Fish oil supplements are amongst the most widely used nutritional products in the UK, valued for their omega-3 fatty acid content. But is fish oil good for you? The answer depends on individual circumstances, existing health conditions, and dietary intake. Whilst fish oil contains beneficial omega-3 fatty acids—eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—that support cardiovascular and brain health, evidence for routine supplementation remains mixed. This article examines the clinical evidence, potential benefits, risks, and guidance on who might benefit from fish oil, helping you make an informed decision about supplementation.

What Is Fish Oil and What Does It Contain?

Fish oil is a dietary supplement derived from the tissues of oily fish, including mackerel, herring, sardines, salmon, and anchovies. It is one of the most commonly consumed nutritional supplements in the UK, valued primarily for its rich content of omega-3 polyunsaturated fatty acids.

The two principal bioactive components in fish oil are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These long-chain omega-3 fatty acids are mainly obtained through diet, as the human body can only convert limited amounts from alpha-linolenic acid (ALA). EPA and DHA play crucial roles in cellular membrane structure, inflammatory regulation, and various metabolic processes throughout the body.

Fish oil supplements are available in several formulations, including soft gel capsules, liquid preparations, and emulsified products. The concentration of EPA and DHA varies considerably between products, with standard supplements typically containing 180–300 mg of EPA and 120–200 mg of DHA per capsule. Prescription-grade preparations contain significantly higher concentrations and are used therapeutically for specific medical conditions. In the UK, licensed prescription products include omega-3-acid ethyl esters (e.g., Omacor) for hypertriglyceridaemia and icosapent ethyl (Vazkepa) for cardiovascular risk reduction in selected patients.

It is important to distinguish fish oil from other marine-derived supplements such as cod liver oil, which contains additional vitamins A and D, or krill oil, which has a different phospholipid structure. For those who avoid fish products, algae-derived omega-3 supplements provide a plant-based alternative, primarily containing DHA with varying amounts of EPA.

Health Benefits of Fish Oil: What the Evidence Shows

The cardiovascular benefits of fish oil have been extensively studied, though evidence remains mixed. Cardiovascular health represents the most researched area, with some studies suggesting that omega-3 fatty acids may help reduce triglyceride levels, lower blood pressure modestly, and potentially decrease the risk of cardiovascular events in certain populations. However, NICE (NG238) does not recommend routine omega-3 supplements for primary prevention of cardiovascular disease. Prescription omega-3-acid ethyl esters may be used for hypertriglyceridaemia when dietary modification alone is insufficient. Separately, prescription icosapent ethyl (a purified EPA product) is recommended by NICE (TA805) for reducing cardiovascular risk in specific statin-treated patients with elevated triglycerides.

Regarding mental health and cognitive function, research indicates that omega-3 fatty acids are integral to brain structure and function. Some studies suggest potential benefits in depression, though results are inconsistent and NICE (NG222) does not recommend omega-3 supplements for treating depression. Fish oil should not replace conventional treatments. Research into whether omega-3 supplementation might support cognitive function in older adults continues, but definitive evidence of preventing dementia or cognitive decline remains lacking.

Anti-inflammatory properties of EPA and DHA have generated interest in conditions such as rheumatoid arthritis. Some patients report reduced joint stiffness and pain with high-dose fish oil supplementation, though effects are generally modest. The mechanism involves the conversion of EPA and DHA into specialised pro-resolving mediators that help regulate inflammatory responses.

For pregnancy and infant development, adequate DHA intake during pregnancy supports foetal brain and eye development. The NHS advises pregnant women to eat no more than 2 portions of oily fish per week, limit tuna, and avoid shark, swordfish and marlin due to potential contaminants. Pregnant women should avoid cod liver oil supplements due to their high vitamin A content, which may harm the unborn baby.

It is crucial to note that whilst observational studies often show associations between fish consumption and health benefits, supplementation trials have produced more variable results. The protective effects observed with dietary fish intake may result from the combination of omega-3 fatty acids with other nutrients, or from fish replacing less healthy dietary choices.

Who Should Consider Taking Fish Oil Supplements?

Fish oil supplementation may be appropriate for specific groups, though dietary sources of omega-3 fatty acids are generally preferred. Individuals with low omega-3 intake who cannot meet the NHS recommendation of at least two portions of fish weekly (one being oily fish) might benefit from supplementation. This includes people with dietary restrictions, fish allergies, or strong aversions to seafood.

Patients with hypertriglyceridaemia may be prescribed high-dose omega-3 preparations under medical supervision. Prescription omega-3-acid ethyl esters (2–4 grams daily) are licensed in the UK as adjunctive therapy alongside lifestyle modifications and other lipid-lowering treatments. Additionally, icosapent ethyl may be prescribed for cardiovascular risk reduction in eligible statin-treated patients with elevated triglycerides, as per NICE guidance (TA805). These prescription products contain significantly higher concentrations than over-the-counter supplements and should only be used under medical supervision.

Pregnant and breastfeeding women require adequate DHA for foetal and infant neurodevelopment. Whilst dietary fish intake is encouraged (within NHS guidelines of no more than 2 portions of oily fish weekly), supplementation may be considered when dietary intake is insufficient. Pregnant women should avoid supplements containing vitamin A (such as cod liver oil) due to teratogenic risks, and should discuss any supplementation with their midwife or GP.

Individuals with inflammatory conditions such as rheumatoid arthritis sometimes use fish oil as complementary therapy, though evidence for significant clinical benefit is limited. Any supplementation should complement, not replace, conventional disease-modifying treatments.

Conversely, fish oil supplementation is not routinely recommended for primary prevention of cardiovascular disease in the general population, as large-scale trials have not demonstrated consistent benefit. The emphasis should remain on a balanced diet rich in oily fish, fruits, vegetables, and whole grains. People considering fish oil supplements should discuss this with their GP or a registered dietitian, particularly if they have existing health conditions or take regular medications.

Potential Risks and Side Effects of Fish Oil

Whilst fish oil supplements are generally well tolerated, several adverse effects and safety considerations warrant attention. Common gastrointestinal effects include fishy aftertaste, belching, nausea, loose stools, and indigestion. These symptoms affect approximately 5–10% of users and can often be minimised by taking supplements with meals, choosing enteric-coated preparations, or refrigerating capsules.

Bleeding risk represents a theoretical concern, as omega-3 fatty acids possess mild antiplatelet effects that may prolong bleeding time. Whilst clinically significant bleeding is rare at standard supplementation doses, caution is advised for patients taking anticoagulants (warfarin, DOACs) or antiplatelet agents (aspirin, clopidogrel). Patients on warfarin should have their INR monitored when initiating or changing doses of fish oil. There is no official contraindication to combined use, but patients should inform their healthcare providers about all supplements they take.

Allergic reactions can occur in individuals with fish allergies, though reactions to highly purified fish oil are uncommon. Fish allergy is distinct from shellfish allergy, but patients with known seafood allergies should consult their GP before starting supplementation and consider algae-derived omega-3 alternatives.

Contaminant exposure is a potential concern, as fish may accumulate environmental pollutants including mercury, polychlorinated biphenyls (PCBs), and dioxins. The UK Food Standards Agency (FSA) sets limits for contaminants in fish and fish products. Reputable manufacturers employ molecular distillation and purification processes to remove contaminants. Consumers should select supplements from established UK or EU suppliers that comply with regulatory standards.

Drug interactions may occur with certain medications. Beyond anticoagulants, fish oil may interact with blood pressure medications and may affect blood glucose control in diabetes, though these effects are generally modest at typical supplementation doses.

Patients should contact their GP if they experience persistent gastrointestinal symptoms, unusual bleeding or bruising, allergic reactions, or any concerning symptoms after starting fish oil supplementation. Suspected side effects can be reported to the MHRA via the Yellow Card scheme (yellowcard.mhra.gov.uk or the Yellow Card app). Those with existing medical conditions or taking multiple medications should seek professional advice before commencing supplementation.

How to Choose and Take Fish Oil Safely

Selecting an appropriate fish oil supplement requires consideration of several factors to ensure safety, quality, and efficacy. Product quality and purity should be the primary concern. Look for supplements from reputable manufacturers that comply with UK and EU regulatory standards. Quality certifications and batch testing information can provide reassurance about product purity and omega-3 content.

Dosage and concentration vary considerably between products. Standard supplements typically provide 250–500 mg combined EPA and DHA per capsule, whilst concentrated preparations may contain 1000 mg or more. For general health, the NHS recommends eating at least two portions of fish weekly (one being oily) rather than specifying a daily omega-3 target. If supplementing, typical daily intakes range from 250–500 mg combined EPA/DHA, though therapeutic doses for specific conditions may be substantially higher and should be determined by a healthcare professional.

Formulation considerations include different forms such as natural triglyceride, ethyl ester, or re-esterified triglyceride. While some evidence suggests differences in absorption between these forms, the clinical significance remains uncertain. Enteric-coated capsules can reduce gastrointestinal side effects.

Storage and handling are important for maintaining supplement quality. Fish oil is susceptible to oxidation, which reduces efficacy and may produce harmful compounds. Store supplements in a cool, dark place, refrigerate after opening if recommended, and check expiration dates. Rancid fish oil has a distinctly unpleasant odour and should be discarded.

Timing and administration: Taking fish oil with meals enhances absorption and reduces gastrointestinal side effects. Consistency in timing helps establish routine adherence.

When to seek professional advice: Consult your GP or a registered dietitian before starting fish oil supplements if you are pregnant, breastfeeding, taking anticoagulant medications, have a bleeding disorder, are scheduled for surgery (inform your surgical team about all supplements), or have any chronic health conditions. Regular medication reviews should include discussion of all supplements taken.

Ultimately, whilst fish oil supplements can be beneficial for specific individuals, they should not replace a balanced diet rich in whole foods, including oily fish where tolerated. The emphasis should remain on overall dietary patterns rather than relying on isolated supplements for health benefits.

Frequently Asked Questions