Fish oil supplements have become increasingly popular in the UK, with many people taking them for perceived heart health and general wellbeing benefits. Derived from oily fish such as mackerel, salmon, and sardines, fish oil contains omega-3 polyunsaturated fatty acids—primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Whilst some health benefits are supported by evidence, particularly for triglyceride reduction, the overall picture is more nuanced than many assume. Understanding what fish oil can and cannot do, who might benefit, and potential risks is essential for making informed decisions 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 has become one of the most popular supplements in the UK, taken regularly for perceived cardiovascular and general health benefits.

The primary active components in fish oil are omega-3 polyunsaturated fatty acids, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These are long-chain omega-3 fatty acids that the body can produce in limited amounts from alpha-linolenic acid (ALA), which is the essential omega-3 fatty acid that must be obtained through diet. A typical fish oil capsule contains approximately 300mg of combined EPA and DHA per 1000mg capsule, though concentrated formulations may provide higher doses (500-800mg).

Fish oil supplements are available in several forms, including:

• Natural triglyceride form – the form naturally found in fish

• Ethyl ester form – a processed form often used in concentrated supplements

• Re-esterified triglyceride form – processed and then converted back to triglyceride structure

• Prescription omega-3 preparations – highly purified formulations licensed as medicines for specific indications

The quality and purity of fish oil supplements can vary considerably between manufacturers. Reputable products undergo processing to remove environmental contaminants such as mercury, polychlorinated biphenyls (PCBs), and dioxins. When selecting a supplement, consumers should look for products from reputable brands with independent testing certification. In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) regulates fish oil products marketed with medicinal claims, whilst those sold as food supplements fall under food law overseen by the Food Standards Agency (FSA) and must comply with UK Nutrition and Health Claims regulations.

Health Benefits of Fish Oil: What the Evidence Shows

The evidence base for fish oil supplementation has evolved considerably over recent decades, with some benefits well-established whilst others remain subject to ongoing research and debate. Understanding what the current evidence actually supports is essential for making informed decisions about supplementation.

Cardiovascular health has been the primary focus of fish oil research. Early observational studies suggested significant cardiovascular benefits, but more recent large-scale randomised controlled trials have produced mixed results. The REDUCE-IT trial demonstrated that high-dose prescription icosapent ethyl (a purified EPA formulation) reduced cardiovascular events in high-risk patients already on statin therapy. However, standard over-the-counter fish oil supplements at typical doses have not consistently shown the same benefits. NICE does not recommend omega-3 supplements for primary or secondary prevention of cardiovascular disease, though NICE TA805 does recommend icosapent ethyl for specific high-risk patients with established cardiovascular disease and raised triglycerides.

Triglyceride reduction represents one of the most robust benefits of fish oil. High-dose omega-3 fatty acids (typically 2-4g daily of EPA and DHA) can reduce serum triglycerides by 20-30% through multiple mechanisms, including reduced hepatic triglyceride synthesis and increased clearance. It should be noted that EPA+DHA combinations may increase LDL-cholesterol in some patients, whereas purified EPA (icosapent ethyl) does not. Prescription omega-3 preparations are licensed for treating hypertriglyceridaemia.

Anti-inflammatory properties of omega-3 fatty acids have generated interest for conditions including rheumatoid arthritis. Some evidence suggests modest benefits in reducing joint pain and stiffness, potentially allowing reduced use of non-steroidal anti-inflammatory drugs (NSAIDs). The proposed mechanism involves EPA and DHA competing with arachidonic acid in inflammatory pathways, producing less inflammatory eicosanoids.

Cognitive function and mental health benefits remain an area of active investigation. Whilst DHA is a structural component of brain tissue and essential for neurodevelopment, evidence for cognitive benefits from supplementation in healthy adults is limited. Some studies suggest potential benefits in mild cognitive impairment, but results are inconsistent. For depression, omega-3 supplementation may provide modest adjunctive benefits, particularly formulations higher in EPA, though it should not replace evidence-based treatments.

Pregnancy and infant development represents an area where omega-3 intake is important. DHA is crucial for foetal brain and retinal development. However, there is no official UK recommendation for routine fish oil supplementation during pregnancy. The NHS advises pregnant women to consume oily fish (up to two portions weekly, avoiding high-mercury species) as part of a balanced diet. Women who don't eat fish may consider supplementation after discussing with their healthcare provider. Cod liver oil should be avoided during pregnancy due to its vitamin A content.

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 are the most frequently reported issues with fish oil supplementation. These include:

• Fishy aftertaste or burping (affecting up to 20% of users)

• Nausea and dyspepsia

• Diarrhoea or loose stools

• Abdominal discomfort or bloating

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

Bleeding risk is a consideration due to omega-3 fatty acids' effects on platelet function and coagulation. Fish oil can prolong bleeding time through reduced platelet aggregation and thromboxane A2 production. However, clinically significant bleeding complications are rare at typical supplementation doses (up to 3g daily). Nevertheless, caution is warranted in patients:

• Taking anticoagulants (warfarin, DOACs) or antiplatelet agents (aspirin, clopidogrel)

• With bleeding disorders or thrombocytopenia

• Scheduled for surgery (consult with the surgical/anaesthetic team about management)

Patients on anticoagulation therapy should consult their GP or anticoagulation clinic before starting fish oil supplements, as monitoring may be appropriate.

Allergic reactions can occur in individuals with fish allergies. Fish oil supplements should be avoided by people with known fish allergies. Those with shellfish allergies should check product labels carefully for potential cross-contamination or shellfish-derived ingredients, though pure fish oil itself does not contain shellfish allergens.

Drug interactions beyond anticoagulants are relatively uncommon but include potential interactions with antihypertensive medications (additive blood pressure-lowering effects). High-dose fish oil may modestly reduce blood pressure, which could be beneficial or require medication adjustment in treated hypertensive patients.

Lipid effects at higher doses may include increased LDL-cholesterol with EPA+DHA combinations, despite triglyceride reduction. This effect is not seen with purified EPA (icosapent ethyl). Patients taking high-dose fish oil should have their lipid profile monitored.

Quality and contamination concerns relate to the source and processing of fish oil. Poorly manufactured products may contain environmental contaminants including PCBs or oxidised (rancid) oils. Oxidised fish oil may have pro-inflammatory rather than anti-inflammatory effects. Consumers should select products from reputable manufacturers with independent testing certification and appropriate storage to prevent oxidation.

Vitamin toxicity is a potential risk with cod liver oil specifically, which contains vitamins A and D. Excessive intake can lead to hypervitaminosis A (causing liver damage, bone problems, and teratogenic effects in pregnancy) or vitamin D toxicity. Standard fish oil supplements derived from fish body tissues do not contain significant amounts of these vitamins.

Any suspected adverse effects from fish oil supplements should be reported through the MHRA Yellow Card scheme.

Who Should Consider Taking Fish Oil Supplements?

The decision to take fish oil supplements should be individualised based on dietary intake, health status, and evidence-based indications. Not everyone requires supplementation, and dietary sources of omega-3 fatty acids are generally preferred for the general population.

Individuals with inadequate dietary omega-3 intake represent the primary group who may benefit from supplementation. The UK Scientific Advisory Committee on Nutrition recommends consuming at least two portions of fish per week, one of which should be oily (approximately 140g per portion). Those who may struggle to meet this recommendation include:

• People who do not consume fish due to dietary preferences, allergies, or cultural reasons

• Vegans and vegetarians (who may consider algae-based omega-3 supplements)

• Individuals with limited access to fresh oily fish

• Those with sensory or texture aversions to fish

For these individuals, supplementation can help ensure adequate omega-3 status, though dietary modification should be explored first where possible.

Patients with specific medical conditions may have evidence-based indications for omega-3 supplementation, ideally under medical supervision:

• Hypertriglyceridaemia – Prescription omega-3 preparations (2-4g daily) are indicated for hypertriglyceridaemia. Patients with triglycerides >10 mmol/L should be referred to a specialist due to pancreatitis risk

• Established cardiovascular disease – NICE recommends icosapent ethyl (Vazkepa) for specific high-risk patients with established cardiovascular disease and raised triglycerides despite statin therapy. Standard over-the-counter fish oil supplements are not recommended by NICE for cardiovascular disease prevention

• Rheumatoid arthritis – Omega-3 supplementation may provide modest symptomatic benefits as adjunctive therapy

Pregnant and breastfeeding women require adequate omega-3 intake for foetal and infant neurodevelopment. The NHS advises consuming up to two portions of oily fish weekly during pregnancy (avoiding high-mercury species like shark, swordfish, and marlin). Women who don't eat fish may consider supplementation after discussing with their healthcare provider. Cod liver oil should be avoided during pregnancy due to its vitamin A content.

Older adults with or at risk of cognitive decline are sometimes advised to ensure adequate omega-3 intake, though evidence for cognitive benefits from supplementation in healthy older adults is limited. Maintaining adequate omega-3 status through diet or supplementation may be reasonable as part of overall nutritional optimisation.

When to seek medical advice: Patients should consult their GP before starting fish oil supplements if they:

• Take anticoagulant or antiplatelet medications

• Have bleeding disorders or upcoming surgery

• Have fish allergies

• Are considering high-dose supplementation (>3g daily)

• Have diabetes (high-dose fish oil may affect lipid profiles)

• Experience persistent side effects from supplementation

Who should avoid fish oil supplements: Certain individuals should exercise caution or avoid fish oil entirely, including those with fish allergies (unless using algae-derived alternatives) and individuals taking medications with significant bleeding risk without medical supervision. Cod liver oil should be avoided in pregnancy due to vitamin A content. During acute episodes of pancreatitis, oral fats including fish oil may be limited, but longer-term triglyceride management under specialist care may include omega-3s.

Ultimately, for most healthy individuals consuming a balanced diet including regular oily fish, supplementation offers limited additional benefit. The focus should remain on achieving adequate omega-3 intake through dietary sources as part of an overall healthy eating pattern aligned with the NHS Eatwell Guide. When supplementation is considered, selecting quality products at appropriate doses and monitoring for adverse effects ensures safe and potentially beneficial use.

Frequently Asked Questions