Fish oil supplements are among the most widely used nutritional products in the UK, valued for their omega-3 fatty acid content. Many people take fish oil to support cardiovascular health, reduce inflammation, and maintain overall wellbeing. The two key omega-3 fatty acids—eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—cannot be produced by the body in adequate amounts and must be obtained through diet or supplementation. Whilst dietary sources remain the preferred option, supplementation may benefit those with insufficient fish intake. Understanding the evidence-based rationale, appropriate usage, and potential risks helps individuals make informed decisions about fish oil 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. These essential fatty acids cannot be synthesised by the human body in sufficient quantities and must therefore be obtained through diet or supplementation.

The two principal omega-3 fatty acids found in fish oil are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The content of EPA and DHA varies considerably between products, so it's important to check labels for the exact amounts of these active ingredients rather than just the total fish oil content. Both EPA and DHA play crucial roles in cellular function, particularly within cell membranes where they influence fluidity, signalling pathways, and inflammatory responses.

It's important to distinguish between fish body oil (the focus of this article) and fish liver oil, which contains vitamins A and D and may not be suitable for pregnant women due to vitamin A content.

Commercially available fish oil supplements are produced through various extraction and purification processes to concentrate omega-3 content and remove potential contaminants such as heavy metals, polychlorinated biphenyls (PCBs), and dioxins. Products may be presented as natural triglycerides, ethyl esters, or re-esterified triglycerides, with potential differences in absorption that are typically minimised when taken with meals. The Medicines and Healthcare products Regulatory Agency (MHRA) regulates certain high-dose omega-3 preparations as medicines, whilst lower-dose products are classified as food supplements.

Quality fish oil supplements should specify the exact amounts of EPA and DHA per capsule. Products meeting Good Manufacturing Practice (GMP) standards can provide additional assurance regarding purity and potency. Understanding these compositional details helps consumers make informed choices when selecting fish oil products.

Why Take Fish Oil: Key Health Benefits

The primary rationale for fish oil supplementation centres on the established physiological roles of omega-3 fatty acids in human health. Cardiovascular effects represent the most extensively researched area, with EPA and DHA demonstrating multiple mechanisms. These include modest reductions in triglyceride levels (particularly at higher prescription doses of 2–4 g daily), slight improvements in blood pressure, reduced platelet aggregation, and favourable effects on endothelial function. It's important to note that NICE guidance does not recommend omega-3 supplements for secondary prevention after myocardial infarction, instead encouraging dietary fish consumption. Prescription-only icosapent ethyl (a purified EPA product) has received NICE approval (TA805) for cardiovascular risk reduction in specific high-risk patients with raised triglycerides despite statin therapy, but this evidence does not apply to over-the-counter fish oil supplements.

Anti-inflammatory properties constitute another significant benefit. Omega-3 fatty acids serve as precursors to specialised pro-resolving mediators (resolvins, protectins, and maresins) that actively promote the resolution of inflammatory processes. This mechanism has implications for various inflammatory conditions, including rheumatoid arthritis, where supplementation may provide modest symptomatic relief and potentially reduce non-steroidal anti-inflammatory drug (NSAID) requirements in some patients.

Neurological and mental health applications have garnered considerable research interest. DHA is a major structural component of brain tissue and retinal membranes, essential for neurodevelopment and cognitive function. Some studies indicate modest improvements in depressive symptoms when EPA-predominant omega-3 supplements are used adjunctively with conventional antidepressants, though fish oil should not replace standard psychiatric treatment.

Additionally, omega-3 fatty acids may support joint health. While some studies have investigated potential benefits for eye health and immune function, the evidence for these applications is less established. It is important to note that whilst these mechanisms are biologically plausible and supported by some clinical evidence, benefits vary considerably between individuals and conditions.

Who Should Consider Taking Fish Oil Supplements?

Fish oil supplementation may be appropriate for several specific population groups, though dietary sources of omega-3 fatty acids should be prioritised where possible. The NHS recommends consuming at least two portions of fish weekly, including one portion of oily fish, which typically provides adequate omega-3 intake for most healthy adults.

Individuals who eat little or no fish represent a primary group who may benefit from supplementation. This includes people with fish allergies (who may require algae-based omega-3 alternatives), those following strict vegetarian or vegan diets (algal DHA is recommended), and individuals with malabsorption disorders affecting fat-soluble nutrient uptake.

Pregnant and breastfeeding women should follow NHS guidance on fish consumption, which includes specific limits on oily fish and tuna due to potential contaminants. The NHS and NICE do not routinely recommend omega-3 supplements during pregnancy. Pregnant women should avoid fish liver oils due to vitamin A content, which may harm the developing baby. Women should consult their midwife or GP before commencing any supplementation during pregnancy.

Patients with hypertriglyceridaemia may be prescribed high-dose omega-3 preparations under medical supervision. Prescription-only icosapent ethyl may be considered for specific high-risk patients with raised triglycerides despite statin therapy, in line with NICE TA805 criteria. However, fish oil should complement rather than replace evidence-based medications such as statins and antihypertensives.

People with rheumatoid arthritis may experience modest symptomatic benefits, though supplementation should be discussed with their rheumatologist as part of comprehensive disease management. Evidence for benefits in inflammatory bowel disease is limited and inconsistent, so specialist advice should be sought.

Conversely, fish oil supplementation is generally unnecessary for healthy individuals consuming adequate dietary omega-3 fatty acids. Those considering supplementation should discuss their individual circumstances with a healthcare professional, particularly if taking anticoagulant medications or managing chronic health conditions.

How to Take Fish Oil Safely and Effectively

Optimal fish oil supplementation requires attention to dosage, timing, product selection, and potential drug interactions to maximise benefits whilst minimising risks.

Dosage considerations vary according to the intended purpose. For general health maintenance in adults with inadequate dietary intake, 250–500 mg combined EPA and DHA daily is typically sufficient. Higher doses (2–4 g daily) for triglyceride reduction are available only on prescription in the UK and should be used under medical supervision. For inflammatory conditions such as rheumatoid arthritis, higher doses (often ≥2.7 g EPA+DHA daily) have been studied, but should be used with clinician oversight. It is crucial to note the actual EPA and DHA content rather than total fish oil weight, as a 1000 mg fish oil capsule may contain only 300 mg of omega-3 fatty acids.

Timing and administration can influence tolerability and absorption. Taking fish oil with meals, particularly those containing other fats, enhances absorption and reduces gastrointestinal side effects such as fishy aftertaste, reflux, or nausea. Dividing higher doses throughout the day rather than taking a single large dose may improve tolerance. Storing capsules in the refrigerator or freezer can minimise fishy burps, and enteric-coated formulations may reduce gastrointestinal symptoms.

Product quality is paramount. Select supplements that specify EPA and DHA content and are produced to Good Manufacturing Practice (GMP) standards. Liquid formulations may offer higher concentrations but require refrigeration after opening and have shorter shelf lives than capsules.

Important drug interactions warrant careful consideration. Fish oil possesses mild antiplatelet effects and may theoretically increase bleeding risk when combined with anticoagulants (warfarin, DOACs), antiplatelet agents (aspirin, clopidogrel), or NSAIDs. Whilst clinically significant bleeding is uncommon, patients taking these medications should consult their GP or anticoagulation clinic before starting fish oil. Additionally, DHA-containing fish oils may slightly elevate LDL cholesterol in some individuals and could theoretically affect blood glucose control in diabetes, necessitating monitoring.

Patients should inform all healthcare providers about fish oil supplementation, particularly before surgical procedures. Rather than routinely stopping supplements, follow the specific advice of your surgical or anaesthetics team, as discontinuation is not always necessary at standard doses.

Potential Side Effects and Risks of Fish Oil

Whilst fish oil is generally well-tolerated, users should be aware of potential adverse effects and circumstances requiring medical attention.

Common gastrointestinal effects represent the most frequent complaints, affecting up to 20% of users. These include fishy aftertaste, belching, heartburn, nausea, loose stools, and abdominal discomfort. Such symptoms are typically mild and often resolve with dose reduction, taking supplements with meals, or switching to enteric-coated preparations. Freezing capsules before consumption may reduce fishy reflux.

Bleeding risk remains a theoretical concern due to fish oil's antiplatelet properties, though clinically significant bleeding is rare at standard supplemental doses. However, individuals taking anticoagulants, those with bleeding disorders, or patients scheduled for surgery should exercise caution and discuss with their healthcare provider.

Allergic reactions can occur in individuals with fish or shellfish allergies, though fish oil allergy is distinct from fish protein allergy and does not always coincide. Symptoms may include rash, itching, swelling, or respiratory difficulties. Anyone experiencing signs of anaphylaxis (difficulty breathing, facial swelling, rapid pulse) should call 999 or go to A&E immediately.

Metabolic effects at very high doses may include modest increases in LDL cholesterol (particularly with DHA-containing products) and potential effects on glycaemic control in diabetes, though evidence is inconsistent. Patients with diabetes taking high-dose fish oil should monitor blood glucose levels.

Atrial fibrillation risk may be increased with high-dose prescription omega-3 products. If you experience palpitations or irregular heartbeat while taking fish oil, seek medical advice.

Contaminant exposure from poor-quality supplements may include heavy metals (mercury, lead), PCBs, or dioxins. Selecting reputable, tested products minimises this risk.

When to contact your GP: Seek medical advice if you experience persistent gastrointestinal symptoms, unusual bleeding or bruising, allergic reactions, or if you are taking anticoagulants and considering fish oil supplementation. Pregnant women, those with chronic health conditions, or individuals taking multiple medications should discuss supplementation with their healthcare provider before commencing fish oil.

If you suspect you've experienced a side effect from fish oil, you can report it through the MHRA Yellow Card Scheme.

Frequently Asked Questions