Omega-3 fish oil is a widely used dietary supplement derived from oily fish such as salmon, mackerel, and sardines. It contains eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), long-chain polyunsaturated fatty acids with anti-inflammatory properties and roles in cardiovascular and neurological health. Whilst prescription omega-3 medicines are licensed for specific conditions such as hypertriglyceridaemia, over-the-counter supplements are commonly taken for general health maintenance. UK guidance does not routinely recommend omega-3 supplements for cardiovascular disease prevention or cognitive health in the general population. Understanding the evidence-based uses, appropriate dosing, and safety considerations is essential for informed decision-making about omega-3 supplementation.

What Is Omega-3 Fish Oil and How Does It Work?

Omega-3 fish oil is a dietary supplement derived from the tissues of oily fish such as mackerel, salmon, sardines, and anchovies. It contains two principal long-chain polyunsaturated fatty acids: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). While alpha-linolenic acid (ALA) is the essential omega-3 fatty acid that must be obtained through diet, EPA and DHA are the biologically active forms that can be synthesised from ALA, albeit inefficiently.

The mechanism of action of omega-3 fatty acids is multifaceted. EPA and DHA are incorporated into cell membranes throughout the body, where they influence membrane fluidity and cellular signalling pathways. They serve as precursors to bioactive lipid mediators known as resolvins and protectins, which possess anti-inflammatory properties. This anti-inflammatory effect is thought to underpin many of the proposed cardiovascular and metabolic benefits associated with omega-3 supplementation.

Omega-3 fatty acids also modulate lipid metabolism by reducing hepatic triglyceride synthesis and increasing fatty acid oxidation. In cardiovascular physiology, they may improve endothelial function, reduce platelet aggregation, and have modest effects on blood pressure. DHA is particularly concentrated in neural and retinal tissues, where it plays a structural role in maintaining cell membrane integrity and supporting neurotransmission.

Whilst the body can convert ALA (found in plant sources like flaxseed and walnuts) into EPA and DHA, this conversion is inefficient—typically around 5-10% for EPA and less than 1% for DHA. Consequently, marine-derived omega-3 supplements provide a more direct and bioavailable source of these fatty acids, particularly for individuals with limited fish consumption in their diet.

It's important to note that over-the-counter omega-3 supplements differ from prescription-only omega-3 medicines, which are licensed for specific medical conditions.

Evidence-Based Uses of Omega-3 Fish Oil

The clinical evidence for omega-3 fish oil varies considerably depending on the indication. For hypertriglyceridaemia, prescription omega-3-acid ethyl esters (2-4 g daily) are licensed for treatment, typically under specialist supervision. These preparations can reduce serum triglyceride levels by 20-30%. Management of severe hypertriglyceridaemia generally requires specialist input rather than routine omega-3 supplementation alone.

In 2022, NICE (TA805) recommended icosapent ethyl (a highly purified EPA) for reducing cardiovascular risk in specific patients with established cardiovascular disease and raised triglycerides, despite maximum tolerated statin therapy. It's important to note that over-the-counter fish oil supplements should not be considered equivalent to this prescription medication.

For general cardiovascular disease prevention, NICE guidance (NG238) does not routinely recommend omega-3 supplements for primary or secondary prevention in the general population. While some studies have shown benefits with high-dose purified EPA, this remains an area of ongoing research.

In rheumatoid arthritis, omega-3 supplementation may provide modest symptomatic relief and reduce the need for non-steroidal anti-inflammatory drugs (NSAIDs), though it should be considered an adjunct rather than a primary treatment. Cochrane reviews suggest doses of at least 2.7 g of EPA and DHA daily may be required for anti-inflammatory effects.

For age-related macular degeneration (AMD), NICE guidance (NG82) does not recommend offering omega-3 supplements to prevent or treat AMD, despite some observational studies suggesting associations between dietary intake and reduced risk.

Regarding cognitive function, NICE guidance (NG97) does not recommend omega-3 supplements to prevent dementia. Similarly, for depression and mental health, NICE (NG222) does not routinely recommend omega-3 supplements as treatment for depression.

Whilst omega-3 fatty acids are important for foetal neurodevelopment, routine supplementation during pregnancy beyond a balanced diet is not universally recommended by UK guidelines, though women may wish to ensure adequate intake through diet.

Recommended Dosage and How to Take Omega-3 Fish Oil

The appropriate dosage of omega-3 fish oil depends on the intended use and individual patient factors. For general health maintenance, UK dietary recommendations advise consuming two portions of fish per week, including one portion of oily fish, providing approximately 450 mg of EPA and DHA daily. Over-the-counter supplements typically provide 250-1000 mg of combined EPA and DHA per capsule.

For hypertriglyceridaemia, prescription omega-3-acid ethyl esters (such as Omacor) are dosed at 2-4 g daily according to the Summary of Product Characteristics (SmPC). This treatment should be initiated and monitored by healthcare professionals. Patients with very high triglycerides (>10 mmol/L) or recurrent levels above 20 mmol/L should be referred for urgent specialist assessment due to pancreatitis risk.

For rheumatoid arthritis, some evidence suggests benefit at doses of 2.7 g or more of EPA and DHA daily as an adjunctive treatment, though this should be discussed with a healthcare professional.

It's important to note that over-the-counter omega-3 supplements should not be used as substitutes for prescribed medications such as statins or icosapent ethyl. High-dose supplementation (above 3 g daily) should only be used under clinical supervision.

Omega-3 supplements are best taken with meals to enhance absorption and reduce gastrointestinal side effects such as fishy aftertaste or reflux. The fat content of food facilitates the absorption of these lipid-soluble nutrients. Dividing the daily dose across multiple meals may improve tolerability.

Patients should be advised to check the EPA and DHA content on supplement labels, as total fish oil content does not reflect the amount of active omega-3 fatty acids. For example, a 1000 mg fish oil capsule may contain only 300 mg of combined EPA and DHA.

Quality and purity are important considerations. Reputable supplements should be tested for contaminants such as mercury, polychlorinated biphenyls (PCBs), and dioxins. Products bearing certification from independent testing organisations provide additional assurance.

Patients taking anticoagulant or antiplatelet medications should consult their GP or pharmacist before starting omega-3 supplements, as high doses may affect bleeding time.

Side Effects, Safety and Who Should Avoid Omega-3 Fish Oil

Omega-3 fish oil supplements are generally well tolerated, with most adverse effects being mild and gastrointestinal in nature. Common side effects include:

• Fishy aftertaste or reflux (affecting up to 10% of users)

• Nausea or loose stools, particularly at higher doses

• Mild gastrointestinal discomfort

These effects can often be minimised by taking supplements with meals, choosing enteric-coated preparations, or refrigerating capsules. Switching to a different brand or formulation may also help.

At higher doses, omega-3 fatty acids may prolong bleeding time. According to the SmPC for prescription omega-3 products, patients taking anticoagulants should be monitored, particularly when starting or changing the dose. INR monitoring is advised for patients on warfarin. Patients should inform healthcare professionals about omega-3 use before surgery or dental procedures.

Patients taking warfarin, direct oral anticoagulants (DOACs), or antiplatelet agents (such as aspirin or clopidogrel) should inform their healthcare provider before starting high-dose omega-3 supplements.

Allergic reactions to fish oil supplements are uncommon but possible, particularly in individuals with fish or shellfish allergies. Patients with known fish allergies should exercise caution and consider algae-based omega-3 supplements as an alternative source of DHA and EPA.

Individuals with atrial fibrillation should be aware that high-dose omega-3 supplementation may be associated with increased risk of atrial fibrillation in certain populations. This signal is stronger with prescription products, particularly icosapent ethyl. Patients with current or previous atrial fibrillation should discuss omega-3 use with their healthcare provider.

Patients should contact their GP if they experience:

• Persistent gastrointestinal symptoms

• Unusual bleeding or bruising

• Allergic reactions (rash, swelling, difficulty breathing)

• Any concerns about interactions with prescribed medications

Pregnant and breastfeeding women should avoid cod liver oil supplements due to their vitamin A (retinol) content, as excessive vitamin A intake during pregnancy can be harmful to the developing baby. NHS advice recommends limiting oily fish consumption to 2 portions weekly during pregnancy and avoiding certain fish with higher mercury content. DHA supplementation during pregnancy and lactation is generally considered safe when using products specifically formulated for pregnancy.

Patients with diabetes should be aware that omega-3 supplements generally have neutral effects on blood glucose control. Those with liver disease should consult their specialist before taking high-dose omega-3, particularly for hypertriglyceridaemia management.

Suspected adverse reactions to omega-3 supplements can be reported via the MHRA Yellow Card Scheme.

Frequently Asked Questions