Fish liver oil is a nutritional supplement extracted from fish livers, most commonly cod, providing omega-3 fatty acids alongside naturally occurring vitamins A and D. Unlike standard fish oil derived from fish flesh, fish liver oil offers a unique nutritional profile with significant amounts of fat-soluble vitamins. Historically used in the UK to prevent rickets, it remains popular for supporting cardiovascular health, bone strength, immune function, and vision. However, the vitamin A content requires careful consideration, particularly for pregnant women and those taking certain medications. Understanding what fish liver oil is good for—and its safe use—helps individuals make informed decisions about supplementation.

What Is Fish Liver Oil and How Does It Differ from Fish Oil?

Fish liver oil is a nutritional supplement derived specifically from the livers of fish, most commonly cod (cod liver oil), halibut, or shark. This distinguishes it fundamentally from standard fish oil, which is extracted from the flesh and tissues of oily fish such as mackerel, sardines, and anchovies. Whilst both products contain beneficial omega-3 fatty acids—primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—fish liver oil has a unique nutritional profile that sets it apart.

The key difference lies in the vitamin content. Fish liver oil is naturally rich in fat-soluble vitamins A and D, which accumulate in fish livers as part of their metabolic processes. A single teaspoon of cod liver oil can provide substantial amounts of these vitamins, sometimes exceeding recommended daily intakes. In contrast, standard fish oil contains negligible amounts of vitamins A and D unless they have been artificially added during manufacturing.

This distinction has important clinical implications. Whilst the omega-3 content may be similar or even lower in fish liver oil compared to concentrated fish oil supplements, the presence of vitamins A and D offers additional potential benefits—but also introduces considerations around safe upper limits, particularly for vulnerable populations. In the UK, fish liver oils are typically regulated as food supplements under food law, with the Food Standards Agency (FSA) and Trading Standards overseeing their safety and labelling. As food supplements, they can only carry authorised nutrition and health claims, not disease treatment claims.

Historically, cod liver oil has been used in the UK since the 19th century, particularly for preventing rickets in children due to its vitamin D content. Today, it remains a popular supplement, though consumers should understand these compositional differences when selecting between fish liver oil and standard fish oil products for their specific health needs. When choosing a product, check the label for vitamin A and D content, consider your total intake from all sources (including diet and other supplements), and select reputable brands that test for environmental contaminants such as dioxins and PCBs.

Key Nutrients in Fish Liver Oil and Their Health Benefits

Fish liver oil contains three principal bioactive components, each with distinct physiological roles supported by varying levels of clinical evidence.

Omega-3 fatty acids (EPA and DHA) are polyunsaturated fats with well-established anti-inflammatory properties. These fatty acids are incorporated into cell membranes throughout the body and serve as precursors for specialised pro-resolving mediators that help regulate inflammatory responses. EPA and DHA support cardiovascular health by reducing triglyceride levels, improving endothelial function, and potentially reducing blood pressure. For hypertriglyceridaemia, prescription omega-3-acid ethyl esters (such as Omacor) are licensed medicines, but over-the-counter fish liver oil supplements are not equivalent or licensed for this purpose. For general heart health, EU authorised claims for EPA/DHA require at least 250 mg daily of combined EPA+DHA, which some cod liver oil products may not provide. Additionally, DHA is crucial for brain structure and function, though evidence for cognitive health benefits in the general population remains inconsistent and requires further research.

Vitamin A (retinol) is essential for vision, immune function, and cellular differentiation. It plays a critical role in maintaining the integrity of epithelial tissues, including the respiratory and gastrointestinal tracts, and supports the production and function of white blood cells. Adequate vitamin A status is particularly important for immune surveillance and response to infections. The NHS recommends 700 micrograms daily for men and 600 micrograms for women, amounts easily met or exceeded by fish liver oil supplementation.

Vitamin D (cholecalciferol) regulates calcium and phosphate metabolism, essential for bone health and preventing conditions such as osteomalacia in adults and rickets in children. Beyond skeletal benefits, vitamin D receptors are present throughout the body, and the vitamin plays immunomodulatory roles, though clinical benefits outside of bone health remain under investigation. Public Health England recommends 10 micrograms (400 IU) daily, particularly during autumn and winter months when sunlight exposure is insufficient for cutaneous vitamin D synthesis. Fish liver oil provides a convenient dietary source, especially relevant given the high prevalence of vitamin D insufficiency in the UK population.

What Conditions May Fish Liver Oil Help With?

Clinical evidence supports the use of fish liver oil and its constituent nutrients for several health conditions, though the strength of evidence varies considerably.

Cardiovascular health represents one of the most studied areas. The omega-3 fatty acids in fish liver oil may help reduce triglyceride levels and support overall heart health. NICE guidance recommends consuming two portions of oily fish weekly as part of a healthy diet. However, NICE specifically does not recommend omega-3 supplements for the primary or secondary prevention of cardiovascular disease. For individuals with hypertriglyceridaemia, prescription omega-3-acid ethyl esters may be used under medical supervision, but over-the-counter supplements are not equivalent to these licensed medicines. Patients with established cardiovascular disease should follow evidence-based treatments recommended by their healthcare team.

Bone and joint health may benefit from the combined vitamin D and omega-3 content. Vitamin D is essential for calcium absorption and bone mineralisation, with supplementation recommended by NICE for at-risk groups including older adults, those with limited sun exposure, and individuals with darker skin. Regarding joint conditions, some clinical trials suggest omega-3 fatty acids may reduce joint pain and stiffness in rheumatoid arthritis, though the doses used in studies are often higher than typical cod liver oil provides. These supplements should complement rather than replace conventional disease-modifying treatments.

Immune function and respiratory health may be supported by vitamins A and D, both crucial for maintaining mucosal barriers and regulating immune responses. Benefits are most clearly demonstrated in vitamin D-deficient populations, where supplementation has been associated with reduced respiratory infection risk. However, the evidence for preventing or treating specific conditions in the general population remains insufficient to make definitive claims.

Eye health benefits from vitamin A, which is essential for rhodopsin production in retinal photoreceptors. Severe vitamin A deficiency causes night blindness, though this is rare in the UK. DHA also concentrates in retinal membranes, supporting visual function, though supplementation benefits for age-related macular degeneration remain uncertain and require further investigation.

How to Take Fish Liver Oil Safely: Dosage and Precautions

Safe and effective use of fish liver oil requires attention to dosage, timing, and individual circumstances.

Typical dosages vary significantly by product, but standard cod liver oil supplements often provide approximately 5ml (one teaspoon) daily, containing roughly 10–20 micrograms of vitamin D, 800–1000 micrograms of vitamin A, and 200–400mg of combined EPA and DHA. Always check the product label for specific amounts, as concentrations vary widely between brands. The NHS advises against exceeding 1.5mg (1500 micrograms) of vitamin A daily from supplements and food combined, as chronic excessive intake may increase fracture risk and cause liver toxicity. Avoid taking vitamin A supplements if you eat liver or liver products like pâté more than once a week, and older adults at risk of osteoporosis should be particularly cautious with vitamin A intake. For vitamin D, do not exceed 100 micrograms (4,000 IU) daily unless under medical supervision.

Timing and administration: Fish liver oil can be taken with or without food, though taking it with meals may improve absorption of fat-soluble vitamins and reduce gastrointestinal side effects such as fishy aftertaste or mild nausea. Some individuals find that refrigerating liquid formulations or choosing enteric-coated capsules minimises these effects. Consistency in timing—taking the supplement at the same time daily—may improve adherence.

Special populations require particular consideration:

• Pregnant women and those trying to conceive should avoid fish liver oil due to vitamin A content. Excessive vitamin A during pregnancy, particularly in the first trimester, is associated with teratogenic effects and birth defects. The NHS specifically advises pregnant women to avoid supplements containing vitamin A. Alternative omega-3 supplements without added vitamins may be appropriate after discussion with a midwife or GP.

• Breastfeeding women should seek medical advice before taking fish liver oil to ensure total vitamin A intake remains within safe limits.

• Children may benefit from vitamin D supplementation, but fish liver oil dosing must be carefully controlled to avoid vitamin A excess. The NHS recommends vitamin drops specifically formulated for children rather than adult fish liver oil products.

• Individuals taking anticoagulants (warfarin, DOACs) or antiplatelet medications should consult their GP before starting fish liver oil, as high-dose omega-3 fatty acids may theoretically increase bleeding risk, though clinical significance at supplement doses remains unclear.

• People taking orlistat or bile acid sequestrants may experience reduced absorption of fat-soluble vitamins, potentially affecting the benefits of fish liver oil.

Quality considerations: Choose products from reputable manufacturers that test for contaminants including heavy metals, PCBs, and dioxins. Look for third-party certification where available.

Potential Side Effects and Who Should Avoid Fish Liver Oil

Whilst fish liver oil is generally well-tolerated, potential adverse effects and contraindications warrant careful consideration.

Common side effects are typically mild and gastrointestinal in nature. These include fishy aftertaste or burping, mild nausea, loose stools, and occasional indigestion. These effects often resolve with continued use or can be minimised by taking the supplement with food, choosing enteric-coated capsules, or refrigerating liquid formulations. Some individuals report a fishy body odour with high doses, though this is uncommon at standard supplementation levels.

Vitamin toxicity risks represent the most significant concern with long-term fish liver oil use. Hypervitaminosis A can develop with chronic excessive intake (above 3000 micrograms daily), causing symptoms including headache, nausea, dizziness, blurred vision, poor muscle coordination, and in severe cases, liver damage and increased intracranial pressure. Bone health may be compromised with chronic excess. Vitamin D toxicity is less common but can occur with very high doses (above 100 micrograms or 4000 IU daily over extended periods), leading to hypercalcaemia with symptoms of nausea, weakness, frequent urination, and kidney problems.

Specific populations who should avoid fish liver oil include:

• Pregnant women and those trying to conceive due to vitamin A teratogenicity risks

• Breastfeeding women should seek medical advice before use to ensure total vitamin A intake remains within safe limits

• Individuals with liver disease, as vitamin A metabolism and storage occur in the liver

• Those with hypercalcaemia or kidney stones should seek medical advice before use

• Patients with fish or seafood allergies, though reactions are rare with highly purified products

• Individuals taking retinoid medications (isotretinoin, acitretin) due to additive vitamin A effects

• Older adults at risk of osteoporosis should be cautious with vitamin A intake

When to seek medical advice: For severe allergic reactions (breathing difficulties, facial swelling), call 999 or go to A&E immediately. Contact your GP if you experience persistent gastrointestinal symptoms, unusual headaches, vision changes, or bone pain. Before starting fish liver oil, discuss with your healthcare provider if you take regular medications, have chronic health conditions, or are planning surgery, as omega-3 fatty acids may theoretically affect bleeding time. Always inform healthcare professionals about all supplements you take, as these can interact with prescribed treatments and diagnostic tests.

If you experience any suspected side effects from fish liver oil, report them to the MHRA Yellow Card Scheme, which monitors the safety of supplements and medicines.

Frequently Asked Questions