Fish oil supplements, derived from oily fish such as mackerel, salmon, and sardines, are amongst the most widely used dietary supplements in the UK. Valued primarily for their omega-3 polyunsaturated fatty acids—eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—fish oil has been studied for various health applications, particularly cardiovascular and inflammatory conditions. Whilst evidence supports specific therapeutic uses such as triglyceride reduction under medical supervision, guidance from NICE and the NHS emphasises that routine supplementation for cardiovascular disease prevention is not recommended. Understanding what fish oil is good for requires careful consideration of the evidence, appropriate dosing, and individual health circumstances.

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 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). EPA typically comprises 18–30% of fish oil content, whilst DHA accounts for 10–25%, though concentrations vary depending on the fish source and processing methods. Both EPA and DHA play crucial roles in cellular function, particularly within cell membranes, where they influence fluidity, signalling pathways, and inflammatory responses.

Fish oil supplements are available in several formulations, including:

• Natural triglyceride form – the form naturally present in fish

• Ethyl ester form – a concentrated, processed version

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

• Prescription omega-3 preparations – highly purified formulations available on prescription for specific medical indications

It's important to note that over-the-counter fish oil supplements are regulated as food supplements in the UK, not as medicines. The quality and purity of fish oil products can vary considerably. Reputable manufacturers test for contaminants such as heavy metals (mercury, lead), polychlorinated biphenyls (PCBs), and dioxins. When selecting a fish oil supplement, look for products from reputable brands that display third-party testing certification to ensure safety and stated omega-3 content.

Important note for pregnant women: Fish liver oil (including cod liver oil) contains vitamin A (retinol) and should be avoided during pregnancy due to potential risks to the developing baby.

Health Benefits of Fish Oil: What the Evidence Shows

The cardiovascular benefits of omega-3 fatty acids have been extensively studied, though the evidence base has evolved considerably in recent years. EPA and DHA exert several physiological effects that may contribute to cardiovascular health, including reducing triglyceride levels, modestly lowering blood pressure, decreasing platelet aggregation, and possessing anti-inflammatory properties. These mechanisms form the biological rationale for fish oil's potential therapeutic applications.

Large-scale epidemiological studies have consistently demonstrated that populations consuming diets rich in oily fish have lower rates of cardiovascular disease. However, randomised controlled trials of fish oil supplementation have produced more mixed results. Current NICE guidance (NG238) advises not to offer omega-3 fatty acid supplements for the prevention of cardiovascular disease events in primary or secondary prevention. While omega-3 fatty acids can significantly reduce triglyceride concentrations (by 20–30% at therapeutic doses), this effect is primarily seen with prescription omega-3 medicines at doses of 2-4g daily.

The REDUCE-IT trial showed that high-dose prescription icosapent ethyl (a purified EPA formulation) reduced cardiovascular events in specific high-risk patients with elevated triglycerides despite statin therapy. However, this applies only to the prescription medicine (Vazkepa), not standard over-the-counter fish oil supplements. It's also important to note that icosapent ethyl was associated with increased risks of atrial fibrillation and bleeding in clinical trials.

Anti-inflammatory effects represent another important mechanism. Omega-3 fatty acids are metabolised into specialised pro-resolving mediators (resolvins and protectins) that help regulate inflammatory responses. This has implications for various inflammatory conditions, though clinical translation varies by condition.

For cognitive function and mental health, DHA is particularly important as it comprises approximately 40% of polyunsaturated fatty acids in the brain. Observational studies suggest associations between omega-3 intake and reduced cognitive decline, though intervention trials have shown inconsistent results. Some evidence supports a modest benefit in mild depression, but fish oil should not replace established treatments for mental health conditions as recommended in NICE guidance.

Conditions Fish Oil May Help With

Hypertriglyceridaemia is the condition with the strongest evidence base for fish oil use. High-dose omega-3 preparations (2–4 grams of EPA/DHA daily) can effectively lower triglyceride levels. For this medical purpose, licensed prescription omega-3 medicines should be used under clinical supervision, not over-the-counter supplements. If triglyceride levels exceed 10 mmol/L on a repeat fasting sample, urgent specialist referral is needed due to the risk of pancreatitis. Specialist assessment should also be considered for levels persistently above 5 mmol/L with other risk factors. Prescription omega-3 preparations are sometimes used alongside statins in patients with persistently elevated triglycerides despite optimal statin therapy.

Rheumatoid arthritis has moderate evidence supporting omega-3 supplementation as an adjunctive therapy. Several systematic reviews indicate that fish oil (typically 2.6–3 grams EPA/DHA daily) may reduce joint pain and morning stiffness, potentially allowing some patients to reduce non-steroidal anti-inflammatory drug (NSAID) use. However, fish oil should complement, not replace, disease-modifying antirheumatic drugs (DMARDs) prescribed by rheumatologists.

For pregnancy and infant development, adequate DHA intake is important for foetal brain and eye development. The NHS advises pregnant women to consume up to two portions of oily fish weekly (avoiding high-mercury species like shark, swordfish and marlin, and limiting tuna to two steaks or four medium-sized cans per week) or consider supplementation if dietary intake is insufficient. Pregnant women should avoid fish liver oil supplements due to their vitamin A (retinol) content. Some evidence suggests omega-3 supplementation during pregnancy may reduce the risk of early preterm birth, though routine supplementation is not universally recommended by NICE.

Other conditions where fish oil has been investigated include:

• Age-related macular degeneration – observational evidence suggests benefit, but supplementation trials show limited effect once the condition is established

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

• Asthma – insufficient evidence to support routine use

• Attention deficit hyperactivity disorder (ADHD) – NICE guidance (NG87) advises not to offer omega-3 supplements for ADHD treatment

It is important to note that for many conditions, there is no official link established between fish oil supplementation and clinical benefit, and patients should discuss potential use with their GP or specialist.

How to Take Fish Oil Safely and Effectively

Dosage recommendations vary depending on the intended purpose. For general health maintenance, consuming two portions (approximately 280 grams) of fish weekly, including one portion of oily fish, provides adequate omega-3 intake for most adults according to NHS guidance. This equates to roughly 450–500 mg of EPA and DHA daily. For those unable or unwilling to consume fish, supplements typically provide 250–500 mg of combined EPA/DHA per capsule.

For therapeutic purposes, higher doses are often required:

• Triglyceride reduction: 2–4 grams EPA/DHA daily (prescription omega-3 medicines only, under clinical supervision)

• Rheumatoid arthritis: 2.6–3 grams EPA/DHA daily

It's important to note that NICE guidance does not recommend routine omega-3 supplementation for cardiovascular disease prevention.

When selecting a supplement, check the actual EPA and DHA content rather than total fish oil content, as a 1000 mg fish oil capsule may contain only 300 mg of omega-3 fatty acids. Products should clearly state EPA and DHA quantities on the label.

Timing and absorption: Fish oil is fat-soluble and best absorbed when taken with meals containing some dietary fat. Dividing doses throughout the day may improve tolerability and reduce gastrointestinal side effects. Enteric-coated formulations can minimise fishy aftertaste and reflux.

Quality considerations are important. Choose supplements that:

• Display independent third-party testing certification

• Specify the fish source and extraction method

• Confirm testing for contaminants (heavy metals, PCBs)

• Provide clear expiration dates (omega-3s oxidise over time)

Patients taking fish oil for specific medical conditions should do so under healthcare professional guidance, particularly when doses exceed 3 grams daily. Regular monitoring of lipid profiles may be appropriate, as DHA-containing products can raise LDL cholesterol in some patients. For patients taking warfarin, INR monitoring may be advisable when starting or changing omega-3 supplement doses.

Potential Side Effects and Who Should Avoid Fish Oil

Fish oil is generally well tolerated at recommended doses, but several common side effects may occur, particularly at higher doses:

• Gastrointestinal symptoms: fishy aftertaste, reflux, nausea, loose stools, and abdominal discomfort are the most frequently reported adverse effects. These can often be minimised by taking supplements with meals, using enteric-coated formulations, or refrigerating capsules.

• Fishy breath or body odour: some individuals notice a characteristic odour, which may be reduced by choosing higher-quality, fresher products.

• Mild bleeding tendency: omega-3 fatty acids have antiplatelet effects, which may prolong bleeding time, though clinically significant bleeding is rare at typical supplemental doses.

Prescription icosapent ethyl has been associated with an increased risk of atrial fibrillation/flutter and bleeding in clinical trials. Seek medical advice if you experience palpitations or other symptoms of arrhythmia.

Drug interactions require careful consideration. Fish oil may enhance the effects of anticoagulant and antiplatelet medications, including warfarin, direct oral anticoagulants (DOACs), aspirin, and clopidogrel. Whilst the clinical significance remains debated, patients taking these medications should consult their GP or anticoagulation clinic before starting fish oil supplements, particularly at doses exceeding 3 grams daily. International normalised ratio (INR) monitoring may be advisable for warfarin users.

Specific populations requiring caution include:

• Individuals with fish or seafood allergies: whilst fish oil is derived from fish tissue rather than protein, allergic reactions remain possible. Algae-based omega-3 supplements provide an alternative source of DHA.

• Patients with bleeding disorders: the antiplatelet effects may pose increased risk.

• Those scheduled for surgery: follow your surgical team's instructions regarding supplement use before procedures.

• Individuals with diabetes: omega-3 supplements have minimal to no clinically meaningful effect on glycaemic control; continue usual monitoring.

When to contact your GP: Seek medical advice if you experience persistent gastrointestinal symptoms, unusual bleeding or bruising, allergic reactions (rash, swelling, difficulty breathing), or if you are considering fish oil alongside prescribed medications. Patients should inform healthcare professionals about all supplements during consultations, as these may influence treatment decisions and monitoring requirements.

Report any suspected side effects to the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk).

Frequently Asked Questions