Cod liver oil supplements are nutritional products derived from the liver of Atlantic cod, providing omega-3 fatty acids (EPA and DHA), vitamin A, and vitamin D. Used in the UK for over a century, these supplements support cardiovascular health, bone strength, and immune function. Unlike standard fish oil, cod liver oil contains higher concentrations of vitamins A and D, which requires careful attention to dosing—particularly in pregnancy, when taking certain medications, or in individuals with liver conditions. This article examines the evidence for cod liver oil supplements, safe usage guidance aligned with NHS and NICE recommendations, potential side effects, and populations who should exercise caution or avoid these products entirely.

What Are Cod Liver Oil Supplements?

Cod liver oil supplements are nutritional products derived from the liver of cod fish, primarily Atlantic cod (Gadus morhua). These supplements have been used for over a century in the United Kingdom and remain popular as a source of essential nutrients, particularly omega-3 fatty acids, vitamin A (in the form of preformed retinol), and vitamin D.

The oil is extracted from cod livers through various processing methods, including traditional steam extraction and modern molecular distillation techniques. High-quality supplements undergo purification to remove potential contaminants such as heavy metals and environmental pollutants, which can accumulate in fish tissues. In the UK, cod liver oil is regulated as a food supplement by the Food Standards Agency (FSA), not as a licensed medicine by the MHRA.

Cod liver oil is available in several formulations, including liquid preparations, soft gel capsules, and flavoured emulsions designed to mask the characteristic fishy taste. The typical composition includes eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) — the two primary omega-3 fatty acids — alongside fat-soluble vitamins A and D. UK products vary, but a typical daily serving contains approximately 5–15 micrograms of vitamin D, 250–800 micrograms of vitamin A (as retinol), and 200–500 milligrams of combined EPA and DHA.

Unlike fish oil supplements, which are extracted from the body tissues of oily fish, cod liver oil specifically comes from the liver and consequently contains higher concentrations of vitamins A and D. This distinction is clinically important when considering appropriate dosing and potential risks of hypervitaminosis, particularly in vulnerable populations.

As with all supplements, follow label instructions, do not exceed the recommended dose, and keep products out of reach of children.

Health Benefits of Cod Liver Oil

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

Cardiovascular health has been extensively studied in relation to omega-3 fatty acids. EPA and DHA contribute to the maintenance of normal blood pressure and triglyceride levels. The Great Britain Nutrition and Health Claims Register recognises that DHA and EPA contribute to normal heart function, with beneficial effects observed at intakes of 250 milligrams daily. However, recent large-scale trials have produced mixed results regarding cardiovascular event prevention. NICE guidance (NG238 and NG185) specifically states that omega-3 supplements should not be offered for primary or secondary prevention of cardiovascular disease, and they should not replace evidence-based cardiovascular medications.

Vitamin D supplementation is particularly relevant in the UK, where sunlight exposure is limited during autumn and winter months. Current NHS guidance recommends that adults and children over one year consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D, especially during October to March. Some groups are advised to take vitamin D year-round, including people with limited sun exposure, those with darker skin, and care home residents. Cod liver oil can contribute to meeting this recommendation, supporting bone health, immune function, and muscle strength.

Joint health and inflammatory conditions represent another area of interest. Cochrane reviews suggest that regular omega-3 intake may help reduce joint stiffness, pain and NSAID use in rheumatoid arthritis, though evidence remains insufficient for cod liver oil to be recommended as a primary treatment. The anti-inflammatory properties of omega-3 fatty acids may modulate inflammatory pathways, but patients with inflammatory arthritis should continue prescribed disease-modifying treatments.

Cognitive function and eye health are supported by DHA, which is a structural component of brain tissue and retinal membranes. The GB Nutrition and Health Claims Register recognises that DHA contributes to the maintenance of normal brain function and vision in the general population, with beneficial effects observed at 250 milligrams daily.

How to Take Cod Liver Oil Supplements Safely

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

Dosage considerations should account for the vitamin A and D content alongside omega-3 fatty acids. The NHS advises that adults should not take more than 1.5 milligrams (1,500 micrograms) of vitamin A daily from supplements. People who regularly eat liver should avoid vitamin A supplements altogether, and older adults should avoid long-term high intakes. Most cod liver oil supplements contain between 250–800 micrograms of vitamin A (as retinol) per serving. For vitamin D, the upper safe limit is 100 micrograms (4,000 IU) daily for adults, though toxicity is rare below this threshold.

Timing and administration can influence tolerability. Taking cod liver oil with meals, particularly those containing other fats, enhances absorption of fat-soluble vitamins and may reduce gastrointestinal discomfort. Some individuals find that taking supplements in the evening minimises fishy aftertaste or reflux. Capsules should be swallowed whole with adequate water, whilst liquid formulations can be mixed with juice or food if palatability is an issue.

Product selection is crucial for safety and efficacy. Choose supplements that:

• Display clear labelling of EPA, DHA, vitamin A, and vitamin D content

• Provide evidence of third-party testing for contaminants (preferably UKAS-accredited)

• Are stored according to manufacturer instructions (typically in cool, dark conditions)

Storage and shelf life require attention, as omega-3 fatty acids are susceptible to oxidation. Rancid cod liver oil loses nutritional value and may cause gastrointestinal upset. Check expiry dates, store bottles away from heat and light, and refrigerate after opening if recommended. Discard any product with an unusually strong or unpleasant odour.

Medication interactions require caution. Do not take cod liver oil with oral retinoids (such as isotretinoin, acitretin, or alitretinoin) due to increased vitamin A toxicity risk. Separate dosing from orlistat or bile acid sequestrants (like cholestyramine) by at least 2 hours, as these can reduce absorption. If taking warfarin, monitor your INR when starting or stopping cod liver oil. Consult your GP or pharmacist before commencing supplementation if you take any prescribed medications or are planning surgery.

Potential Side Effects and Risks

Whilst cod liver oil supplements are generally well tolerated, users should be aware of potential adverse effects and circumstances requiring medical attention.

Gastrointestinal effects are the most commonly reported side effects. These include nausea, loose stools, indigestion, and a fishy aftertaste or 'repeating'. These symptoms are typically mild and dose-dependent, often resolving with dose reduction or by taking the supplement with food. Enteric-coated capsules may reduce upper gastrointestinal symptoms by delaying release until the supplement reaches the small intestine. Persistent or severe gastrointestinal symptoms warrant medical review to exclude other causes.

Hypervitaminosis A represents a more serious concern with long-term, excessive intake. Chronic vitamin A toxicity can cause liver damage, bone pain, skin changes, and increased intracranial pressure. Pregnant women are particularly vulnerable, as excessive vitamin A intake during pregnancy is associated with teratogenic effects and birth defects. The NHS specifically advises pregnant women to avoid vitamin A supplements and liver products. Symptoms of vitamin A toxicity include headache, blurred vision, nausea, dizziness, and skin peeling — any of these warrant immediate cessation and medical consultation.

Bleeding risk may be increased with high-dose omega-3 supplementation, particularly in individuals taking anticoagulants (warfarin, DOACs), antiplatelet agents (aspirin, clopidogrel), or non-steroidal anti-inflammatory drugs. At standard doses, the bleeding risk is generally small, but patients should inform healthcare providers about supplement use before surgical procedures and follow local pre-operative instructions. If you take warfarin, your INR should be monitored when starting or stopping cod liver oil supplements.

Allergic reactions are rare but possible, particularly in individuals with fish or shellfish allergies. Symptoms may include rash, itching, swelling, or respiratory difficulty. Anaphylaxis is extremely uncommon but constitutes a medical emergency requiring immediate treatment with intramuscular adrenaline and emergency services contact (999).

Contaminant exposure from poor-quality products may include heavy metals (mercury, lead), polychlorinated biphenyls (PCBs), or dioxins. Reputable manufacturers employ purification processes to minimise these risks, but consumers should verify third-party testing and choose products from established suppliers.

If you experience any suspected side effects from cod liver oil supplements, you can report them through the MHRA Yellow Card scheme.

Who Should Avoid Cod Liver Oil Supplements?

Certain populations should exercise caution or avoid cod liver oil supplements entirely due to specific health risks or lack of safety data.

Pregnant women should avoid vitamin A supplements, including cod liver oil, and liver products due to potential risks to the developing foetus. The NHS and Royal College of Obstetricians and Gynaecologists advise against taking vitamin A supplements during pregnancy, as excessive intake is associated with congenital malformations. Pregnant women should instead consider pregnancy-specific supplements containing appropriate doses of vitamin D and omega-3 DHA without added vitamin A. Standard prenatal vitamins and fish oil supplements (without added vitamin A) represent safer alternatives.

Breastfeeding women should avoid high-vitamin A supplements, including most cod liver oil products, unless advised by a healthcare professional. Alternative omega-3 sources such as DHA-only fish oil or algal DHA supplements may be more suitable during breastfeeding.

Individuals taking oral retinoids (isotretinoin, acitretin, alitretinoin) should not take cod liver oil due to increased risk of vitamin A toxicity when these medications are combined.

Individuals with bleeding disorders or those taking anticoagulant medications should consult their GP or anticoagulation clinic before using cod liver oil. Omega-3 fatty acids possess mild antiplatelet effects that may potentiate the action of warfarin, direct oral anticoagulants, or antiplatelet drugs, potentially increasing bleeding risk. Whilst there is no official contraindication at standard doses, medical supervision ensures appropriate monitoring of clotting parameters (INR for warfarin users) and clinical bleeding signs.

Patients with fish or seafood allergies should avoid cod liver oil unless allergy testing confirms tolerance. Although refined cod liver oil contains minimal protein (the typical allergen), cross-contamination during processing or residual proteins may trigger allergic reactions in sensitised individuals. Alternative omega-3 sources include algal oil supplements, which provide DHA and EPA without fish-derived ingredients.

People with liver disease should exercise caution, as the liver metabolises vitamin A and stores excess amounts. Individuals with hepatic impairment may have reduced capacity to process and eliminate vitamin A, increasing toxicity risk. Medical supervision is advisable for anyone with chronic liver conditions considering cod liver oil supplementation.

People with conditions predisposing to hypercalcaemia (such as sarcoidosis) or a history of kidney stones should seek medical advice before taking vitamin D-containing supplements, including cod liver oil.

Children under 12 months should not receive cod liver oil supplements unless specifically recommended by a paediatrician. Infant vitamin drops containing appropriate doses of vitamins A, C, and D are recommended by the NHS for breastfed babies and formula-fed infants consuming less than 500 millilitres daily.

Frequently Asked Questions