Cod liver oil is a traditional dietary supplement derived from cod fish liver, valued for its rich content of omega-3 fatty acids (EPA and DHA) alongside naturally occurring vitamins A and D. These nutrients support cardiovascular function, bone health, immune regulation, and inflammatory processes through multiple biological pathways. Whilst cod liver oil has been used for generations in the UK, particularly during winter months, it is important to understand both its potential health benefits and safety considerations. This article examines the evidence-based benefits of cod liver oil, appropriate dosing, potential side effects, and guidance on who should exercise caution or avoid supplementation altogether.

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 flesh 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, particularly during winter months.

The primary active constituents in cod liver oil are omega-3 polyunsaturated fatty acids, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These long-chain omega-3 fatty acids are synthesised inefficiently by the human body from alpha-linolenic acid (ALA) and are often obtained through diet or supplementation. EPA and DHA contribute to normal cardiovascular function, brain health, and inflammatory regulation. 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 crucial for calcium absorption, bone mineralisation, and immune modulation.

The mechanism of action involves multiple pathways. Omega-3 fatty acids are incorporated into cell membranes, where they influence membrane fluidity and cellular signalling. They serve as precursors for anti-inflammatory mediators called resolvins and protectins, which help resolve inflammatory processes. EPA and DHA also modulate lipid metabolism, potentially affecting triglyceride synthesis in the liver. The vitamins A and D in cod liver oil act through nuclear receptors to regulate gene expression, affecting immune responses, bone metabolism, and cellular differentiation. This combination of nutrients provides a broad spectrum of potential health benefits, though the evidence base varies across different clinical applications.

It's important to note that cod liver oil is classified as a food supplement, not a medicine, in the UK.

Health Benefits of Cod Liver Oil

Cod liver oil has been associated with several health benefits, though the strength of evidence varies. Cardiovascular health represents one of the most studied areas. Omega-3 fatty acids, particularly EPA and DHA, have been shown to reduce serum triglyceride levels, though significant reductions of 20–30% are typically observed only at prescription doses (2–4g/day of EPA/DHA), not at standard supplement doses. It's important to note that NICE guidance (NG238) does not recommend omega-3 supplements for primary or secondary cardiovascular disease prevention. Prescription-only icosapent ethyl (a purified EPA preparation) may be considered for specific high-risk patients as per NICE TA805, but this differs from over-the-counter cod liver oil supplements.

Bone and joint health may benefit from the vitamin D content in cod liver oil. Vitamin D is essential for calcium homeostasis and bone mineralisation, and supplementation of 10 micrograms (400 IU) daily is recommended by Public Health England for certain populations, particularly during autumn and winter months. Some observational studies suggest associations between cod liver oil consumption and bone health, but no causal link has been established through randomised controlled trials. The high vitamin A content in cod liver oil should be considered carefully, as excessive vitamin A intake may adversely affect bone health with prolonged use. The anti-inflammatory properties of omega-3 fatty acids may provide modest symptomatic relief in inflammatory joint conditions such as rheumatoid arthritis, with some patients reporting reduced joint stiffness and pain.

Cognitive function and mental health represent emerging areas of interest. DHA is a major structural component of brain tissue, and adequate intake during pregnancy and early life is considered important for neurodevelopment. In adults, some studies suggest potential benefits for mood regulation, though evidence remains inconsistent and insufficient to recommend routine supplementation for these purposes. The immune system may also benefit from the combined effects of omega-3 fatty acids and vitamins A and D, which play regulatory roles in both innate and adaptive immunity. However, more robust clinical evidence is needed to establish definitive therapeutic benefits in these areas.

Recommended Dosage and How to Take Cod Liver Oil

Dosage recommendations for cod liver oil vary depending on the intended health benefit and individual circumstances. In the UK, there is no single official recommended dose, but guidance from the NHS and scientific advisory bodies provides useful frameworks. For general health maintenance, typical doses range from 5–10 ml (approximately one to two teaspoons) of liquid cod liver oil daily, or equivalent capsule formulations.

The Scientific Advisory Committee on Nutrition (SACN) recommends that adults consume at least 450 mg of combined EPA and DHA daily, ideally through dietary sources such as oily fish. This recommendation is best met by consuming two portions of fish weekly, one of which should be oily. For individuals who do not regularly consume fish, supplementation with cod liver oil can help meet this target. However, it is important to note that cod liver oil contains significant amounts of vitamins A and D, which are fat-soluble and can accumulate in the body. The NHS advises that adults should not consume more than 1.5 mg (5000 IU) of vitamin A daily from supplements and should avoid liver products if taking supplements containing vitamin A. This is particularly important for pregnant women, who should avoid supplements containing vitamin A (including cod liver oil) altogether. For vitamin D, the UK recommendation is 10 micrograms (400 IU) daily for adults during autumn and winter, with a safe upper limit generally not exceeding 100 micrograms (4000 IU) daily unless under medical supervision.

Cod liver oil is best taken with food to enhance absorption of fat-soluble vitamins and reduce the likelihood of 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. When selecting a product, look for supplements that have been tested for purity, particularly regarding heavy metal contamination (mercury, lead) and environmental pollutants such as polychlorinated biphenyls (PCBs). Reputable manufacturers should provide third-party testing certificates.

Patients should carefully check product labels for vitamin A content and calculate their total intake from all supplements to avoid exceeding safe upper limits. Those taking orlistat (a weight-loss medication) should be aware that it may reduce the absorption of fat-soluble vitamins, potentially affecting the benefits of cod liver oil.

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 gastrointestinal effects include nausea, indigestion, diarrhoea, and a fishy aftertaste or 'fish burps'. These symptoms are usually mild and can often be minimised by taking the supplement with meals, choosing enteric-coated capsules, or refrigerating liquid formulations. Some individuals may experience loose stools, particularly at higher doses.

Bleeding risk represents a consideration, as omega-3 fatty acids possess mild antiplatelet effects that may modestly prolong bleeding time. At standard supplemental doses, this effect is generally minimal for most people. However, patients taking anticoagulant medications (such as warfarin, apixaban, or rivaroxaban) or antiplatelet agents (including aspirin, clopidogrel) should exercise caution. There is no official contraindication to combined use, but patients should inform their GP or anticoagulation clinic when starting or stopping cod liver oil, and INR monitoring may need to be more frequent for those on warfarin. Similarly, individuals scheduled for surgery should follow their surgeon's or anaesthetist's instructions; many recommend discontinuing cod liver oil approximately one week beforehand to minimise bleeding risk.

Vitamin toxicity poses a particular concern with cod liver oil due to its high vitamin A and D content. Chronic excessive intake of vitamin A (hypervitaminosis A) can cause liver damage, bone pain, hair loss, and in severe cases, increased intracranial pressure. Pregnant women are especially vulnerable, as excessive vitamin A intake during pregnancy is associated with teratogenic effects and birth defects. The NHS specifically advises pregnant women to avoid cod liver oil and other supplements containing vitamin A. Excessive vitamin D intake (hypervitaminosis D) can lead to hypercalcaemia, presenting with nausea, weakness, kidney stones, and potentially cardiac arrhythmias.

Allergic reactions are possible in individuals with fish or shellfish allergies, though cod liver oil generally has low allergenic potential as it contains minimal protein. Patients should contact their GP if they experience symptoms such as rash, swelling, difficulty breathing, or other signs of hypersensitivity. Quality and purity concerns also merit consideration, as fish-derived products may contain environmental contaminants. Choosing products from reputable manufacturers with appropriate quality assurance is advisable.

If you experience any side effects from taking cod liver oil, you can report them through the MHRA Yellow Card Scheme, which helps monitor the safety of supplements and medicines.

Who Should Avoid Cod Liver Oil?

Certain populations should avoid cod liver oil or use it only under medical supervision due to specific safety concerns. Pregnant women represent the most important group requiring caution. The NHS explicitly advises pregnant women to avoid cod liver oil and any supplements containing vitamin A, as well as liver and liver products, because excessive vitamin A may cause birth defects. Whilst omega-3 fatty acids are beneficial during pregnancy, alternative sources such as standard fish oil supplements (without added vitamin A) or dietary intake of oily fish (up to two portions weekly, avoiding high-mercury species) are preferred. Breastfeeding women should similarly exercise caution and discuss supplementation with their midwife or GP.

Individuals with fish or seafood allergies should avoid cod liver oil, although severe allergic reactions are uncommon as the oil contains minimal allergenic protein. Those with documented allergies should consult an allergist before considering use. Patients with bleeding disorders (such as haemophilia or von Willebrand disease) or those taking anticoagulant or antiplatelet medications should seek medical advice before starting cod liver oil, given its potential effects on haemostasis.

People with liver disease should use cod liver oil cautiously, as the liver is the primary storage site for vitamin A, and impaired hepatic function may increase the risk of vitamin A toxicity. Similarly, individuals with hypercalcaemia or conditions predisposing to elevated calcium levels (such as sarcoidosis or hyperparathyroidism) should avoid cod liver oil due to its vitamin D content, which enhances calcium absorption.

Children may benefit from cod liver oil supplementation, particularly for vitamin D status, but dosing must be carefully adjusted to avoid vitamin toxicity. Parents should consult their GP or health visitor before giving cod liver oil to children, and should not exceed age-appropriate doses. The NHS provides specific vitamin D supplementation guidance for infants and children.

Patients taking medications that interact with vitamins A or D should discuss cod liver oil use with their GP or pharmacist. This includes those taking retinoids (such as isotretinoin or acitretin for acne or psoriasis) or medications affecting calcium metabolism. Anyone experiencing unexplained symptoms after starting cod liver oil—including persistent nausea, bone pain, unusual fatigue, or changes in vision—should discontinue use and seek medical advice promptly. When in doubt, consulting a healthcare professional before starting any new supplement remains the safest approach.

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