The recommended dose of fish oil depends on whether it is used for general health or specific medical conditions. For adults maintaining general wellbeing, 250–500mg of combined EPA and DHA daily is typical, often achievable through diet alone by consuming oily fish twice weekly. Higher prescription doses of 2–4g daily may be indicated for conditions such as hypertriglyceridaemia, but these require medical supervision. Fish oil supplements contain omega-3 fatty acids—primarily EPA and DHA—which support cell membrane function and help modulate inflammation. Understanding appropriate dosing is essential for safe and effective use.

What Is Fish Oil and Why Is It Used?

Fish oil is a dietary supplement derived from the tissues of oily fish such as mackerel, herring, sardines, and salmon. It is rich in long-chain omega-3 polyunsaturated fatty acids, primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These omega-3s are derived from alpha-linolenic acid (ALA), which is the essential fatty acid that cannot be synthesised by the human body and must be obtained through diet, with limited conversion to EPA and DHA.

Omega-3 fatty acids play roles in maintaining cell membrane structure, supporting brain function, and modulating inflammatory responses throughout the body. EPA and DHA are incorporated into cell membranes, where they influence membrane fluidity and cellular signalling pathways. They also serve as precursors for bioactive lipid mediators called resolvins and protectins, which help resolve inflammation.

Common reasons for fish oil supplementation include:

• Managing elevated triglyceride levels under medical supervision

• Supporting joint health in conditions such as rheumatoid arthritis (as an adjunct to conventional treatments)

• Supplementing dietary intake when fish consumption is inadequate

It's important to note that the National Institute for Health and Care Excellence (NICE) advises against routine use of omega-3 supplements for cardiovascular disease prevention.

Whilst fish oil supplements are widely available over the counter in the UK, it is important to recognise that they are not regulated as medicines unless prescribed for specific medical conditions. The Medicines and Healthcare products Regulatory Agency (MHRA) oversees prescription omega-3 preparations, whilst food supplements fall under different regulatory frameworks. Prescription icosapent ethyl (an EPA-only medicine) has specific indications and is not interchangeable with over-the-counter fish oil supplements. Individuals should discuss their specific circumstances with a healthcare professional before commencing supplementation, particularly if they have existing medical conditions or take other medications.

Recommended Dose of Fish Oil for Adults in the UK

The recommended dose of fish oil varies significantly depending on whether it is being used for general health maintenance or for specific medical conditions. There is no single universally agreed dosage, and recommendations differ between organisations and clinical contexts.

For general health and dietary supplementation, the NHS suggests that most people can obtain sufficient omega-3 fatty acids by consuming at least two portions of fish per week, including one portion of oily fish (approximately 140g). The European Food Safety Authority (EFSA) considers an adequate daily intake of combined EPA and DHA to be around 250mg for adults. For those who do not consume fish regularly, standard over-the-counter fish oil supplements commonly contain between 250mg and 1,000mg of combined EPA and DHA per capsule.

EFSA considers supplemental intakes of combined EPA and DHA up to 5g daily to be safe for adults. However, most general health supplements contain considerably less than this upper limit. A typical maintenance dose for adults ranges from 250mg to 500mg of combined EPA and DHA daily.

For specific medical indications, higher doses may be recommended:

• Hypertriglyceridaemia: Prescription omega-3-acid ethyl esters typically contain 2–4g of EPA and DHA daily, as per the Summary of Product Characteristics (SmPC), for use under medical supervision

• Cardiovascular disease in specific high-risk groups: NICE Technology Appraisal (TA805) recommends icosapent ethyl (a prescription EPA-only medicine) at a dose of 2g twice daily for certain adults with raised triglycerides despite statin therapy. Standard fish oil supplements are not recommended by NICE for routine cardiovascular disease prevention

• Rheumatoid arthritis: Doses of 2.6g or more of EPA and DHA daily have been investigated for modest anti-inflammatory effects as an adjunct to disease-modifying treatments

It is essential to check the EPA and DHA content on supplement labels, as the total fish oil content may be higher than the active omega-3 fatty acid content. Individuals considering doses above 1g daily should consult their GP or a registered dietitian, particularly if they have existing health conditions or take anticoagulant medications.

How to Take Fish Oil Safely and Effectively

To maximise the benefits and minimise potential adverse effects of fish oil supplementation, several practical considerations should be observed.

Timing and administration:

Fish oil supplements are fat-soluble and are generally better absorbed when taken with meals containing dietary fat. Taking capsules with food also helps reduce common gastrointestinal side effects such as fishy aftertaste, reflux, and nausea. Dividing higher doses throughout the day (for example, taking capsules with breakfast and dinner) may improve tolerability compared to taking the entire dose at once.

Quality and storage considerations:

• Choose supplements from reputable manufacturers that provide third-party testing certificates to ensure purity and accurate labelling

• Look for products that specify EPA and DHA content clearly on the label

• Store fish oil supplements in a cool, dark place to prevent oxidation, which can reduce efficacy and create rancid odours

• Check expiry dates regularly and discard any capsules that smell unpleasant or appear discoloured

Important safety precautions:

Anticoagulant interactions: Fish oil has mild antiplatelet effects and may theoretically increase bleeding risk when combined with anticoagulant medications such as warfarin, apixaban, or antiplatelet agents like clopidogrel. If you take warfarin, your GP may arrange additional INR monitoring when starting or changing omega-3 doses. Whilst clinically significant bleeding is uncommon with standard doses, individuals taking these medications should inform their GP before starting fish oil supplements.

Surgical procedures: Follow your surgical team's specific instructions regarding supplements before surgery. Some healthcare professionals recommend discontinuing fish oil supplements 1–2 weeks before planned surgery, particularly with higher doses or concurrent anticoagulant use.

Heart rhythm concerns: High-dose prescription omega-3 preparations have been associated with increased risk of atrial fibrillation (irregular heartbeat). Those with existing heart rhythm disorders should seek medical advice before taking high-dose supplements.

Allergies: Individuals with fish or shellfish allergies should exercise caution. Whilst highly purified fish oil supplements typically contain minimal allergenic protein, allergic reactions remain possible. Alternative omega-3 sources such as algal oil supplements may be more appropriate for those with fish allergies.

Pregnant and breastfeeding women should consult their midwife or GP before taking fish oil supplements. Specifically, cod liver oil should be avoided during pregnancy due to its vitamin A content. Pregnant women should also follow NHS guidance on limiting oily fish and tuna consumption.

Potential Side Effects and When to Seek Medical Advice

Fish oil supplements are generally well tolerated by most individuals, but several side effects may occur, particularly at higher doses.

Common side effects include:

• Gastrointestinal disturbances: Fishy aftertaste, belching, heartburn, nausea, and loose stools are the most frequently reported adverse effects. These symptoms are usually mild and often improve with dose reduction or by taking capsules with meals. Enteric-coated formulations may reduce upper gastrointestinal symptoms.

• Fishy body odour or breath: Some individuals notice a fishy smell in their breath or sweat, which can be socially bothersome though medically insignificant.

• Mild increases in LDL cholesterol: High-dose fish oil supplementation (particularly above 3g daily) may modestly increase low-density lipoprotein (LDL) cholesterol levels in some individuals, though this effect is inconsistent and typically small.

Less common but more significant concerns:

• Bleeding complications: Whilst rare, excessive fish oil intake combined with anticoagulant medications may increase bleeding risk. Signs include unusual bruising, prolonged bleeding from minor cuts, blood in urine or stools, or unexplained nosebleeds.

• Heart rhythm disturbances: High-dose prescription omega-3 preparations have been associated with increased risk of atrial fibrillation. Seek medical advice if you experience palpitations or irregular heartbeat.

• Allergic reactions: Though uncommon with purified supplements, symptoms may include rash, itching, swelling, or difficulty breathing.

When to contact your GP:

You should seek medical advice if you experience:

• Persistent or severe gastrointestinal symptoms that do not improve with dose adjustment

• Signs of bleeding or unusual bruising, particularly if taking anticoagulant medications

• Symptoms suggestive of an allergic reaction

• Palpitations or irregular heartbeat

• If you have diabetes and notice any changes in blood glucose control (although clinically meaningful effects are uncommon with standard doses)

Seek urgent medical attention if you develop signs of a severe allergic reaction, including facial swelling, difficulty breathing, or widespread rash.

Before starting fish oil supplementation, inform your GP about all medications and supplements you currently take. This is particularly important if you have diabetes, liver disease, pancreatic disorders, or take medications affecting blood clotting. Regular monitoring may be appropriate for individuals taking prescription-strength omega-3 preparations for medical conditions.

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

Frequently Asked Questions