Are fish oil supplements good for you? This question concerns millions of UK adults who regularly take omega-3 supplements, yet the answer depends significantly on individual circumstances and the specific product used. Fish oil supplements contain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), omega-3 polyunsaturated fatty acids with established biological effects. Whilst robust evidence supports high-dose prescription omega-3 preparations for specific cardiovascular conditions, standard over-the-counter supplements are not recommended by NICE for cardiovascular disease prevention. The NHS emphasises obtaining omega-3s primarily through dietary sources—at least two portions of fish weekly, including one portion of oily fish. This article examines the evidence for fish oil supplementation, identifies who may benefit, and outlines important safety considerations to help you make an informed decision.

What Are Fish Oil Supplements and What Do They Contain?

Fish oil supplements are dietary products derived from the tissues of oily fish, most commonly including mackerel, herring, sardines, anchovies, and salmon. These supplements have become widely consumed nutritional products in the UK, with significant numbers of adults taking them regularly for perceived health benefits.

The primary active constituents in fish oil are omega-3 polyunsaturated fatty acids , specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These long-chain fatty acids are important nutrients; while alpha-linolenic acid (ALA) is the essential omega-3 fatty acid in UK nutrition guidance, the body can only convert limited amounts to EPA and DHA, which is why many people consider supplementation. A typical fish oil capsule contains between 180–500 mg of combined EPA and DHA, though high-strength preparations may provide up to 1,000 mg or more per dose.

Fish oil supplements are available in several formulations in the UK, including:

• Standard fish oil capsules – containing natural triglyceride forms of omega-3s

• Concentrated or pharmaceutical-grade preparations – with higher EPA/DHA content

• Ethyl ester formulations – chemically modified to allow higher concentration of EPA/DHA, though absorption may differ from triglyceride forms and can be influenced by taking with meals

• Cod liver oil – which also contains vitamins A and D but lower omega-3 concentrations

Beyond EPA and DHA, fish oil supplements may contain small amounts of other fatty acids, vitamin E (added as a preservative to prevent oxidation), and in some products, additional vitamins or flavourings to mask the characteristic fishy taste. The quality, purity, and concentration of omega-3 fatty acids can vary considerably between brands and price points, making informed selection important for consumers seeking potential benefits.

Proven Health Benefits of Fish Oil Supplements

The evidence base for fish oil supplementation has been extensively studied, though the strength of evidence varies considerably across different health conditions. The most robust clinical evidence supports the use of high-dose omega-3 fatty acids in specific cardiovascular contexts, whilst claims for other benefits remain more contentious.

Cardiovascular benefits have been the subject of significant research. It's important to note that NICE does not recommend standard over-the-counter fish oil supplements for cardiovascular disease prevention. However, NICE (TA805) does recommend prescription icosapent ethyl (a purified EPA-only preparation) for reducing cardiovascular risk in specific high-risk patients with elevated triglycerides despite statin therapy. The REDUCE-IT study demonstrated that high-dose purified EPA significantly reduced cardiovascular events in this defined population. Standard fish oil supplements at typical doses (250–500 mg daily) are not recommended by NICE for primary or secondary cardiovascular prevention.

Triglyceride reduction is a well-documented effect, with omega-3 fatty acids reducing serum triglycerides by 20–30% at prescription doses of 2–4 grams daily under clinical supervision. This mechanism involves decreased hepatic synthesis of very-low-density lipoproteins (VLDL) and enhanced triglyceride clearance. For patients with severe hypertriglyceridaemia (>10 mmol/L), prescription omega-3 preparations may be recommended alongside lifestyle modifications. It should be noted that DHA-containing omega-3 supplements may raise LDL-cholesterol in some people.

Anti-inflammatory properties have been demonstrated in laboratory studies, with EPA and DHA serving as precursors to specialised pro-resolving mediators that help regulate inflammatory responses. Some evidence suggests modest benefits in rheumatoid arthritis, with patients experiencing reduced joint tenderness and morning stiffness, though fish oil should complement rather than replace conventional disease-modifying treatments.

Evidence for other commonly claimed benefits—including cognitive function, depression, and eye health—remains mixed, with systematic reviews showing inconsistent results. Whilst omega-3s are structurally important for brain and retinal tissue, supplementation trials have not consistently demonstrated clinical improvements in healthy populations.

Who Should Consider Taking Fish Oil Supplements?

The decision to take fish oil supplements should be individualised based on dietary intake, cardiovascular risk factors, and specific medical conditions. Current UK guidance emphasises obtaining omega-3 fatty acids primarily through dietary sources, with supplementation reserved for particular circumstances.

Individuals with inadequate dietary intake represent the primary target group. The NHS recommends consuming at least two portions of fish weekly, including one portion of oily fish (approximately 140g), which provides omega-3 fatty acids (amounts vary by fish species). People who do not meet these recommendations due to dietary preferences, allergies, or vegetarian/vegan lifestyles may benefit from supplementation. However, plant-based alternatives containing alpha-linolenic acid (ALA) from flaxseed or algal-derived EPA/DHA supplements may be more appropriate for those avoiding fish products.

Patients with specific cardiovascular risk profiles may benefit from prescription omega-3 preparations in certain circumstances. NICE (TA805) recommends icosapent ethyl (a purified EPA-only preparation) for adults with elevated triglycerides, LDL-cholesterol controlled by statins, and established cardiovascular disease or diabetes with additional risk factors. Such decisions should be made in consultation with a GP or cardiologist, as standard over-the-counter fish oil supplements are not recommended by NICE for cardiovascular disease prevention.

Pregnant and breastfeeding women have increased omega-3 requirements for foetal neurodevelopment, particularly DHA. Whilst dietary sources are preferred, supplementation may be considered if fish consumption is limited. The NHS advises pregnant women to limit oily fish to 2 portions weekly and avoid shark, marlin, and swordfish (due to mercury). Cod liver oil should be avoided during pregnancy due to high vitamin A content, which poses teratogenic risks. Standard fish oil or specific pregnancy formulations are safer alternatives.

People who should exercise caution include those taking anticoagulant medications (warfarin, DOACs), individuals with fish allergies, and patients scheduled for surgery, as omega-3s may affect bleeding risk. Those scheduled for surgical procedures should follow their surgical team's specific instructions regarding supplement discontinuation. Anyone considering supplementation whilst taking prescribed medications should consult their GP or pharmacist to assess potential interactions and appropriateness.

Potential Risks and Side Effects of Fish Oil

Whilst fish oil supplements are generally well-tolerated, they are not without potential adverse effects and safety considerations. Understanding these risks enables informed decision-making and appropriate monitoring.

Common gastrointestinal side effects affect approximately 10–20% of users and include:

• Fishy aftertaste or burping (eructation)

• Nausea and mild abdominal discomfort

• Loose stools or diarrhoea, particularly at higher doses

• Indigestion or reflux symptoms

These effects are typically dose-dependent and may be minimised by taking supplements with meals, choosing enteric-coated formulations, or refrigerating capsules. If gastrointestinal symptoms persist or worsen, discontinuation and medical review are advisable.

Bleeding risk is a consideration with omega-3 supplementation. Omega-3 fatty acids can inhibit platelet aggregation and may affect bleeding time. Patients taking anticoagulants (warfarin, apixaban, rivaroxaban) or antiplatelet agents (aspirin, clopidogrel) should consult their GP before commencing fish oil, as enhanced monitoring may be warranted. Those scheduled for surgical procedures should follow their surgical team's specific instructions regarding supplement discontinuation.

Lipid profile effects can include increases in LDL-cholesterol in some individuals, particularly with DHA-containing supplements. Patients taking omega-3s for hypertriglyceridaemia should have their complete lipid profile monitored. Additionally, high-dose prescription icosapent ethyl has been associated with an increased risk of atrial fibrillation in clinical trials.

Allergic reactions can occur in individuals with fish allergies. Symptoms may include urticaria, facial swelling, or respiratory difficulties. Anyone with known fish allergies should seek medical advice before using fish oil supplements. Those with shellfish allergies should be cautious with krill oil supplements specifically.

Contaminant concerns have historically included mercury, polychlorinated biphenyls (PCBs), and dioxins, though reputable manufacturers employ molecular distillation and purification processes to minimise these risks. Choosing products tested by independent organisations provides additional assurance.

Drug interactions beyond anticoagulation should be discussed with a healthcare professional. Patients should inform healthcare providers of all supplements taken to enable comprehensive medication review.

If you experience any suspected side effects from fish oil supplements, report them to the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

How to Choose Quality Fish Oil Supplements in the UK

The UK supplement market offers numerous fish oil products with considerable variation in quality, purity, and value. Selecting an appropriate product requires understanding key quality indicators and regulatory standards.

Omega-3 content and concentration should be the primary consideration. Product labels must clearly state the amounts of EPA and DHA per serving, not merely total fish oil content. A 1,000 mg fish oil capsule typically contains only 300–500 mg of combined EPA/DHA, with the remainder comprising other fatty acids and carrier oils. For potential benefits, aim for products providing at least 250–500 mg combined EPA/DHA daily, or higher doses if recommended by a healthcare professional. Concentrated formulations reduce pill burden and often provide better value.

Third-party testing and certification offer assurance of quality and safety. Look for products tested by:

• IFOS (International Fish Oil Standards) – providing independent testing for contaminants and potency

• Nutrasource – offering third-party testing and verification

• Friend of the Sea or MSC (Marine Stewardship Council) – confirming sustainable sourcing practices (not purity or potency)

These testing programmes indicate that products have undergone assessment for heavy metals, PCBs, dioxins, and oxidation markers (peroxide value, totox), ensuring they meet safety thresholds.

Formulation type affects absorption and stability. Triglyceride forms (natural or re-esterified) and ethyl ester forms have different properties; absorption can vary based on whether supplements are taken with meals. Enteric-coated capsules may reduce fishy aftertaste and improve gastrointestinal tolerance. Products should contain vitamin E or other antioxidants to prevent rancidity.

Storage and expiry dates are critical, as omega-3 fatty acids are susceptible to oxidation. Purchase products well within their expiry date, store in cool, dark conditions, and refrigerate after opening if recommended. Rancid fish oil develops an unpleasant odour and may contain harmful oxidation products.

Regulatory status in the UK means fish oil supplements are classified as food supplements rather than medicines (except prescription omega-3 preparations). Whilst manufacturers must comply with food safety regulations, therapeutic claims are restricted. In the EU/UK, permitted health claims for omega-3s are limited and specific (e.g., 250 mg daily EPA+DHA contributes to normal heart function). Be sceptical of products making exaggerated health claims, and consult a GP or registered dietitian for personalised advice, particularly if considering supplementation for specific medical conditions.

Frequently Asked Questions