Should I take fish oil? This is a common question among UK adults considering dietary supplements. Fish oil, derived from oily fish such as mackerel and salmon, contains omega-3 fatty acids (EPA and DHA) that support various bodily functions. Whilst dietary sources remain preferable, supplementation may benefit certain individuals. However, NICE does not recommend standard omega-3 supplements for cardiovascular disease prevention, and evidence for other conditions varies. Understanding who may benefit, potential risks, and how fish oil compares to dietary intake is essential for making 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 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 18–31% of fish oil content, whilst DHA accounts for 12–23%, though these proportions vary depending on the fish source and processing methods. These long-chain omega-3 fatty acids are derived from alpha-linolenic acid (ALA), which is the essential omega-3 fatty acid that humans cannot synthesise and must obtain through diet. The body converts only limited amounts of ALA to EPA and DHA, making direct consumption beneficial.

Fish oil supplements are available in various formulations, including soft gel capsules, liquid preparations, and emulsions. Some products contain additional ingredients such as vitamin E (added as an antioxidant to prevent rancidity) or vitamin D. It is important to distinguish fish oil from cod liver oil, which contains vitamins A and D but lower concentrations of omega-3 fatty acids. The NHS advises that pregnant women should avoid cod liver oil and other supplements containing vitamin A (retinol), as high doses may harm the unborn baby.

The quality and purity of fish oil supplements can vary between manufacturers. Reputable products should comply with UK food law regarding contaminant limits. Many undergo purification processes to reduce environmental contaminants such as mercury, polychlorinated biphenyls (PCBs), and dioxins, which can accumulate in fish tissues.

Potential Health Benefits of Fish Oil Supplements

Research into fish oil supplementation has explored numerous potential health benefits, though the strength of evidence varies across different conditions and findings must be interpreted in the context of UK clinical guidance.

Regarding cardiovascular health, it's important to note that NICE guideline NG238 does not recommend omega-3 fatty acid supplements (EPA+DHA) for primary or secondary prevention of cardiovascular disease. However, NICE has approved icosapent ethyl (a prescription-only, purified EPA product) for specific high-risk patients with established cardiovascular disease and raised triglycerides who are already taking statins (TA805). Omega-3 fatty acids may influence several cardiovascular mechanisms, including reducing triglyceride levels, modest effects on blood pressure, and anti-inflammatory properties, though these surrogate effects do not necessarily translate to clinical outcome benefits in all populations.

For hypertriglyceridaemia, prescription omega-3-acid ethyl esters (not standard over-the-counter fish oil supplements) may be prescribed under medical supervision according to the product's licensed indication. These prescription products are typically used at higher doses than dietary supplements and should only be used under clinician oversight.

For mental health conditions, evidence remains inconclusive. Some studies suggest omega-3 supplementation may provide modest benefits as an adjunct treatment for depression, particularly formulations higher in EPA. However, there is insufficient evidence to recommend fish oil as a standalone treatment for depression, anxiety, or other psychiatric disorders. The Royal College of Psychiatrists notes that whilst omega-3s may have a supportive role, they should not replace evidence-based treatments.

Rheumatoid arthritis patients may experience modest reductions in joint pain and morning stiffness with fish oil supplementation, potentially allowing reduced use of non-steroidal anti-inflammatory drugs (NSAIDs). The anti-inflammatory properties of EPA and DHA may contribute to these effects, though fish oil should complement rather than replace disease-modifying treatments.

Claims regarding cognitive function, age-related macular degeneration, and cancer prevention remain unproven. Whilst observational studies suggest associations between dietary fish consumption and reduced disease risk, supplementation trials have generally failed to demonstrate significant protective effects in these areas.

Who Should Consider Taking Fish Oil?

The decision to take fish oil supplements should be individualised, considering dietary intake, health status, and evidence-based indications. Dietary sources of omega-3 fatty acids are generally preferable to supplementation. The NHS recommends consuming at least two portions of fish weekly, including one portion of oily fish (approximately 140 grams), which provides adequate omega-3 intake for most healthy adults.

Individuals with severe hypertriglyceridaemia may be prescribed prescription-only omega-3-acid ethyl esters by their GP or lipid specialist. These prescription medicines are different from standard over-the-counter fish oil supplements and should be used under medical supervision according to the product's licensed indication.

For people with established cardiovascular disease, NICE does not recommend standard omega-3 supplements for primary or secondary prevention. However, a specific prescription-only EPA product (icosapent ethyl) may be considered for eligible patients with established cardiovascular disease who have raised triglycerides (≥1.7 mmol/L) despite statin therapy, in line with NICE technology appraisal guidance (TA805). This should be discussed with a cardiologist or GP.

Pregnant and breastfeeding women require adequate DHA for foetal and infant neurodevelopment. Whilst dietary sources are preferred, supplementation may be appropriate for those who do not consume fish due to dietary restrictions or preferences. The NHS specifically advises pregnant women to avoid cod liver oil and other supplements containing vitamin A (retinol), as these may harm the unborn baby. Standard prenatal supplements often contain appropriate amounts of DHA.

Conversely, individuals who regularly consume oily fish, those taking anticoagulant medications (without medical supervision), and people with fish or shellfish allergies should exercise caution. Vegans and vegetarians may consider algae-based omega-3 supplements as an alternative source of EPA and DHA. Anyone considering supplementation should discuss this with their GP or a registered dietitian, particularly if they have existing health conditions or take regular medications.

Possible Side Effects and Risks of Fish Oil

Fish oil supplements are generally well-tolerated when taken at recommended doses, but several side effects and potential risks warrant consideration. The most commonly reported adverse effects are gastrointestinal disturbances, including nausea, loose stools, indigestion, and abdominal discomfort. These symptoms typically affect a proportion of users and are usually mild. Taking fish oil with meals or using enteric-coated formulations may reduce gastrointestinal upset.

Many users report a persistent fishy aftertaste, halitosis (bad breath), or fishy-smelling burps, which, whilst not medically concerning, can affect compliance. Refrigerating capsules or choosing flavoured preparations may minimise these effects. Some individuals also notice fishy body odour with high-dose supplementation.

A significant consideration is the antiplatelet effect of omega-3 fatty acids, which may influence blood clotting. This is particularly relevant at higher doses, such as those used in prescription omega-3 medicines. Patients taking anticoagulants (warfarin, DOACs), antiplatelet agents (aspirin, clopidogrel), or NSAIDs should consult their GP before starting fish oil, as combined use may increase bleeding risk. Those taking warfarin should have their INR checked when starting or changing doses of omega-3 supplements. There is no established link between standard-dose fish oil and major bleeding events in most individuals, but caution is warranted in those with bleeding disorders or upcoming surgery.

Allergic reactions are possible, particularly in individuals with fish allergies. While having a shellfish allergy does not necessarily mean you will be allergic to fish oil, caution is advised due to potential cross-contamination. Symptoms may include rash, itching, or, rarely, anaphylaxis. Seek immediate medical attention if you experience difficulty breathing, facial or throat swelling, or dizziness/collapse after taking fish oil. Anyone with known fish allergy should seek medical advice before use and consider alternative omega-3 sources.

High doses may cause minor elevations in LDL cholesterol in some individuals, and there is evidence suggesting potential increases in atrial fibrillation risk with high-dose prescription omega-3 preparations, particularly in certain high-risk populations.

When to contact your GP: Seek medical advice if you experience unusual bleeding or bruising, severe abdominal pain, allergic symptoms, or if you are considering fish oil whilst taking anticoagulant medications or before planned surgery. Suspected side effects can be reported to the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or via the Yellow Card app).

How to Choose and Take Fish Oil Safely

Selecting a high-quality fish oil supplement requires attention to several key factors. Purity and testing are important—choose products from reputable manufacturers that undergo third-party testing to verify EPA and DHA content and check for contaminants (mercury, PCBs, dioxins). Reputable brands should comply with UK food safety standards.

Examine the EPA and DHA content carefully, as this varies significantly between products. Labels should clearly state the amount of EPA and DHA per serving, not just total fish oil content. For general health maintenance, combined EPA and DHA intake of approximately 250 mg daily is typically sufficient, in line with European Food Safety Authority recommendations. Higher therapeutic doses (1 gram or more daily) are available as prescription medicines for specific conditions under medical supervision. Be aware that a 1000 mg fish oil capsule may contain only 300 mg of combined omega-3 fatty acids.

Consider the formulation type: natural triglyceride forms may be better absorbed than ethyl ester forms, though both are effective. Enteric-coated capsules can reduce gastrointestinal side effects. Check expiration dates and storage requirements, as omega-3 fatty acids are susceptible to oxidation. Supplements should smell mildly fishy but not rancid.

Dosing recommendations depend on the indication. For general supplementation, approximately 250 mg of combined EPA and DHA daily is reasonable for those not consuming oily fish regularly. Higher doses should only be taken under medical supervision. Take fish oil with meals to enhance absorption and reduce gastrointestinal upset. Consistency is important—benefits, if present, typically require several weeks to months of regular use.

Important safety considerations include informing your GP and surgical team about fish oil use, particularly before planned procedures, as they may advise discontinuation beforehand. Store supplements in a cool, dark place or refrigerate to prevent oxidation. Avoid exceeding recommended doses without medical advice, as very high intakes provide no additional benefit and may increase risks. Pregnant women should specifically avoid cod liver oil and other supplements containing vitamin A (retinol).

Patients should be aware that fish oil supplements are not regulated as medicines in the UK and are not subject to the same rigorous standards as prescription medications. The MHRA does not evaluate most supplements for efficacy, emphasising the importance of choosing reputable brands and maintaining realistic expectations about potential benefits.

Frequently Asked Questions