Cod liver oil and fish oil are both popular omega-3 supplements, yet they differ significantly in composition and suitability. Fish oil, extracted from oily fish flesh, provides concentrated EPA and DHA with minimal vitamins. Cod liver oil, derived from cod liver, contains omega-3s alongside substantial vitamin A and vitamin D. Whilst both offer omega-3 benefits, cod liver oil's vitamin content requires careful consideration—particularly the vitamin A, which makes it unsuitable during pregnancy and for those already taking vitamin supplements. Understanding these distinctions is essential for safe, appropriate supplementation aligned with UK guidance and individual nutritional requirements.

What Is the Difference Between Cod Liver Oil and Fish Oil?

Cod liver oil and fish oil are both marine-derived food supplements rich in omega-3 fatty acids, yet they differ significantly in their source, composition, and nutritional profile. Fish oil is typically extracted from the flesh of oily fish species such as mackerel, sardines, anchovies, and herring. The extraction process concentrates the omega-3 polyunsaturated fatty acids—primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—whilst containing minimal amounts of fat-soluble vitamins.

Cod liver oil, by contrast, is derived specifically from the liver of Atlantic cod (Gadus morhua). Because the liver is a storage organ for fat-soluble vitamins, cod liver oil naturally contains substantial amounts of vitamin A (retinol) and vitamin D (cholecalciferol) alongside omega-3 fatty acids. The vitamin content can vary between brands and may be standardised during manufacturing.

Historically, cod liver oil was widely used in the UK as a nutritional supplement, particularly for children, to prevent rickets and support growth during periods when dietary vitamin D was insufficient. Fish oil supplements gained prominence later as research highlighted the cardiovascular and anti-inflammatory benefits of omega-3 fatty acids. Today, both supplements are available over the counter in various formulations, including liquid, capsule, and flavoured preparations.

It's important to note that these are food supplements and should not replace a balanced diet. The choice between these supplements depends on individual nutritional needs, existing vitamin status, and specific health objectives. Understanding their distinct compositions is essential for making an informed decision that aligns with clinical guidance and personal health requirements.

Nutritional Content: Omega-3, Vitamins A and D Compared

The nutritional profiles of cod liver oil and standard fish oil differ considerably, which has important implications for supplementation strategies. Standard fish oil supplements typically provide:

• EPA and DHA: Combined omega-3 content usually ranges from 250–1000 mg per capsule or serving, depending on concentration

• Minimal vitamins A and D: Negligible amounts, as these are not stored in fish muscle tissue

• Variable purity: Quality products undergo molecular distillation to remove contaminants such as heavy metals and persistent organic pollutants

Cod liver oil provides a broader nutritional spectrum:

• Omega-3 fatty acids: Generally lower concentrations than concentrated fish oil—typically 200–400 mg combined EPA and DHA per serving

• Vitamin A: Content varies by brand, typically ranging from 700–1500 micrograms per serving, which can represent a significant proportion of the daily requirement

• Vitamin D: Typically 5–10 micrograms (200–400 IU) per serving, contributing toward the UK recommended 10 micrograms daily intake

The vitamin A content in cod liver oil warrants particular attention. The NHS advises that adults should not take more than 1500 micrograms of vitamin A daily from supplements. Excessive vitamin A intake during pregnancy is teratogenic and associated with birth defects, which is why pregnant women are advised to avoid cod liver oil and vitamin A supplements.

Vitamin D content can be beneficial, especially given the high prevalence of vitamin D insufficiency in the UK population. The UK Government recommends that everyone should consider taking 10 micrograms of vitamin D daily during autumn and winter months when sunlight exposure is limited. However, individuals already taking vitamin D supplements should account for the additional vitamin D in cod liver oil to avoid exceeding safe upper limits (100 micrograms daily for adults according to the European Food Safety Authority).

Health Benefits and Clinical Evidence for Each Supplement

Both supplements contain omega-3 fatty acids (EPA and DHA), though the strength of evidence varies across different clinical outcomes:

Cardiovascular health: It's important to note that NICE does not recommend over-the-counter omega-3 supplements for primary or secondary prevention of cardiovascular disease. While omega-3 fatty acids can reduce triglyceride levels, significant reductions (15–30%) typically require prescription-strength products at doses of 2–4 grams daily. NICE has approved icosapent ethyl (Vazkepa) for reducing cardiovascular risk in specific high-risk patients with raised triglycerides who are already on statin therapy.

Anti-inflammatory effects: Omega-3 fatty acids compete with arachidonic acid in inflammatory pathways, potentially reducing production of pro-inflammatory eicosanoids. Some evidence supports their use as adjunctive therapy in rheumatoid arthritis, though effects are modest and should not replace conventional disease-modifying treatments.

Cognitive and mental health: Research into omega-3 supplementation for depression, cognitive decline, and dementia prevention has yielded mixed results. Whilst some observational studies suggest associations between higher omega-3 intake and better cognitive outcomes, intervention trials have generally not demonstrated significant benefits in preventing cognitive decline or treating depression.

Cod liver oil's additional vitamins provide distinct benefits:

• Vitamin D: Essential for bone health, immune function, and calcium homeostasis. Supplementation is particularly important for at-risk groups including older adults, those with limited sun exposure, and individuals with darker skin

• Vitamin A: Supports vision, immune function, and cellular differentiation. However, most UK adults obtain adequate vitamin A from diet

There is no robust clinical evidence that cod liver oil provides superior health outcomes compared to fish oil when omega-3 intake is the primary objective. The choice should be guided by individual vitamin requirements and safety considerations.

Safety Considerations and Potential Side Effects

Both supplements are generally well-tolerated, but several safety considerations merit attention. Common adverse effects affecting both fish oil and cod liver oil include:

• Gastrointestinal symptoms: Fishy aftertaste, belching, nausea, loose stools, and dyspepsia occur in approximately 5–15% of users. Taking supplements with meals or using enteric-coated preparations may reduce these effects

• Bleeding risk: Omega-3 fatty acids have mild antiplatelet effects. Whilst clinically significant bleeding is rare at typical supplementation doses (up to 3 grams daily), caution is advised in individuals taking anticoagulants (warfarin, DOACs) or antiplatelet agents (aspirin, clopidogrel). Patients taking warfarin should inform their healthcare provider when starting omega-3 supplements as INR monitoring may be advisable

• Allergic reactions: Individuals with fish or shellfish allergies should avoid these supplements unless specifically advised by their healthcare provider

Specific concerns with cod liver oil relate to its vitamin content:

Vitamin A toxicity: Chronic excessive intake can cause hepatotoxicity, bone abnormalities, and central nervous system effects. Long-term high intake of vitamin A may also reduce bone mineral density, particularly in older adults and postmenopausal women. Pregnant women must avoid cod liver oil due to teratogenic risks. The NHS explicitly advises against cod liver oil supplementation during pregnancy and when planning conception.

Vitamin D excess: Whilst less common, hypervitaminosis D can occur with combined supplementation, causing hypercalcaemia, renal impairment, and vascular calcification. Individuals taking prescribed vitamin D (for osteoporosis, deficiency) should consult their GP before adding cod liver oil.

Drug interactions to consider:

• Blood pressure medications: Omega-3s may have additive hypotensive effects

Quality and purity: Choose supplements certified by independent testing organisations. Reputable manufacturers test for heavy metals (mercury, lead), dioxins, and PCBs. Products should meet European Pharmacopoeia or similar standards.

When to contact your GP: Seek medical advice if you experience persistent gastrointestinal symptoms, unusual bleeding or bruising, signs of vitamin toxicity (severe headache, vision changes, bone pain), or if you're taking multiple supplements or medications.

If you suspect an adverse reaction to any supplement, report it through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

Which Supplement Is Right for You? UK Guidance

Selecting between cod liver oil and fish oil requires consideration of individual nutritional status, health objectives, and safety factors. Fish oil is generally preferable when:

• The primary goal is increasing omega-3 (EPA and DHA) intake

• You are pregnant, planning pregnancy, or breastfeeding

• You already take vitamin A or multivitamin supplements

• You require higher doses of omega-3 fatty acids (>1 gram daily)

• You have liver disease or conditions affecting vitamin A metabolism

Cod liver oil may be appropriate when:

• You want a supplement containing both omega-3 and vitamin D (but note it is not suitable for treating vitamin D deficiency, which requires licensed vitamin D products at therapeutic doses)

• You have limited dietary intake of both omega-3 fatty acids and fat-soluble vitamins

• You are not pregnant and do not take other vitamin A or D supplements

• You prefer traditional supplementation approaches

UK public health guidance emphasises that:

• Dietary sources are preferred: The NHS recommends consuming at least two portions of fish weekly, including one portion of oily fish (salmon, mackerel, sardines)

• Vitamin D supplementation: UK Government advice is that everyone should consider taking 10 micrograms of vitamin D daily, particularly during autumn and winter

• Pregnancy precautions: Avoid cod liver oil and vitamin A supplements; limit oily fish to two portions weekly due to potential pollutant exposure

Practical recommendations:

• Check existing supplement and medication lists to avoid excessive vitamin intake or interactions

• Choose products with third-party quality certification

• Start with standard doses and take with food to minimise gastrointestinal effects

• If you have cardiovascular disease, diabetes, or take regular medications, discuss supplementation with your GP or pharmacist

• Vitamin D testing is not routinely needed but may be recommended if you're at risk of deficiency or have symptoms

For most UK adults, standard fish oil provides omega-3 benefits without the vitamin A concerns associated with cod liver oil. However, it's important to note that NICE does not recommend over-the-counter omega-3 supplements for cardiovascular disease prevention. Individual circumstances vary, and personalised advice from a healthcare professional ensures safe, appropriate supplementation aligned with your specific health needs and existing treatment regimens.

Frequently Asked Questions