Fish oil supplements, rich in omega-3 fatty acids EPA and DHA, are among the most widely used dietary supplements in the UK. The benefits of fish oil include supporting cardiovascular health, particularly through triglyceride reduction, and contributing to anti-inflammatory processes throughout the body. Omega-3 fatty acids play essential structural and functional roles in cell membranes, influencing everything from heart function to brain health. While fish oil is generally well tolerated and available over the counter, not all products are equivalent in quality or concentration. Understanding the evidence-based benefits, appropriate dosing, and safety considerations is essential for anyone considering supplementation. This article examines the clinical evidence, recommended uses, and practical guidance for fish oil supplementation in line with UK health guidance.

What Is Fish Oil and How Does It Work?

Fish oil is a dietary supplement derived from the tissues of oily fish, including salmon, mackerel, sardines, and anchovies. It is one of the most widely consumed supplements in the UK, valued primarily for its high content of long-chain omega-3 fatty acids—specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These fatty acids cannot be synthesised efficiently by the human body from other dietary sources and must therefore be obtained through diet or supplementation.

The therapeutic effects of fish oil are attributed to the biological activity of EPA and DHA. Once ingested, these omega-3 fatty acids are incorporated into cell membranes throughout the body, where they influence cellular function and signalling pathways. EPA and DHA exert anti-inflammatory effects by modulating the production of eicosanoids—bioactive lipid mediators derived from arachidonic acid. Omega-3 fatty acids compete with arachidonic acid in enzymatic pathways, leading to the production of less inflammatory prostaglandins and leukotrienes.

Additionally, fish oil has been shown to affect lipid metabolism. It can reduce circulating triglyceride levels by decreasing hepatic synthesis of very-low-density lipoproteins (VLDL) and enhancing the clearance of triglyceride-rich particles from the bloodstream. However, it's important to note that DHA-containing products may increase LDL-cholesterol in some individuals. DHA also plays a structural role in neuronal and retinal tissues, contributing to cognitive function and visual health.

While fish oil supplements are available over the counter in various formulations—including capsules, liquids, and emulsions—it is important to recognise that not all products are equivalent in purity, concentration, or bioavailability. Consumers should look for products that specify EPA and DHA content and have been tested for contaminants such as heavy metals and polychlorinated biphenyls (PCBs). Reputable manufacturers comply with UK food supplement regulations and employ third-party testing to ensure quality and safety.

Health Benefits of Fish Oil Supplements

Fish oil supplementation has been investigated extensively for its potential health benefits, with varying levels of evidence supporting its use across different clinical contexts. One of the most well-established benefits is its effect on cardiovascular health, specifically for triglyceride reduction. High-dose omega-3 fatty acids (typically 2–4 grams of EPA and DHA daily) have been shown to reduce serum triglyceride levels by approximately 20–30%, which may be beneficial for individuals with severe hypertriglyceridaemia. The MHRA has approved prescription omega-3 preparations for this indication. However, it's important to note that NICE does not recommend omega-3 supplements for the primary or secondary prevention of cardiovascular disease in the general population.

There is also evidence suggesting that regular fish oil intake may contribute to a modest reduction in blood pressure (typically 2-3 mmHg), particularly in individuals with hypertension. The anti-inflammatory properties of omega-3 fatty acids may help improve endothelial function and reduce arterial stiffness, although these effects are generally modest and should not replace standard antihypertensive therapy.

Mental health and cognitive function represent another area of interest. DHA is a major structural component of brain tissue, and adequate omega-3 status is considered important for neurodevelopment and cognitive maintenance. Some observational studies have linked higher fish consumption with a reduced risk of depression and cognitive decline, though clinical trial evidence remains mixed. There is no official link established between fish oil supplementation and the prevention or treatment of dementia, and further research is needed.

In inflammatory conditions such as rheumatoid arthritis, fish oil may provide symptomatic relief by reducing joint pain and stiffness, potentially allowing for reduced use of non-steroidal anti-inflammatory drugs (NSAIDs). Cochrane reviews support modest benefits for joint symptoms. However, it should be viewed as a complementary approach rather than a primary treatment. Evidence for benefits in other inflammatory or autoimmune conditions is less robust and requires further investigation.

Who Should Consider Taking Fish Oil?

Fish oil supplementation may be appropriate for several groups, though individual circumstances and existing health conditions should always be considered. Individuals with severe hypertriglyceridaemia (triglycerides ≥10 mmol/L) may benefit from high-dose omega-3 supplementation, but this should be under specialist advice. NICE recommends urgent assessment if triglycerides exceed 20 mmol/L or if there are symptoms of pancreatitis. It's important to understand that omega-3 supplements are not a substitute for statins or other lipid-lowering therapies for most people.

Pregnant and breastfeeding women are often advised to ensure adequate omega-3 intake to support foetal and infant neurodevelopment. DHA is particularly important during the third trimester and early infancy for brain and retinal development. However, pregnant women should avoid supplements containing vitamin A (such as cod liver oil), as excessive vitamin A intake during pregnancy can be harmful. The NHS advises that pregnant and breastfeeding women can safely consume up to two portions of oily fish per week, should limit tuna intake, and should avoid shark, marlin, and swordfish entirely. A low-dose supplement may be considered if dietary intake is insufficient.

People with limited dietary intake of oily fish—whether due to dietary preferences, allergies, or cultural practices—may consider supplementation to meet recommended omega-3 levels. The Scientific Advisory Committee on Nutrition (SACN) recommends that adults consume at least two portions of fish per week, including one portion (approximately 140 grams) of oily fish, which provides around 450 mg of EPA and DHA daily.

Individuals with inflammatory joint conditions, such as rheumatoid arthritis, may find fish oil a useful adjunct to conventional treatment, though they should discuss this with their rheumatologist or GP. It is important to note that fish oil is not a substitute for disease-modifying antirheumatic drugs (DMARDs) or other prescribed therapies.

Conversely, fish oil supplementation may not be necessary for individuals who already consume adequate amounts of oily fish regularly or those without specific risk factors or deficiencies.

Recommended Dosage and How to Take Fish Oil

The appropriate dosage of fish oil depends on the intended purpose and individual health status. For general health maintenance, a daily intake of approximately 450 mg of combined EPA and DHA is typically recommended in the UK, which aligns with the Scientific Advisory Committee on Nutrition (SACN) guidance. This can often be achieved through dietary sources alone—specifically, consuming at least one portion of oily fish per week as part of two portions of fish overall. For individuals who do not consume oily fish regularly, a standard over-the-counter fish oil supplement may be appropriate.

For triglyceride reduction, higher doses are usually required. Clinical evidence supports the use of 2–4 grams of EPA and DHA daily for significant triglyceride-lowering effects. At these doses, fish oil should be considered a therapeutic intervention and taken under medical supervision, ideally using licensed prescription products such as omega-3-acid ethyl esters. It's worth noting that DHA-containing products may increase LDL-cholesterol levels in some individuals, which should be monitored.

When selecting a fish oil supplement, it is important to check the EPA and DHA content per capsule, as total fish oil content can be misleading. For example, a 1000 mg fish oil capsule may contain only 300 mg of combined EPA and DHA. Consumers should read labels carefully and calculate the number of capsules needed to achieve the desired dose.

Timing and administration: Fish oil is best taken with meals to enhance absorption and reduce the likelihood of gastrointestinal side effects such as reflux or fishy aftertaste. Splitting the dose across two or three meals may improve tolerability, particularly at higher doses. Some individuals find enteric-coated capsules or refrigerated liquid formulations more palatable.

It is advisable to store fish oil supplements in a cool, dark place or refrigerate them to prevent oxidation, which can reduce efficacy and lead to rancidity. Supplements should be used before the expiry date, and any capsules with an unusual odour should be discarded.

Patients taking anticoagulant or antiplatelet medications should consult their GP or pharmacist before starting fish oil, as high doses may theoretically increase bleeding risk. Those on high-dose omega-3 should inform their healthcare providers before surgical procedures.

Potential Side Effects and Safety Considerations

Fish oil is generally well tolerated, but like all supplements, it can cause side effects in some individuals. The most common adverse effects are gastrointestinal, including nausea, diarrhoea, indigestion, and a fishy aftertaste or burping. These symptoms are usually mild and can often be minimised by taking fish oil with food, using enteric-coated capsules, or switching to a different formulation. Freezing capsules before ingestion is another strategy some people find helpful.

At higher doses (above 3 grams daily), there is a risk of increased bleeding due to the antiplatelet effects of omega-3 fatty acids. While clinically significant bleeding is uncommon, individuals taking anticoagulants (such as warfarin or direct oral anticoagulants) or antiplatelet agents (such as aspirin or clopidogrel) should inform their healthcare provider before starting fish oil supplementation. Additional monitoring may be warranted, and it's advisable to inform surgeons or anaesthetists about high-dose omega-3 use before procedures.

Recent clinical trials have identified an increased risk of atrial fibrillation with higher-dose omega-3 supplementation, particularly with prescription products. People with existing heart rhythm disorders or risk factors for atrial fibrillation should discuss this with their clinician before taking high doses of fish oil.

As mentioned previously, DHA-containing omega-3 products may increase LDL-cholesterol levels in some individuals, which should be monitored in people with existing cardiovascular risk factors.

Allergic reactions to fish oil are uncommon but can occur, particularly in individuals with fish allergies. Fish oil is typically derived from fish, not shellfish, though cross-contamination is possible. Symptoms may include rash, itching, swelling, or, in rare cases, anaphylaxis. Anyone with a known fish allergy should avoid fish oil supplements or seek advice from an allergist. Alternative sources of omega-3 fatty acids, such as algal oil (derived from microalgae), are available and suitable for vegetarians, vegans, and those with fish allergies.

Contaminants such as mercury, PCBs, and dioxins can be present in fish oil, particularly in lower-quality products. Reputable manufacturers employ molecular distillation and testing to ensure purity.

When to seek medical advice: Patients should contact their GP if they experience persistent gastrointestinal symptoms, signs of an allergic reaction, unusual bruising or bleeding, or if they are unsure whether fish oil is appropriate given their medication regimen or health conditions. Any suspected adverse reactions should be reported to the MHRA Yellow Card Scheme. Fish oil should not be viewed as a replacement for prescribed medications or lifestyle modifications.

Frequently Asked Questions