Cod liver oil and fish oil are both popular omega-3 supplements, yet they differ significantly in their nutritional composition and clinical applications. Whilst fish oil is extracted from the tissues of oily fish and provides concentrated EPA and DHA, cod liver oil comes specifically from cod livers and uniquely contains substantial amounts of vitamins A and D alongside omega-3 fatty acids. Understanding these key differences is essential for selecting the most appropriate supplement for your individual health needs, particularly given the specific safety considerations surrounding vitamin A intake during pregnancy and the varying omega-3 concentrations between products.

What Are Cod Liver Oil and Fish Oil?

Cod liver oil and fish oil are both marine-derived supplements rich in omega-3 fatty acids, yet they differ fundamentally in their source and nutritional composition. Understanding these distinctions is essential for making informed decisions about supplementation.

Fish oil is extracted from the tissues of oily fish species such as mackerel, herring, sardines, anchovies, and salmon. The primary active constituents are the long-chain omega-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These fatty acids cannot be synthesised efficiently by the human body and must be obtained through diet or supplementation. Fish oil supplements are standardised primarily for their EPA and DHA content, typically ranging from 250 mg to 1000 mg of combined omega-3s per capsule.

Cod liver oil, by contrast, is specifically extracted from the liver of cod fish (primarily Gadus morhua). Whilst it also contains EPA and DHA, cod liver oil is particularly notable for its naturally high concentrations of fat-soluble vitamins A and D. The vitamin A, vitamin D and omega-3 content varies substantially between brands, so it's important to check product labels. Typically, a 5 ml teaspoon may provide vitamin A (retinol), vitamin D3 (cholecalciferol), and omega-3 fatty acids, though amounts differ between products.

Both supplements have been used traditionally in the UK for decades, with cod liver oil having particular historical significance in addressing vitamin D deficiency and rickets in children during the early-to-mid 20th century. Today, both remain widely available as over-the-counter food supplements regulated under UK food law rather than as medicines, with health claims governed by the GB Nutrition and Health Claims Register (in Great Britain) and EU regulations (in Northern Ireland).

Key Nutritional Differences Between Cod Liver Oil and Fish Oil

The most clinically significant difference between these supplements lies in their vitamin A and D content. Standard fish oil contains negligible amounts of these vitamins, as they are not concentrated in fish muscle tissue. Cod liver oil, however, provides substantial quantities of both, which can be beneficial but also requires careful consideration regarding total dietary intake.

Vitamin A (retinol) in cod liver oil exists in its preformed state, which is readily absorbed but can accumulate in the body. The UK Reference Nutrient Intake (RNI) for vitamin A is 700 µg (approximately 2,300 IU) for adult men and 600 µg (approximately 2,000 IU) for adult women. The NHS advises not regularly taking more than 1.5 mg (1,500 µg) of vitamin A per day from supplements and diet, especially if you eat liver. For context, the European Food Safety Authority (EFSA) sets an upper limit of 3,000 µg per day. Regular cod liver oil supplementation can contribute significantly to this intake, and excessive vitamin A consumption is associated with hepatotoxicity, bone health concerns, and teratogenic effects during pregnancy.

Vitamin D3 content in cod liver oil can be advantageous, particularly in the UK where sunlight exposure is limited during autumn and winter months. The NHS and Scientific Advisory Committee on Nutrition (SACN) recommend that adults and children over one year consider taking a daily supplement containing 10 µg (400 IU) of vitamin D, especially during October to March. Some groups are advised to take this supplement year-round. Cod liver oil can contribute meaningfully to meeting this recommendation, though the amount varies between products.

Regarding omega-3 content, fish oil supplements typically contain higher concentrations of EPA and DHA per dose compared to cod liver oil. High-strength fish oil preparations may provide 1,000–2,000 mg of omega-3s per serving, whereas cod liver oil generally provides 500–1,200 mg. For individuals specifically seeking higher doses of omega-3 fatty acids, concentrated fish oil may be more appropriate, though therapeutic doses (2-4 g daily) for conditions like hypertriglyceridaemia should be prescribed as medicines rather than self-administered through supplements.

The ratio of EPA to DHA also varies between products. Some fish oils are formulated with higher EPA content (beneficial for inflammatory conditions), whilst others emphasise DHA (important for cognitive and visual function). Cod liver oil typically contains both in relatively balanced proportions, though at lower absolute concentrations.

Health Benefits: Cod Liver Oil vs Fish Oil

Both supplements share common benefits attributable to their omega-3 fatty acid content, whilst cod liver oil offers additional advantages related to its vitamin content. The evidence base for omega-3 supplementation has evolved considerably in recent years.

Cardiovascular health has been extensively studied in relation to omega-3 supplementation. EPA and DHA contribute to the maintenance of normal blood pressure (at 3 g/day) and normal blood triglyceride levels (at 2 g/day), as recognised by the GB Nutrition and Health Claims Register. The mechanism involves modulation of eicosanoid synthesis, reduction in hepatic triglyceride production, and mild anti-inflammatory effects on vascular endothelium. However, recent large-scale trials have shown mixed results regarding cardiovascular event reduction. NICE guideline NG238 does not recommend omega-3 supplements for cardiovascular disease prevention in the general population. For specific high-risk patients with raised triglycerides, NICE Technology Appraisal 805 recommends prescription icosapent ethyl (a purified EPA preparation) rather than over-the-counter supplements.

Anti-inflammatory properties of omega-3 fatty acids are well-established. EPA and DHA are incorporated into cell membranes and serve as precursors for specialised pro-resolving mediators (resolvins and protectins) that help resolve inflammatory processes. This has implications for conditions such as rheumatoid arthritis, where systematic reviews suggest fish oil supplements may provide modest symptomatic benefit, though they should not replace conventional disease-modifying therapy.

Cognitive and visual function are supported by DHA, which is a major structural component of neuronal membranes and retinal photoreceptors. Adequate DHA intake during pregnancy and early childhood is important for neurodevelopment. In older adults, there is ongoing research into omega-3 supplementation for cognitive decline, though current evidence does not support routine use for dementia prevention.

Vitamin D benefits specific to cod liver oil include support for bone health, immune function, and muscle function. Vitamin D deficiency remains prevalent in the UK, particularly among certain ethnic groups, older adults, and those with limited sun exposure. The additional vitamin D in cod liver oil may help address this, though dedicated vitamin D supplements often provide higher, more standardised doses.

Vitamin A in cod liver oil supports normal vision, immune function, and skin health. However, most UK adults obtain adequate vitamin A from dietary sources, and supplementation is not routinely necessary except in specific deficiency states.

Which Supplement Is Right for You?

Selecting between cod liver oil and fish oil depends on individual health status, dietary intake, existing supplementation, and specific therapeutic goals. A systematic approach considers several key factors.

For general cardiovascular and anti-inflammatory support in adults without vitamin deficiencies, standard fish oil is often more appropriate. It provides higher concentrations of EPA and DHA without the risk of excessive vitamin A intake. Individuals with specific conditions such as hypertriglyceridaemia should consult their GP rather than self-treating with over-the-counter supplements, as prescription options may be more appropriate.

For individuals with vitamin D insufficiency or deficiency, particularly during UK winter months, cod liver oil offers the dual benefit of omega-3s alongside vitamin D. However, it is important to calculate total vitamin D intake from all sources (diet, other supplements, and cod liver oil) to ensure appropriate dosing. Those requiring therapeutic vitamin D doses (e.g., 20 µg or 800 IU daily) may find dedicated vitamin D supplements more suitable, as they provide standardised amounts without the variability seen in cod liver oil products.

Pregnant women and those planning pregnancy should avoid cod liver oil due to its vitamin A content. Excessive vitamin A intake during pregnancy is associated with teratogenic effects, particularly affecting craniofacial and cardiac development. The NHS specifically advises pregnant women to avoid cod liver oil supplements, liver products, and any supplements containing vitamin A (retinol). Standard fish oil or algae-based omega-3 supplements are safer alternatives during pregnancy, providing DHA for foetal brain development without vitamin A concerns.

Children may benefit from either supplement, but dosing must be carefully considered. For children aged 6 months to 5 years, the NHS recommends vitamin drops containing vitamins A, C and D, available through the Healthy Start scheme. Cod liver oil should be used cautiously in children to avoid excessive vitamin A intake. Fish oil products specifically formulated for children, with appropriate EPA/DHA ratios and pleasant flavouring, are available but not routinely necessary for children eating a balanced diet.

Older adults with limited sun exposure and dietary omega-3 intake may benefit from cod liver oil's combined nutritional profile. However, those taking multiple supplements should audit their total vitamin A intake to avoid exceeding safe limits, as this population may be at increased risk of vitamin A-related bone health concerns.

People taking retinoid medicines (such as isotretinoin or acitretin) should avoid cod liver oil and other vitamin A-containing supplements due to the risk of additive toxicity.

Safety Considerations and UK Guidance

Both cod liver oil and fish oil are generally well-tolerated, but several safety considerations warrant attention, particularly regarding interactions, contraindications, and quality assurance.

Common adverse effects of both supplements include:

• Gastrointestinal disturbances (nausea, diarrhoea, fishy aftertaste)

• Belching and reflux symptoms

• Mild antiplatelet effects at high doses

These effects are typically mild and can be minimised by taking supplements with food or using enteric-coated preparations. Refrigeration after opening may help preserve freshness and reduce fishy taste.

Drug interactions require consideration, particularly with anticoagulant and antiplatelet medications. Omega-3 fatty acids possess mild antiplatelet properties, and whilst clinically significant bleeding is rare, patients taking warfarin, direct oral anticoagulants (DOACs), or antiplatelet agents (aspirin, clopidogrel) should consult their GP or pharmacist before commencing high-dose omega-3 supplementation. The British National Formulary advises caution when using prescription omega-3 products with anticoagulants.

Vitamin A toxicity is the primary concern specific to cod liver oil. Chronic excessive intake can lead to hepatotoxicity, bone demineralisation, and hypercalcaemia. Symptoms of vitamin A toxicity include headache, nausea, dry skin, hair loss, and bone pain. Patients taking other vitamin A-containing supplements, consuming high amounts of liver, or using retinoid medicines should avoid cod liver oil or choose low-vitamin A formulations.

Fish and shellfish allergies: Individuals with known allergies to fish or shellfish should exercise caution with these supplements and consult their GP before use.

Quality and purity are important considerations for all fish-derived supplements. Concerns about environmental contaminants (mercury, polychlorinated biphenyls, dioxins) have been raised. Reputable manufacturers employ molecular distillation and purification processes to remove contaminants. Look for products that:

• Display third-party testing certification (e.g., IFOS, Friend of the Sea)

• Specify EPA and DHA content clearly

• Provide batch-specific purity data

• Are manufactured to GMP (Good Manufacturing Practice) standards

UK regulatory framework: Fish oils and cod liver oil are classified as food supplements rather than medicines, regulated under food law. They are not subject to the same rigorous pre-market authorisation as medicines, though they must comply with food safety regulations and labelling requirements. Health claims made on packaging must be authorised by the GB Nutrition and Health Claims Register (in Great Britain) or the EU Register (in Northern Ireland).

When to seek medical advice: Patients should consult their GP before starting supplementation if they:

• Are pregnant or planning pregnancy

• Have bleeding disorders or take anticoagulant medications

• Have liver disease

• Are scheduled for surgery (follow your surgical team's instructions regarding supplements)

• Experience unusual symptoms after starting supplementation

• Have fish or shellfish allergies

• Are taking retinoid medicines

Suspected adverse reactions to prescription omega-3 medicines should be reported via the MHRA Yellow Card scheme.

The NHS emphasises that supplements should complement, not replace, a balanced diet rich in oily fish (at least two portions weekly, one of which should be oily fish such as salmon, mackerel, or sardines). For most individuals, dietary sources of omega-3s remain the preferred approach, with supplementation reserved for those unable to meet requirements through diet alone or with specific clinical indications.

Frequently Asked Questions