Cod liver oil is a traditional nutritional supplement derived from the livers of Atlantic cod, valued for its unique combination of omega-3 fatty acids and fat-soluble vitamins A and D. Unlike standard fish oil, cod liver oil provides significant amounts of these vitamins alongside EPA and DHA, making it particularly useful for individuals with limited dietary omega-3 intake or those at risk of vitamin D deficiency. Whilst it offers potential benefits for cardiovascular, bone, and immune health, it's important to understand appropriate usage, safe dosing, and contraindications—particularly for pregnant women and those taking certain medications.

What Is Cod Liver Oil and What Does It Contain?

Cod liver oil is a nutritional supplement derived from the liver of Atlantic cod (Gadus morhua) and related species. Unlike standard fish oil supplements, which are extracted from the body tissue of oily fish, cod liver oil is specifically sourced from fish livers, resulting in a distinct nutritional profile that has made it a traditional remedy in the UK and Scandinavia for generations.

The primary active constituents of cod liver oil are omega-3 fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), alongside significant amounts of fat-soluble vitamins A and D. A typical 5ml teaspoon of cod liver oil contains approximately 1,000mg of omega-3 fatty acids, 300–800 micrograms of vitamin A (retinol), and 5–10 micrograms (200–400 IU) of vitamin D3 (cholecalciferol). The exact concentrations vary between manufacturers and formulations, so it's important to check product labels.

The omega-3 fatty acids in cod liver oil possess anti-inflammatory properties and play essential roles in cardiovascular health, brain function, and cellular membrane integrity. Vitamin A supports immune function, vision, and skin health, whilst vitamin D is crucial for calcium absorption, bone mineralisation, and immune regulation. This combination of nutrients distinguishes cod liver oil from other marine oil supplements.

Cod liver oil is available in liquid form, capsules, and emulsified preparations. It's important to note that cod liver oil is classified as a food supplement, not a medicine, and cannot make disease treatment claims. Modern manufacturing processes typically include molecular distillation to remove environmental contaminants such as heavy metals, polychlorinated biphenyls (PCBs), and dioxins, ensuring products meet the safety standards set by UK food law and overseen by the Food Standards Agency (FSA).

Health Benefits of Cod Liver Oil

The health benefits of cod liver oil are primarily attributed to its omega-3 fatty acid content and fat-soluble vitamins, though the strength of evidence varies across different clinical applications.

Cardiovascular health has been extensively researched in relation to omega-3 fatty acids. EPA and DHA may help reduce triglyceride levels, though significant reductions typically require prescription-strength doses (2-4g daily of EPA/DHA), not the amounts found in standard cod liver oil supplements. It's important to note that NICE does not recommend omega-3 supplements for the prevention of cardiovascular disease. The anti-inflammatory effects of omega-3s may benefit vascular health, but cod liver oil supplements should not replace evidence-based cardiovascular medications.

Bone and joint health benefits from the vitamin D content, which enhances calcium absorption and bone mineralisation. Some limited evidence suggests omega-3s may provide modest symptomatic relief in osteoarthritis and rheumatoid arthritis, though cod liver oil should complement rather than replace conventional treatments. Vitamin D deficiency is prevalent in the UK, particularly during winter months, and supplementation through cod liver oil may help maintain adequate levels.

Immune function and respiratory health may be supported by vitamins A and D, both of which play regulatory roles in immune responses. Historical use of cod liver oil to prevent rickets in children reflects its vitamin D content. Some observational data suggest potential benefits for respiratory health, though robust clinical trial evidence is limited.

Cognitive function and mental health have been investigated in relation to omega-3 intake. DHA is a structural component of brain tissue, and adequate intake during early life supports neurodevelopment. However, pregnant women should not take cod liver oil due to its vitamin A content, which can harm the developing foetus. Evidence for cognitive benefits in adults remains inconclusive, though some studies suggest potential mood effects warranting further research.

Eye health benefits from vitamin A, which is essential for retinal function and night vision. While omega-3 fatty acids are present in retinal tissue, current evidence does not support cod liver oil supplementation for preventing age-related macular degeneration.

Who Should Consider Taking Cod Liver Oil?

Cod liver oil may be particularly beneficial for specific population groups, though individual circumstances should always be discussed with a healthcare professional.

Individuals with limited dietary omega-3 intake who do not regularly consume oily fish may benefit from supplementation. The NHS recommends at least two portions of fish per week (about 140g per portion), including one portion of oily fish such as salmon, mackerel, sardines, or herring. Many UK adults fail to meet this target. Cod liver oil provides an alternative source of EPA and DHA for those who dislike fish or follow certain dietary patterns. Those following vegetarian or vegan diets might consider algal-based omega-3 supplements instead.

People at risk of vitamin D deficiency represent another key group. This includes individuals with limited sun exposure, those with darker skin (which reduces cutaneous vitamin D synthesis), elderly people with reduced synthetic capacity, and those who are housebound or institutionalised. During the UK winter months (October to March), sunlight intensity is insufficient for adequate vitamin D production, and Public Health England recommends that everyone consider taking 10 micrograms of vitamin D daily during this period.

Older adults may benefit from cod liver oil's vitamin D content for maintaining bone health. However, they should be particularly mindful of vitamin A intake, as long-term consumption exceeding 1.5mg daily (from diet and supplements combined) may increase the risk of bone fractures. Those at risk of osteoporosis should carefully check total vitamin A (retinol) intake.

Individuals with certain inflammatory conditions such as rheumatoid arthritis may experience modest symptomatic benefits from the anti-inflammatory properties of omega-3 fatty acids, though cod liver oil should be used as an adjunct to, not a replacement for, prescribed medications.

Certain groups should avoid or exercise caution with cod liver oil, including pregnant women (due to vitamin A risks to the foetus), individuals taking anticoagulant medications, and those with fish allergies. People with liver disease should consult their GP before supplementation due to concerns about vitamin A accumulation. Children should not be given cod liver oil unless specifically advised by a healthcare professional, due to the vitamin A content.

How to Take Cod Liver Oil Safely

Safe and effective use of cod liver oil requires attention to dosage, timing, and product quality to maximise benefits whilst minimising potential risks.

Recommended dosage varies depending on the specific product formulation and individual nutritional needs. Most manufacturers recommend 5ml of liquid cod liver oil daily (providing approximately 1,000mg of combined EPA and DHA), or 1-2 capsules (typically providing 100-250mg of EPA and DHA each). It is essential to check the product label for specific vitamin A and D content and not exceed the recommended daily amount. The NHS advises that adults should not consume more than 1.5mg (1,500 micrograms) of vitamin A daily from food and supplements combined over the long term, as higher amounts may increase the risk of bone fractures and other adverse effects.

Timing and administration can influence tolerability. Taking cod liver oil with meals, particularly those containing some fat, enhances absorption of the fat-soluble vitamins and omega-3 fatty acids whilst reducing the likelihood of gastrointestinal discomfort or fishy aftertaste. Some individuals prefer taking it in the morning with breakfast, whilst others find evening administration more convenient. Liquid formulations can be mixed with juice or taken directly from a spoon, whilst capsules should be swallowed whole with water.

Product selection should prioritise quality and purity. Look for products that:

• Display third-party testing certification for contaminants

• Specify EPA, DHA, vitamin A, and vitamin D content clearly

• Are manufactured to Good Manufacturing Practice (GMP) standards

• Come from reputable manufacturers with clear labelling

Refrigeration of liquid cod liver oil after opening helps preserve freshness and prevent oxidation, which can reduce nutritional value and create an unpleasant taste.

Duration of supplementation can be long-term for most individuals, provided dosage remains within safe limits. However, periodic review with a healthcare professional is advisable, particularly for those taking multiple supplements or medications, to ensure continued appropriateness and avoid nutrient excess.

Potential Side Effects and Precautions

Whilst cod liver oil is generally well-tolerated, users should be aware of potential adverse effects and important contraindications to ensure safe supplementation.

Common side effects are typically mild and gastrointestinal in nature. These include:

• Fishy aftertaste or burping (eructation)

• Nausea or mild stomach upset

• Loose stools or diarrhoea, particularly at higher doses

• Halitosis (bad breath) with a fishy odour

These effects can often be minimised by taking cod liver oil with food, choosing enteric-coated capsules, or refrigerating liquid formulations. If gastrointestinal symptoms persist or worsen, discontinuation and consultation with a GP is recommended.

Vitamin A toxicity represents a significant concern with excessive cod liver oil consumption. Chronic hypervitaminosis A can cause headaches, dizziness, nausea, skin changes, hair loss, bone pain, and in severe cases, liver damage. Pregnant women must avoid cod liver oil due to the teratogenic effects of vitamin A, which can cause serious birth defects. Women who are pregnant or planning pregnancy should instead take pregnancy-specific supplements containing the recommended 400 micrograms of folic acid and 10 micrograms of vitamin D, without vitamin A.

Bleeding risk may be slightly increased due to the antiplatelet effects of omega-3 fatty acids. Individuals taking anticoagulant medications (such as warfarin, apixaban, or rivaroxaban) or antiplatelet agents (such as aspirin or clopidogrel) should consult their GP or pharmacist before starting cod liver oil. If you're scheduled for surgery, inform your surgical team about all supplements you're taking and follow their specific advice.

Drug interactions can occur with certain medications, including:

• Retinoid medications (e.g., isotretinoin, acitretin) – avoid concurrent use due to additive vitamin A effects

• Orlistat or cholestyramine – may reduce absorption of fat-soluble vitamins

• Thiazide diuretics – potential risk of hypercalcaemia with vitamin D

• Digoxin – vitamin D-related hypercalcaemia may affect cardiac function

When to seek medical advice: Contact your GP if you experience persistent nausea, unusual bruising or bleeding, severe headaches, vision changes, bone pain, or any other concerning symptoms after starting cod liver oil. Seek immediate medical attention if you develop signs of an allergic reaction such as wheezing, facial swelling, or rash. Individuals with existing liver disease, bleeding disorders, or fish allergies should not take cod liver oil without medical supervision.

Suspected side effects can be reported through the MHRA Yellow Card Scheme, which helps monitor the safety of supplements and medications.

Frequently Asked Questions