Fish oil benefits and side effects are important considerations for the millions of UK adults who take this popular dietary supplement. Derived from oily fish such as salmon and mackerel, fish oil is valued primarily for its omega-3 polyunsaturated fatty acids—eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These fatty acids support cardiovascular health, reduce inflammation, and contribute to brain function. Whilst generally well tolerated, fish oil can cause gastrointestinal disturbances and may interact with anticoagulant medications. The NHS recommends obtaining omega-3s primarily through dietary sources rather than supplements for general health maintenance.

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 commonly consumed supplements in the UK, valued primarily for its high content of omega-3 polyunsaturated fatty acids—specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). While alpha-linolenic acid (ALA) is the only truly essential omega-3 fatty acid, EPA and DHA are important long-chain omega-3s that the body can only produce in limited amounts from ALA.

The therapeutic effects of fish oil are attributed to the biological actions of EPA and DHA. These omega-3 fatty acids are incorporated into cell membranes throughout the body, where they influence membrane fluidity and cellular signalling. EPA and DHA serve as precursors to bioactive lipid mediators, including resolvins, protectins, and maresins, which play crucial roles in modulating inflammatory responses. Omega-3 fatty acids generally exert anti-inflammatory effects, helping to resolve chronic low-grade inflammation associated with various health conditions, while omega-6 fatty acids can have varying effects depending on the specific fatty acid and physiological context.

Additionally, omega-3 fatty acids contribute to cardiovascular health through multiple mechanisms. They help reduce triglyceride levels, lower blood pressure modestly, decrease platelet aggregation, and may improve endothelial function. DHA is particularly concentrated in the brain and retina, where it supports neuronal membrane integrity and visual function. The body can convert ALA from plant sources into EPA and DHA, but this conversion is inefficient—typically around 5-10% for EPA and often less than 1% for DHA—which is why direct consumption of marine-derived omega-3s is often recommended for those seeking their specific benefits.

Health Benefits of Fish Oil Supplements

Fish oil supplementation has been studied extensively for its potential health benefits, with varying levels of evidence supporting different clinical applications. Cardiovascular health remains the most researched area. High-dose prescription omega-3 preparations (omega-3-acid ethyl esters) are licensed medicines for hypertriglyceridaemia, reducing triglyceride levels by 20–30%. It's important to note that over-the-counter fish oil supplements are not equivalent to these prescription products. NICE guidance (NG238) does not recommend standard fish oil supplements for cardiovascular disease prevention in the general population, though a specific high-purity icosapent ethyl product (Vazkepa) has been approved by NICE for certain high-risk patients with raised triglycerides.

Mental health and cognitive function represent another area of interest. DHA is important for brain structure and function, but evidence that omega-3 supplementation prevents cognitive decline or dementia is inconclusive. Some research suggests EPA-rich formulations may have modest adjunctive benefits in managing depression, though this should not replace conventional treatment and is not currently recommended by NICE for depression management.

There is some evidence for anti-inflammatory effects that may help manage rheumatoid arthritis symptoms, with some patients reporting reduced joint pain and stiffness, though omega-3s should not replace disease-modifying treatments. Regarding eye health, despite DHA's presence in retinal tissue, large clinical trials (including AREDS2 for age-related macular degeneration and DREAM for dry eye) have not demonstrated significant benefits of supplementation. During pregnancy, adequate omega-3 intake supports foetal brain and eye development, and evidence suggests it may reduce the risk of preterm birth, though dietary sources are preferred.

The NHS and NICE emphasise obtaining omega-3s primarily through dietary sources—consuming at least two portions of fish weekly, including one oily fish portion—rather than relying on supplements for general health maintenance.

Potential Side Effects and Risks of Fish Oil

Fish oil supplements are generally well tolerated by most individuals, but they can cause side effects, particularly at higher doses. The most commonly reported adverse effects are gastrointestinal disturbances, including fishy aftertaste, belching, nausea, loose stools, and indigestion. These symptoms can often be minimised by taking supplements with meals, choosing enteric-coated formulations, or refrigerating capsules to reduce fishy odour.

Bleeding risk is a theoretical concern due to omega-3 fatty acids' antiplatelet effects, which reduce blood clotting. The NHS advises that taking up to 3g daily of omega-3 supplements is unlikely to cause harm in most people. However, caution is warranted in individuals taking anticoagulant medications such as warfarin or direct oral anticoagulants (DOACs), or antiplatelet drugs like aspirin or clopidogrel. Patients on these medications should consult their GP or anticoagulation clinic before starting supplementation, and those on warfarin should have their INR monitored appropriately.

Other potential side effects include a slight increase in LDL cholesterol in some individuals, though this is typically offset by triglyceride reduction. Contamination concerns have historically included environmental pollutants (PCBs, dioxins), though reputable manufacturers employ purification processes to minimise these risks and should comply with UK and EU contaminant limits.

Patients should contact their GP if they experience persistent gastrointestinal symptoms, unusual bruising or bleeding, allergic reactions (rash, swelling, difficulty breathing), or any concerning symptoms after starting fish oil supplements. Any suspected adverse reactions can be reported through the MHRA Yellow Card scheme. It is essential to purchase supplements from reputable sources that provide third-party testing certificates to ensure purity and quality.

Who Should Avoid Fish Oil Supplements?

Whilst fish oil is safe for most people, certain groups should exercise caution or seek medical advice before supplementation. Individuals with fish or seafood allergies should consult their GP or allergy specialist before using fish oil supplements, as some products may contain trace fish proteins that could trigger allergic reactions. Alternative omega-3 sources, such as algae-based supplements (which provide DHA and EPA without fish proteins), may be suitable alternatives for this group.

People taking anticoagulant or antiplatelet medications should consult their GP or anticoagulation clinic before using fish oil supplements. This includes those prescribed warfarin, direct oral anticoagulants (DOACs such as apixaban or rivaroxaban), aspirin, clopidogrel, or other blood-thinning agents. The theoretical risk of increased bleeding warrants medical oversight, particularly if combining multiple agents that affect coagulation.

Individuals scheduled for surgery should follow their surgical team's advice regarding fish oil supplementation; many clinicians recommend discontinuing supplements at least one week before the procedure due to theoretical bleeding risk concerns. Patients should inform their surgical team about all supplements they are taking. Those with bleeding disorders such as haemophilia or von Willebrand disease should avoid fish oil unless specifically recommended by their haematologist.

People with diabetes can generally take fish oil supplements, as most studies show little to no clinically meaningful effect on glycaemic control, though monitoring is advisable when starting high doses. Individuals with liver disease should seek medical advice, as high-dose omega-3s are metabolised hepatically. Pregnant and breastfeeding women should avoid fish liver oil supplements (which contain vitamin A) and follow NHS guidance limiting oily fish consumption to two portions weekly and tuna to two tuna steaks or four medium-sized cans weekly due to potential pollutant exposure. Any omega-3 supplementation during pregnancy should be discussed with a healthcare professional.

Safe Dosage and How to Take Fish Oil

The appropriate fish oil dosage varies depending on the intended purpose and individual health status. For general health maintenance, the NHS recommends obtaining omega-3 fatty acids primarily through diet—consuming at least two portions (approximately 140g each) of fish weekly, with one being an oily fish such as salmon, mackerel, or sardines. The European Food Safety Authority (EFSA) suggests a minimum of 250mg of combined EPA and DHA daily for adults, which can typically be achieved through a balanced diet.

For those using supplements, standard supplemental doses range from 250mg to 1,000mg of combined EPA and DHA daily. Products vary considerably in their omega-3 content; a typical 1,000mg fish oil capsule may contain only 300mg of actual omega-3 fatty acids (EPA plus DHA), so it is essential to check the label for the specific omega-3 content rather than total fish oil weight. The NHS advises that taking up to 3g of omega-3 supplements daily is unlikely to cause harm, but higher doses should only be taken under medical supervision.

Higher therapeutic doses for specific conditions such as hypertriglyceridaemia are available as prescription-only medicines (omega-3-acid ethyl esters or icosapent ethyl) and should only be used under medical supervision. These prescription products are not interchangeable with over-the-counter supplements.

To optimise absorption and minimise side effects, take fish oil supplements with meals, preferably those containing some dietary fat, as omega-3s are fat-soluble. This also helps reduce gastrointestinal discomfort and fishy aftertaste. Dividing the daily dose between meals can further improve tolerability. Store supplements in a cool, dark place or refrigerate them to prevent oxidation and maintain freshness.

Quality considerations are paramount. Choose supplements that have been third-party tested for purity, potency, and contaminant levels, and comply with UK and EU standards for environmental contaminants. Check expiration dates and avoid products with a strong fishy odour, which may indicate oxidation. Patients should inform their GP about all supplements they take, as omega-3s can interact with certain medications. If you are considering fish oil for a specific health condition, consult your healthcare provider to determine the appropriate dose and ensure it is suitable for your individual circumstances.

Frequently Asked Questions