations such as algal oil or krill oil may prove better tolerated. Some individuals find liquid formulations mixed with juice or smoothies more palatable than capsules. High-dose, concentrated prescription products should only be used under medical supervision.

Patients should maintain adequate hydration when taking omega-3 supplements, as this supports digestion and may reduce the intensity of fishy aftertaste. If side effects persist despite these interventions, consulting a GP or pharmacist can help identify suitable alternatives or determine whether supplementation remains appropriate for individual circumstances.

Frequently Asked Questions