Fish oil supplements, derived from oily fish such as salmon and mackerel, are amongst the most popular dietary supplements in the UK. Valued for their omega-3 fatty acid content—particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—fish oil is widely used to support cardiovascular health, brain function, and inflammatory conditions. Whilst the NHS recommends obtaining omega-3 through dietary sources, supplementation may benefit those with inadequate fish intake. Understanding what fish oil does for you, its evidence-based benefits, appropriate dosing, and potential risks enables informed decisions about whether supplementation is suitable for your individual health needs.

What Is Fish Oil and What Does It Contain?

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

The two principal omega-3 fatty acids found in fish oil are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). EPA typically comprises 18–30% of fish oil content, whilst DHA accounts for 10–25%, though these proportions vary depending on the fish species and extraction method. Both EPA and DHA are long-chain omega-3 fatty acids that play crucial roles in cellular function. Unlike alpha-linolenic acid (ALA), which is the essential omega-3 fatty acid found in plant sources, EPA and DHA are conditionally essential as the body can only convert limited amounts from ALA.

Fish oil supplements are available in various formulations, including:

• Natural triglyceride form – the form naturally present in fish

• Ethyl ester form – a concentrated, processed version

• Re-esterified triglycerides – processed oil converted back to triglyceride form

• Prescription preparations – such as omega-3-acid ethyl esters and icosapent ethyl, which are prescription-only medicines for specific conditions

Standard fish oil supplements are classified as food supplements, not medicines. The quality and purity of fish oil supplements can vary considerably between manufacturers. Reputable products undergo purification processes to remove environmental contaminants such as mercury, polychlorinated biphenyls (PCBs), and dioxins. When selecting a fish oil supplement, consumers should look for products that clearly state the EPA and DHA content per serving, rather than simply the total fish oil content.

For those following vegetarian or vegan diets, algae-derived EPA and DHA supplements are available as an alternative to fish oil.

Health Benefits: What Does Fish Oil Do for Your Body?

Fish oil exerts its physiological effects primarily through the actions of EPA and DHA on multiple body systems. These omega-3 fatty acids are incorporated into cell membranes, where they influence membrane fluidity, cellular signalling, and gene expression. The evidence base for fish oil's health benefits varies considerably depending on the condition and population studied.

Cardiovascular health represents the most extensively researched area. EPA and DHA contribute to cardiovascular function through several mechanisms: reducing triglyceride levels (typically by 20-30% at doses of 2-4g daily), modestly lowering blood pressure, decreasing platelet aggregation, and potentially reducing inflammation. An authorised GB Nutrition and Health Claim states that 250mg daily of EPA and DHA contributes to normal heart function. However, large-scale trials have produced mixed results regarding fish oil's ability to prevent cardiovascular events in the general population. NICE guidance (NG238) suggests that omega-3 supplements should not routinely be offered for the primary or secondary prevention of cardiovascular disease. There is one important exception: NICE Technology Appraisal 805 recommends icosapent ethyl (a prescription-only, highly purified EPA) with statins for reducing cardiovascular risk in specific high-risk groups.

Brain and cognitive function may be supported by DHA, which is a major structural component of brain tissue. Adequate DHA intake during pregnancy and early childhood is associated with normal brain development. Some observational studies suggest associations between higher omega-3 intake and reduced cognitive decline in older adults, though intervention trials have not consistently demonstrated that fish oil supplementation prevents dementia or improves cognitive function in those with established cognitive impairment.

Anti-inflammatory properties of omega-3 fatty acids have generated interest in conditions characterised by chronic inflammation. EPA and DHA are precursors to specialised pro-resolving mediators (resolvins and protectins) that help regulate inflammatory responses. Some evidence suggests potential benefits in rheumatoid arthritis, with studies indicating that high-dose fish oil may modestly reduce joint pain and stiffness, potentially allowing reduction in non-steroidal anti-inflammatory drug (NSAID) use. However, there is no official link established for many other inflammatory conditions, and further research is needed.

Eye health may be influenced by DHA, which is highly concentrated in retinal tissue. Adequate omega-3 intake has been associated with reduced risk of age-related macular degeneration in some observational studies, though supplementation trials (including the AREDS2 study) have not demonstrated protective effects. Some evidence suggests fish oil may provide modest benefit for dry eye symptoms, though results remain inconclusive.

How to Take Fish Oil Supplements Safely

When considering fish oil supplementation, several practical factors influence both efficacy and safety. The appropriate dosage depends on the intended purpose and individual health status. For general health maintenance, commonly used amounts in supplements range from 250–500mg of combined EPA and DHA daily, though there is no official UK recommended nutrient intake for EPA and DHA. The NHS emphasises obtaining omega-3 fatty acids through dietary sources, recommending at least two portions of fish weekly, including one portion of oily fish.

Fish oil supplements are best absorbed when taken with meals containing fat, as the dietary fat enhances omega-3 absorption. Taking supplements with food also reduces the likelihood of gastrointestinal side effects such as reflux or fishy aftertaste. Dividing the daily dose between two meals may further improve tolerability and absorption.

Important considerations for safe use include:

• Check the EPA and DHA content – product labels should clearly state the amount of EPA and DHA per capsule, not just total fish oil content

• Storage – keep supplements in a cool, dark place and refrigerate after opening if recommended by the manufacturer, as omega-3 fatty acids are susceptible to oxidation

• Quality assurance – select products that have been tested for contaminants

• Timing – consistency in timing of supplementation may help with adherence and reduce gastrointestinal symptoms

• Upper intake – avoid taking more than 5g per day of combined EPA and DHA from supplements without medical supervision

Patients should inform their GP or pharmacist before starting fish oil supplements, particularly if they are taking prescribed medications. This is especially important for individuals on anticoagulant or antiplatelet therapy, as fish oil may have additive effects on bleeding risk. Those scheduled for surgery should discuss fish oil supplementation with their surgical team, who may advise stopping before procedures depending on individual bleeding risk.

Pregnant and breastfeeding women considering fish oil supplementation should consult their midwife or GP. Whilst omega-3 fatty acids are important for foetal development, pregnant women should avoid fish oil supplements containing vitamin A (such as cod liver oil) due to potential risks to the unborn baby. The NHS advises specific limits on oily fish consumption during pregnancy due to potential contaminants. Algae-derived DHA/EPA products labelled as suitable for pregnancy may be an alternative.

Potential Side Effects and Risks of Fish Oil

Fish oil supplements are generally well tolerated by most individuals, but several adverse effects and potential risks warrant consideration. Understanding these helps patients make informed decisions and recognise when medical advice should be sought.

Common side effects are typically mild and gastrointestinal in nature:

• Fishy aftertaste or reflux – the most frequently reported complaint, often manageable by taking supplements with meals, choosing enteric-coated formulations, or freezing capsules before consumption

• Nausea and gastrointestinal discomfort – may occur particularly at higher doses

• Loose stools or diarrhoea – more common with doses exceeding 3g daily

• Fishy body odour or breath – occasionally reported with regular use

Bleeding risk represents the most clinically significant concern. Omega-3 fatty acids possess antiplatelet properties that may prolong bleeding time. Whilst this effect is generally modest at standard supplementation doses, it becomes more relevant for individuals taking anticoagulants (warfarin, direct oral anticoagulants) or antiplatelet agents (aspirin, clopidogrel). Patients on these medications should not commence fish oil supplementation without medical supervision. There is no official link established between standard-dose fish oil and clinically significant bleeding in otherwise healthy individuals, but caution is warranted in those with bleeding disorders.

Other potential concerns:

• LDL-cholesterol may rise in some people taking omega-3 supplements, particularly those containing DHA. If using for triglyceride lowering, lipid levels should be monitored.

• Atrial fibrillation – high prescription doses have been associated with a small increase in atrial fibrillation risk. Seek medical advice if you have a history of arrhythmia.

• Allergic reactions are possible, particularly in individuals with fish or shellfish allergies. People with fish allergy should avoid fish-derived oils and seek professional advice; algae-derived omega-3 may be an alternative.

When to seek medical advice:

• Unusual bruising or bleeding

• Persistent gastrointestinal symptoms

• Signs of allergic reaction (rash, swelling, breathing difficulties)

• Any concerns about interactions with prescribed medications

Call 999 if you have signs of a severe allergic reaction (anaphylaxis), such as breathing difficulty, throat swelling or collapse.

Patients should report suspected side effects via the MHRA Yellow Card scheme (website or app).

Who Should Consider Taking Fish Oil Supplements?

The decision to take fish oil supplements should be individualised, considering dietary intake, health status, and evidence-based indications. Not everyone requires supplementation, and obtaining omega-3 fatty acids through dietary sources remains the preferred approach where possible.

Individuals who may benefit from fish oil supplementation include:

Those with inadequate dietary intake – The NHS recommends consuming at least two portions of fish weekly, including one portion of oily fish (approximately 140g), which typically provides around 1-3g of EPA and DHA depending on the fish species. Individuals who do not meet this recommendation due to dietary preferences, allergies, or vegetarian/vegan diets may benefit from supplementation. Plant-based omega-3 sources (flaxseed, walnuts) provide alpha-linolenic acid (ALA), which converts to EPA and DHA inefficiently in the body. Vegetarians and vegans may consider algae-derived EPA and DHA supplements.

Pregnant and breastfeeding women – Adequate DHA intake supports foetal brain and eye development. However, pregnant women should avoid fish oil supplements containing vitamin A (such as cod liver oil) due to potential harm to the unborn baby. The NHS provides specific guidance on fish consumption during pregnancy, including limits on oily fish and high-mercury fish. Specific pregnancy-safe omega-3 supplements are available, and women should discuss supplementation with their midwife or GP.

Individuals with hypertriglyceridaemia – Prescription omega-3-acid ethyl esters are licensed as an adjunct to diet when dietary measures alone are insufficient for treating hypertriglyceridaemia. Very high triglyceride levels (>10 mmol/L) pose a risk of pancreatitis and warrant urgent specialist assessment. These medications should only be used under medical supervision.

Patients with rheumatoid arthritis – Some evidence supports high-dose fish oil (>2.7g EPA and DHA daily) as an adjunctive therapy for reducing joint pain and stiffness, though this should be discussed with a rheumatologist.

Specific cardiovascular risk groups – While general omega-3 supplementation is not recommended for cardiovascular prevention, NICE Technology Appraisal 805 recommends icosapent ethyl with statins for reducing cardiovascular risk in specific high-risk groups.

Individuals who should exercise caution or avoid fish oil:

• Those with fish or seafood allergies

• Patients taking anticoagulant or antiplatelet medications (without medical supervision)

• Individuals with bleeding disorders

• Those with a history of atrial fibrillation

• Those scheduled for surgery (discuss with surgical team)

For most healthy adults consuming adequate oily fish, supplementation offers limited additional benefit. The emphasis should remain on a balanced diet rich in whole foods. Individuals considering fish oil supplements should discuss their specific circumstances with a healthcare professional to determine whether supplementation is appropriate and to establish the optimal dose for their needs.

Frequently Asked Questions