Fish oil supplements, derived from oily fish such as mackerel, salmon, and sardines, are amongst the most widely consumed nutritional products in the UK. Valued primarily for their omega-3 fatty acid content—particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—these supplements are often considered for cardiovascular health, inflammatory conditions, and general wellbeing. However, the evidence supporting fish oil varies considerably across different health applications. This article examines what fish oil helps with, who may benefit from supplementation, appropriate dosing, safety considerations, and the current evidence base to help you make informed decisions about omega-3 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 are long-chain omega-3 fatty acids that play crucial roles in cellular membrane structure, inflammatory modulation, and various metabolic processes throughout the body. They are derived from alpha-linolenic acid (ALA), which is the essential omega-3 fatty acid that cannot be synthesised by the human body. EPA and DHA typically comprise about 30% of fish oil content, though this varies by species and processing method.

Fish oil supplements are available in several formulations, including:

• Natural triglyceride form – the naturally occurring structure in fish

• Ethyl ester form – a concentrated, processed version

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

• Licensed medicinal products – highly purified pharmaceutical-grade products with specific clinical indications

The quality and purity of fish oil supplements can vary considerably. Reputable manufacturers test for contaminants such as heavy metals (mercury, lead), polychlorinated biphenyls (PCBs), and dioxins. In the UK, most fish oil products are regulated as food supplements, while specific products like omega-3-acid ethyl esters and icosapent ethyl are licensed medicines with approved indications. Consumers should look for products from reputable brands that clearly state the EPA and DHA content per serving and comply with UK food supplement regulations.

Importantly, fish liver oils (such as cod liver oil) contain fat-soluble vitamins A and D in addition to omega-3 fatty acids. Products containing vitamin A should be avoided during pregnancy due to potential risks to the developing foetus.

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 health conditions. Individuals who may benefit most include those with inadequate dietary omega-3 intake, people who consume little or no oily fish, and patients with certain medical conditions where evidence supports supplementation.

The NHS recommends that most people should aim for at least two portions of fish per week, including one portion (about 140g) of oily fish. However, certain population groups may struggle to meet this target through diet alone, including vegetarians, vegans (who may prefer algal oil alternatives), individuals with fish allergies, or those with cultural or personal dietary restrictions. Pregnant and breastfeeding women are advised to consume oily fish but should limit intake to two portions weekly due to potential pollutant exposure, making supplementation an alternative consideration under medical guidance.

Clinical populations where fish oil supplementation may be discussed with healthcare professionals include:

• Patients with severe hypertriglyceridaemia (triglycerides >10 mmol/L), where specialist referral is typically indicated

• Eligible high-risk patients with cardiovascular disease who meet NICE criteria for prescription icosapent ethyl

• Individuals with inflammatory conditions such as rheumatoid arthritis

• People with dry eye syndrome, though evidence is inconsistent and high-quality trials have not shown clear benefit

It is important to note that fish oil supplementation is not suitable for everyone. Individuals taking anticoagulant medications (warfarin, DOACs) should consult their GP before starting fish oil, as higher doses may increase bleeding risk. Those with fish or shellfish allergies should exercise caution. If you are scheduled for surgery, check with your surgical team about whether to continue fish oil supplements. Patients should always inform their healthcare provider about all supplements they are taking to avoid potential drug interactions and ensure appropriate monitoring.

NICE does not recommend standard fish oil supplements for the prevention or treatment of depression or for routine cardiovascular disease prevention.

Recommended Dosage and Safety Considerations in the UK

Dosage recommendations for fish oil vary depending on the intended purpose and individual health status. For general health maintenance, the European Food Safety Authority (EFSA) suggests a daily intake of approximately 250mg of combined EPA and DHA for adults, which can typically be achieved through dietary sources. However, therapeutic doses for specific conditions may be considerably higher and should only be undertaken under medical supervision.

In the UK, there are important distinctions between over-the-counter supplements and prescription products:

• Food supplements: Standard over-the-counter fish oil supplements typically provide 300–1,000mg of combined EPA and DHA per capsule. Consumers should carefully check product labels to determine actual omega-3 content rather than total fish oil content.

• Licensed medicines: For severe hypertriglyceridaemia, omega-3-acid ethyl esters may be prescribed at doses of 2–4g daily. For cardiovascular risk reduction in eligible high-risk patients, icosapent ethyl (Vazkepa) is prescribed at 2g twice daily in combination with statin therapy, as per NICE Technology Appraisal 805.

Safety considerations include:

• Gastrointestinal effects – fishy aftertaste, belching, nausea, and loose stools are common, particularly at higher doses. Taking supplements with meals or using enteric-coated formulations may reduce these effects.

• Bleeding risk – whilst fish oil has mild antiplatelet effects, clinically significant bleeding is rare at standard doses. However, caution is warranted in patients on anticoagulants or antiplatelet agents.

• Atrial fibrillation risk – higher doses of omega-3 supplements have been associated with increased risk of atrial fibrillation in clinical trials. Patients should seek medical advice if they experience palpitations or irregular heartbeat.

• Vitamin toxicity – some fish oil products, particularly fish liver oils, contain added vitamins A and D. Excessive intake of these fat-soluble vitamins can be harmful, particularly during pregnancy. Pregnant women should avoid fish liver oils due to vitamin A content.

• Allergic reactions – though uncommon, individuals with fish allergies should seek medical advice before use.

• Quality concerns – oxidised (rancid) fish oil may be ineffective and potentially harmful. Store supplements in cool, dark conditions and check expiry dates.

Patients should contact their GP if they experience unusual bruising, prolonged bleeding, palpitations, severe gastrointestinal symptoms, or allergic reactions whilst taking fish oil supplements. Pregnant women should not exceed recommended doses without medical guidance, and children's dosing should be age-appropriate and discussed with a healthcare professional.

If you suspect you've experienced a side effect from a fish oil supplement, you can report it through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

What Does Fish Oil Help With? Evidence-Based Benefits

The evidence base for fish oil supplementation varies considerably across different health conditions, ranging from well-established benefits to areas where research remains inconclusive. Understanding the strength of evidence helps patients and clinicians make informed decisions about supplementation.

Cardiovascular health represents one of the most extensively studied areas. Omega-3 fatty acids can reduce triglyceride levels by 20–30% at therapeutic doses. However, standard mixed EPA/DHA fish oil supplements have not consistently demonstrated reduction in cardiovascular events in large-scale trials. NICE does not recommend routine use of fish oil supplements for primary or secondary prevention of cardiovascular disease. An important exception is icosapent ethyl (an EPA-only preparation), which NICE recommends for specific high-risk patients already on statins who meet criteria outlined in Technology Appraisal 805.

Inflammatory conditions, particularly rheumatoid arthritis, show moderate evidence for benefit. Fish oil's anti-inflammatory properties stem from EPA and DHA competing with arachidonic acid in inflammatory pathways, reducing production of pro-inflammatory eicosanoids. Some patients with rheumatoid arthritis report reduced joint pain and morning stiffness with fish oil supplementation (typically 2.6–3g EPA/DHA daily), though effects are generally modest and should complement rather than replace conventional disease-modifying therapy.

Mental health applications have generated considerable research interest. However, NICE does not recommend omega-3 supplements for the treatment of depression. Some evidence suggests omega-3 supplementation, particularly EPA-rich formulations, may provide modest benefits as adjunctive treatment in some cases, though results are inconsistent. The evidence for cognitive decline prevention and dementia risk reduction remains inconclusive, with observational studies showing associations but intervention trials producing disappointing results.

Pregnancy and infant development represents an important area where DHA plays a crucial role in foetal brain and eye development. Pregnant women with adequate omega-3 intake may have reduced risk of preterm birth according to Cochrane reviews, though supplementation trials show variable results. UK guidance does not strongly support routine supplementation beyond ensuring adequate dietary intake and following NHS advice on fish consumption during pregnancy.

Other potential benefits with varying evidence levels include:

• Dry eye syndrome – evidence is inconsistent, with large randomised controlled trials such as the DREAM study showing no clear benefit over placebo

• Age-related macular degeneration – observational evidence suggests protective effects, but intervention trials like AREDS2 did not show added benefit from omega-3 supplementation

• Asthma – limited evidence for benefit; not recommended as prevention or treatment

• Inflammatory bowel disease – inconsistent evidence; not recommended as primary therapy

Patients considering fish oil supplementation should discuss their individual circumstances with their GP or pharmacist. Whilst fish oil is generally safe at recommended doses, it should not replace evidence-based medical treatments. Those with specific health conditions should seek personalised advice, and anyone experiencing unexpected symptoms whilst taking supplements should seek medical review promptly. The most reliable approach remains consuming oily fish as part of a balanced diet, with supplementation reserved for those unable to meet requirements through food or with specific clinical indications supported by their healthcare team.

Frequently Asked Questions