Cod liver oil is a nutritional supplement extracted from the livers of cod fish, valued for its unique combination of omega-3 fatty acids and fat-soluble vitamins A and D. Used for centuries in Northern Europe to prevent rickets and support health during winter months, it remains a popular supplement in the UK today. Unlike standard fish oil, cod liver oil provides substantial amounts of vitamins A and D alongside EPA and DHA omega-3 fatty acids. Available as liquid or capsules, it is regulated as a food supplement under UK food safety legislation. Understanding its composition, benefits, and safety considerations is essential for informed use.

What Is Cod Liver Oil and Where Does It Come From?

Cod liver oil is a nutritional supplement derived from the livers of cod fish, primarily Atlantic cod (Gadus morhua) and Pacific cod (Gadus macrocephalus). This pale yellow oil has been used for centuries as a traditional remedy, particularly in Northern European and Scandinavian countries, where it was historically administered to prevent rickets and support general health during long winters with limited sunlight exposure.

The extraction process involves steaming or cold-pressing cod livers to release the oil, which is then refined and purified to remove contaminants such as heavy metals, polychlorinated biphenyls (PCBs), and other environmental pollutants that may accumulate in fish tissues. In the UK, cod liver oil is regulated as a food supplement under food law, with oversight from the Food Standards Agency (FSA) and local authorities to ensure products comply with food safety legislation.

Cod liver oil is distinct from standard fish oil supplements. Whilst both contain omega-3 fatty acids, cod liver oil is uniquely rich in fat-soluble vitamins A and D, which are naturally concentrated in fish liver tissue. This combination of omega-3 polyunsaturated fatty acids and vitamins distinguishes cod liver oil from other marine-derived supplements and accounts for its specific nutritional profile.

The supplement is available in various formulations including liquid oil, soft gel capsules, and flavoured preparations designed to improve palatability. In the UK, cod liver oil is classified as a food supplement rather than a medicine, with any health claims subject to the Great Britain Nutrition and Health Claims Register.

Nutritional Content and Active Ingredients in Cod Liver Oil

The primary bioactive components in cod liver oil are omega-3 polyunsaturated fatty acids, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These long-chain fatty acids play essential roles in cellular membrane structure, inflammatory modulation, and cardiovascular function. The EPA and DHA content varies considerably between products, so consumers should check product labels for the specific amounts in their chosen supplement.

Vitamin A (retinol) is present in substantial quantities in cod liver oil, with amounts varying significantly between products. This fat-soluble vitamin is crucial for vision, immune function, cellular differentiation, and epithelial tissue maintenance. Vitamin A exists in cod liver oil as preformed retinol, which is more readily bioavailable than plant-derived carotenoids but also carries a greater risk of toxicity at excessive doses. The UK safe upper level for vitamin A from supplements is 1,500 micrograms per day for adults.

Vitamin D (cholecalciferol or vitamin D3) is the third major constituent, with concentrations typically ranging from 5–25 micrograms (200–1,000 IU) per serving. Vitamin D is essential for calcium homeostasis, bone mineralisation, immune regulation, and numerous other physiological processes. Given the high prevalence of vitamin D insufficiency in the UK population, particularly during autumn and winter months, this component is of particular clinical relevance.

Cod liver oil also contains smaller amounts of vitamin E (tocopherols), which acts as an antioxidant to prevent rancidity of the polyunsaturated fatty acids. The exact nutritional composition varies depending on the fish species, geographical origin, processing methods, and whether the product has been fortified with additional vitamins. Consumers should carefully review product labels to understand the specific nutrient content of their chosen supplement.

Health Benefits and Clinical Evidence

The health benefits attributed to cod liver oil primarily relate to its constituent nutrients rather than unique properties of the oil itself. Cardiovascular health has been extensively studied in relation to omega-3 fatty acids. EPA and DHA contribute to the maintenance of normal blood triglyceride levels, blood pressure, and cardiac function. The Great Britain Nutrition and Health Claims Register permits the claim that EPA and DHA contribute to normal heart function at intakes of 250mg daily. However, large-scale randomised controlled trials have produced mixed results regarding cardiovascular event prevention, and NICE guidance (NG238) does not routinely recommend omega-3 supplementation for primary or secondary prevention of cardiovascular disease in the general population. It's worth noting that NICE has recommended a specific prescription-only omega-3 product (icosapent ethyl) for certain high-risk patients, but this is distinct from cod liver oil supplements.

Bone and musculoskeletal health benefits are primarily mediated through vitamin D content. Adequate vitamin D status is essential for calcium absorption and bone mineralisation. The UK Government recommends that all adults consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D, particularly during autumn and winter. Cod liver oil can contribute to meeting this requirement, though individuals should ensure they do not exceed safe upper limits when combining multiple vitamin D sources.

Immune function and inflammatory conditions have been areas of research interest. Vitamin A plays a critical role in maintaining mucosal barriers and immune cell function, whilst omega-3 fatty acids possess anti-inflammatory properties through their conversion to specialised pro-resolving mediators. Some observational studies have suggested potential benefits in rheumatoid arthritis, though evidence remains insufficient for formal clinical recommendations. There is no official link established between cod liver oil supplementation and prevention of common infections, despite historical use for this purpose.

Cognitive function and mental health have been investigated, given the importance of DHA in brain structure and function. Whilst adequate omega-3 intake is important for neurodevelopment and may support cognitive health, systematic reviews have not demonstrated consistent benefits of supplementation in preventing cognitive decline or treating depression in the general population. The evidence base continues to evolve, and further high-quality trials are needed.

Recommended Dosage and How to Take Cod Liver Oil

When taking cod liver oil, it is essential to follow the specific dosage instructions provided by the manufacturer on the product label. Dosages vary significantly between products depending on their concentration of omega-3 fatty acids, vitamin A, and vitamin D. It is particularly important to consider the vitamin A and D content when determining appropriate dosage, as these fat-soluble vitamins can accumulate in body tissues.

The safe upper limit for vitamin A in adults is 1,500 micrograms daily according to UK guidance. Chronic excessive intake can lead to hypervitaminosis A, with potential hepatotoxicity and teratogenic effects. Pregnant women or those planning pregnancy should avoid cod liver oil supplements entirely, as advised by the NHS, due to the risk of excessive vitamin A intake during pregnancy being associated with congenital malformations. Instead, they should choose pregnancy-specific supplements containing appropriate nutrient levels.

For vitamin D, the safe upper limit is 100 micrograms (4,000 IU) daily for adults. Individuals taking multiple supplements or fortified foods should calculate their total vitamin D intake to avoid exceeding this threshold. Those with conditions affecting fat absorption (such as coeliac disease, Crohn's disease, or chronic pancreatitis) should not self-supplement with higher doses of cod liver oil but should seek medical advice for appropriate vitamin D supplementation under healthcare supervision.

Administration advice includes taking cod liver oil with food to enhance absorption of fat-soluble vitamins and reduce potential gastrointestinal discomfort. Liquid formulations should be measured accurately using the provided dosing device. The supplement should be stored in a cool, dark place to prevent oxidation, and refrigeration after opening is often recommended. Some individuals find that taking cod liver oil in the evening or freezing capsules before consumption reduces fishy aftertaste or reflux. Children's dosages should be specifically formulated for paediatric use, with lower vitamin concentrations appropriate for body weight and developmental stage. Parents should be particularly cautious about vitamin A content in children's formulations.

Potential Side Effects and Safety Considerations

Cod liver oil is generally well tolerated when taken at recommended doses, but several common adverse effects may occur. Gastrointestinal symptoms including nausea, loose stools, dyspepsia, and fishy aftertaste or reflux are the most frequently reported complaints. These effects are typically mild and may be minimised by taking the supplement with meals, choosing enteric-coated capsules, or refrigerating the product. Some individuals experience halitosis (bad breath) with a fishy odour.

Bleeding risk is a theoretical concern with omega-3 fatty acid supplementation due to their antiplatelet effects and potential to prolong bleeding time. Whilst clinically significant bleeding is rare at standard supplementation doses, individuals taking anticoagulant medications (such as warfarin, apixaban, or rivaroxaban) or antiplatelet agents (including aspirin or clopidogrel) should consult their GP or pharmacist before commencing cod liver oil. If you are scheduled for surgery, discuss your cod liver oil use with your surgical team during pre-operative assessment rather than stopping it without medical advice.

Hypervitaminosis represents a significant safety consideration with regular cod liver oil use. Chronic excessive vitamin A intake can cause hepatotoxicity, bone demineralisation, hair loss, dry skin, and central nervous system effects including headache and dizziness. Vitamin D toxicity, though less common, can lead to hypercalcaemia with symptoms including nausea, weakness, confusion, and renal impairment. Individuals taking other supplements containing vitamins A or D must carefully monitor their total intake.

Drug interactions may occur beyond anticoagulants. Vitamin A can interact with retinoid medications used for acne or skin conditions, potentially increasing toxicity risk. Vitamin D absorption may be reduced by certain medications including orlistat and bile acid sequestrants. Contraindications include known allergy to fish. Those with shellfish allergy should check individual products for potential cross-contamination.

When to seek medical advice: Individuals should contact their GP if they experience persistent gastrointestinal symptoms, unusual bleeding or bruising, severe headaches, visual changes, bone pain, or signs of hypercalcaemia. Pregnant women, those planning pregnancy, individuals with liver disease, and anyone taking multiple medications should discuss cod liver oil supplementation with a healthcare professional before commencing use. Quality assurance is important—consumers should select products from reputable manufacturers that provide third-party testing certificates confirming purity and absence of contaminants.

If you suspect you have experienced a side effect from cod liver oil, you can report this through the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Frequently Asked Questions