Cod liver oil is a traditional dietary supplement derived from cod fish liver, valued for its omega-3 fatty acids (EPA and DHA) and naturally occurring vitamins A and D. Unlike standard fish oil, cod liver oil provides these fat-soluble vitamins alongside cardiovascular and anti-inflammatory benefits. Whilst it has been used for generations to support bone health, joint function, and general wellbeing, evidence for specific health claims varies. Understanding appropriate dosing, potential interactions, and safety considerations—particularly regarding vitamin A during pregnancy—is essential for safe use. This article examines what cod liver oil is good for, how it works, and important precautions for UK users.

What Is Cod Liver Oil and How Does It Work?

Cod liver oil is a dietary supplement derived from the liver of cod fish, primarily Atlantic cod (Gadus morhua) and Pacific cod (Gadus macrocephalus). It is distinct from standard fish oil supplements, which are typically extracted from the body tissue of oily fish such as mackerel, sardines, or anchovies. Cod liver oil has been used for centuries as a traditional remedy and remains popular in the UK as a food supplement for its nutritional properties.

The primary active constituents in cod liver oil are omega-3 polyunsaturated fatty acids, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These essential fatty acids cannot be synthesised efficiently by the human body and must be obtained through diet or supplementation. EPA and DHA play crucial roles in cellular membrane structure, inflammatory pathways, and cardiovascular function. Additionally, cod liver oil is naturally rich in fat-soluble vitamins A and D, which distinguishes it from other fish oil products. Vitamin A supports immune function, vision, and skin health, whilst vitamin D is essential for calcium absorption, bone health, and immune regulation.

The mechanism of action of omega-3 fatty acids involves their incorporation into cell membranes, where they influence membrane fluidity and cellular signalling. EPA and DHA serve as precursors to specialised pro-resolving mediators (resolvins and protectins) that help modulate inflammatory responses. They also affect lipid metabolism by reducing triglyceride synthesis in the liver and may influence blood pressure through modest, dose-dependent effects on vascular endothelial function. The vitamins A and D in cod liver oil exert their effects through nuclear receptor-mediated gene transcription, influencing numerous physiological processes including immune response, bone metabolism, and cellular differentiation.

It is important to note that evidence and indications differ between dietary omega-3 intake, food supplements like cod liver oil, and prescription omega-3 medicines.

Health Benefits of Cod Liver Oil

Cod liver oil has been associated with several potential health benefits, though the strength of evidence varies across different conditions. Cardiovascular health represents one of the most studied areas. Omega-3 fatty acids may help reduce triglyceride levels, lower blood pressure modestly, and potentially decrease the risk of certain cardiovascular events in specific populations. However, it's important to note that NICE does not recommend omega-3 supplements for the prevention of cardiovascular disease. The NHS encourages consumption of dietary sources of omega-3s as part of a Mediterranean-style diet pattern, with oily fish being the preferred source.

Bone and joint health may benefit from cod liver oil's vitamin D content, which is essential for calcium absorption and bone mineralisation. This is particularly relevant in the UK, where vitamin D deficiency is common, especially during winter months. UK Government advice recommends that adults and children over one year consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D, particularly during autumn and winter. Some randomised controlled trials suggest omega-3 fatty acids may help reduce joint stiffness and pain in rheumatoid arthritis, providing modest symptomatic relief, though they have no disease-modifying effects and should not replace conventional treatments.

Cognitive function and mental health have also been investigated. DHA is a major structural component of brain tissue, and adequate omega-3 intake during pregnancy and early life supports neurodevelopment. Some research suggests omega-3 supplementation may have modest benefits for mild depression, though there is insufficient evidence to recommend it as a primary treatment. The relationship between cod liver oil and cognitive decline in older adults remains uncertain, with mixed findings from clinical trials.

Immune function may be supported by vitamins A and D, both of which play regulatory roles in immune responses. Vitamin D deficiency has been associated with increased susceptibility to respiratory infections, though supplementation studies show variable results. It is important to note that whilst cod liver oil provides nutritional support, there is no established link between its use and prevention of specific diseases such as COVID-19 or influenza.

Recommended Dosage and How to Take Cod Liver Oil

Dosage recommendations for cod liver oil vary depending on the specific product formulation and the individual's nutritional needs. Most commercially available cod liver oil supplements in the UK provide approximately 5-10 ml (one to two teaspoons) of liquid oil or equivalent capsule doses daily. It is essential to check the product label for specific EPA, DHA, vitamin A, and vitamin D content, as concentrations vary significantly between brands.

A population dietary intake estimate for omega-3 fatty acids is around 450 mg of combined EPA and DHA daily, though this is not a supplement safety limit. For vitamin D, UK Government advice recommends a daily intake of 10 micrograms (400 IU) for most adults, with an upper limit of 100 micrograms (4000 IU) daily from all sources. Vitamin A intake should not exceed 1.5 mg (1,500 micrograms) daily from food and supplements combined to avoid toxicity. This applies to everyone, with particular caution needed during pregnancy.

Practical administration tips include taking cod liver oil with meals to enhance absorption of fat-soluble vitamins and reduce potential gastrointestinal discomfort. Liquid formulations should be stored in a cool, dark place and refrigerated after opening to prevent oxidation. Capsules offer a convenient alternative for those who find the taste of liquid oil unpalatable. Some products are flavoured (commonly with lemon or mint) to improve palatability.

Special populations require particular consideration. Pregnant women should avoid cod liver oil and other supplements containing vitamin A due to potential risks to the developing foetus. Instead, they should focus on dietary sources of omega-3 or consider pregnancy-specific omega-3 supplements containing only EPA and DHA if recommended by a healthcare professional. Children can take cod liver oil, but doses should be adjusted according to age and weight, following manufacturer guidance or healthcare professional advice, with careful attention to vitamin A content. Individuals taking anticoagulant medications (such as warfarin), blood pressure medications, or retinoid treatments (such as isotretinoin or acitretin) should consult their GP before starting cod liver oil due to potential interactions.

High-dose omega-3 therapy for conditions such as hypertriglyceridaemia is typically provided through prescription-only medicines under specialist supervision, not through standard cod liver oil supplements.

Potential Side Effects and Safety Considerations

Cod liver oil is generally well tolerated when taken at recommended doses, but several side effects and safety considerations warrant attention. Common adverse effects include gastrointestinal symptoms such as nausea, indigestion, diarrhoea, and a fishy aftertaste or 'fish burps'. These effects are usually mild and can often be minimised by taking the supplement with food, choosing enteric-coated capsules, or refrigerating liquid formulations. Some individuals may experience loose stools, particularly at higher doses.

Vitamin toxicity represents a significant concern with long-term or excessive cod liver oil consumption. Hypervitaminosis A can occur with chronic excessive intake, leading to symptoms including headache, dizziness, nausea, skin changes, bone pain, and in severe cases, liver damage. Pregnant women should avoid cod liver oil and other vitamin A supplements due to potential foetal abnormalities. Vitamin D toxicity is less common but can result in hypercalcaemia, causing nausea, weakness, kidney problems, and cardiac arrhythmias. It is crucial not to exceed recommended doses and to account for vitamin A and D intake from other supplements, fortified foods, and dietary sources such as liver.

Drug interactions require careful consideration. Omega-3 fatty acids may enhance the effects of anticoagulant and antiplatelet medications (warfarin, aspirin, clopidogrel), potentially increasing bleeding risk. Patients taking these medications should consult their GP or pharmacist before starting cod liver oil. There may also be interactions with blood pressure medications, as omega-3s can have modest hypotensive effects. Individuals taking orlistat or other fat-absorption inhibitors may have reduced absorption of fat-soluble vitamins from cod liver oil. Particular caution is needed with systemic or topical retinoids (isotretinoin, acitretin) due to additive vitamin A toxicity.

Contraindications and precautions include fish or seafood allergy (cod liver oil should be avoided unless advised otherwise by an allergy specialist), active bleeding disorders, and scheduled surgery (follow your surgeon's or anaesthetist's specific advice regarding continuation or discontinuation). Individuals with liver disease should use caution due to potential vitamin A accumulation. Quality and purity concerns exist with fish oil supplements, so choose products from reputable manufacturers that comply with UK and EU food supplement regulations and provide third-party testing certificates.

When to seek medical advice: Contact your GP if you experience persistent gastrointestinal symptoms, unusual bleeding or bruising, severe headaches, vision changes, bone pain, or signs of allergic reaction (rash, swelling, difficulty breathing). Regular monitoring may be appropriate for individuals taking high doses or those with pre-existing medical conditions. Always inform healthcare professionals about all supplements you are taking, as this information is essential for safe prescribing and monitoring of potential interactions. If you suspect you have experienced a side effect from cod liver oil, you can report it through the MHRA Yellow Card Scheme.

Frequently Asked Questions