Cod liver oil is a widely used nutritional supplement in the UK, valued for its omega-3 fatty acids and vitamins A and D. Whilst many people tolerate it well, cod liver oil side effects can occur, ranging from mild gastrointestinal discomfort to more serious complications with excessive use. Understanding these potential adverse effects, recognising warning signs, and knowing who should avoid this supplement are essential for safe consumption. This article examines the common and serious side effects of cod liver oil, identifies at-risk groups, and provides practical guidance on safe dosage and minimising unwanted effects in line with UK health recommendations.

What Is Cod Liver Oil and How Does It Work?

Cod liver oil is a nutritional supplement derived from the liver of Atlantic cod (Gadus morhua) and related species. It has been used for centuries as a source of essential nutrients, particularly in northern European countries including the UK. The supplement is rich in omega-3 fatty acids—specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—as well as fat-soluble vitamins A and D.

The potential effects of cod liver oil stem from its constituent nutrients working through several biological mechanisms. Omega-3 fatty acids are incorporated into cell membranes throughout the body, where they influence inflammatory pathways by competing with omega-6 fatty acids in the production of eicosanoids. This may result in the generation of less inflammatory mediators, which could contribute to cardiovascular health and joint function. EPA and DHA also support neurological function, though evidence for cognitive benefits in healthy adults is limited.

Vitamin A (retinol) in cod liver oil plays essential roles in vision, immune function, and cellular differentiation. It acts by binding to nuclear receptors that regulate gene expression. Vitamin D supports calcium absorption in the gut, bone mineralisation, and immune system modulation. It functions as a steroid hormone after conversion to its active form, calcitriol, which binds to vitamin D receptors in various tissues.

Whilst cod liver oil is widely available as an over-the-counter supplement in liquid and capsule forms, it is important to recognise that it is regulated as a food supplement in the UK. The Food Standards Agency (FSA) and local authorities oversee food supplement safety, but potency and purity can vary between manufacturers. Understanding both the potential benefits and possible adverse effects is essential for safe use.

Common Side Effects of Cod Liver Oil

Most individuals tolerate cod liver oil well when taken at recommended doses, but several common side effects may occur, particularly during initial use or with higher doses. These effects are generally mild and often resolve with continued use or dosage adjustment.

Gastrointestinal disturbances are the most frequently reported side effects and include:

• Fishy aftertaste and belching: Many users experience a persistent fishy taste or fish-flavoured reflux, particularly with liquid formulations

• Nausea: Mild queasiness may occur, especially when taken on an empty stomach

• Diarrhoea or loose stools: The high fat content can have a laxative effect in some individuals

• Indigestion and heartburn: Gastric discomfort may develop, particularly in those with sensitive digestive systems or gastro-oesophageal reflux disease (GORD)

• Abdominal bloating: Some people report feelings of fullness or gas

These gastrointestinal effects can often be minimised by taking cod liver oil with meals, choosing enteric-coated capsules, or refrigerating liquid preparations. Starting with a lower dose and gradually increasing may also improve tolerance.

Other common side effects include:

• Halitosis (bad breath): The fishy odour may be noticeable on the breath

• Mild headache: Occasionally reported, particularly when first starting supplementation

These effects are typically transient and do not indicate serious harm. However, if gastrointestinal symptoms are severe or persistent, it may be advisable to discontinue use and consult a healthcare professional. Switching to a different brand or formulation, such as flavoured varieties or capsules rather than liquid, may improve tolerability for some users.

Serious Side Effects and When to Seek Medical Advice

Whilst serious adverse effects from cod liver oil are uncommon at recommended doses, certain complications can occur, particularly with excessive intake or in susceptible individuals. Recognising these warning signs is crucial for safety.

Vitamin A toxicity (hypervitaminosis A) represents the most significant risk associated with regular cod liver oil consumption. Because vitamin A is fat-soluble and stored in the liver, excessive intake can accumulate to toxic levels. Symptoms of chronic vitamin A toxicity include:

• Severe headaches and dizziness

• Blurred vision or other visual disturbances

• Nausea and vomiting

• Bone and joint pain

• Dry, itchy skin and hair loss

• Liver damage (hepatotoxicity)

• In severe cases, increased intracranial pressure

Pregnant women should avoid cod liver oil due to the risk of vitamin A to foetal development. If you are pregnant and have been taking cod liver oil, stop taking it and discuss this with your midwife or GP at your next appointment.

Vitamin D toxicity (hypervitaminosis D), though rarer, can occur with very high doses over prolonged periods. This leads to hypercalcaemia (elevated blood calcium), which may cause confusion, excessive thirst, kidney stones, and cardiac arrhythmias.

Bleeding complications may arise because omega-3 fatty acids have mild antiplatelet effects. At typical supplement doses, clinically significant bleeding is unlikely, but caution is warranted if you take anticoagulants or antiplatelet medications. Warning signs include:

• Unexplained bruising or prolonged bleeding from minor cuts

• Blood in urine or stools

• Nosebleeds that are difficult to stop

• Unusual bleeding from gums

When to seek medical advice:

Contact your GP promptly if you experience persistent gastrointestinal symptoms, signs of vitamin toxicity, or unusual bleeding. Seek immediate medical attention (call 999 or attend A&E) if you develop severe symptoms such as significant bleeding, severe abdominal pain, difficulty breathing, or signs of allergic reaction including facial swelling or widespread rash. For symptoms such as yellowing of skin or eyes (jaundice), contact your GP urgently or call NHS 111 for advice.

Who Should Avoid Cod Liver Oil?

Certain groups of individuals should exercise caution or avoid cod liver oil entirely due to increased risk of adverse effects or potential interactions with existing medical conditions.

Pregnant women should not take cod liver oil due to the risk of excessive vitamin A intake, which can cause congenital abnormalities. The NHS and NICE recommend that pregnant women avoid supplements containing vitamin A, including cod liver oil and fish liver oils. Alternative omega-3 supplements derived from fish body oils (rather than liver) may be safer options, but should only be taken after discussion with a midwife or GP.

Breastfeeding women should exercise caution with cod liver oil due to its vitamin A content and discuss its use with a healthcare professional.

Individuals with bleeding disorders or those taking anticoagulant medications (such as warfarin, apixaban, rivaroxaban) or antiplatelet drugs (including aspirin, clopidogrel) should use cod liver oil only under medical supervision. The omega-3 fatty acids may increase bleeding risk. If you are scheduled for surgery, inform your surgical team about cod liver oil use, as they may advise you to stop taking it before your procedure.

People with fish or seafood allergies may experience allergic reactions to cod liver oil, ranging from mild skin reactions to severe anaphylaxis. Those with known allergies should avoid this supplement entirely.

Individuals with liver disease should exercise caution, as the liver is responsible for vitamin A storage and metabolism. Impaired hepatic function increases the risk of vitamin A accumulation and toxicity.

Older adults (over 65) should be particularly careful about vitamin A intake, as excessive amounts may increase the risk of bone fractures.

Other groups requiring caution include those taking medications that interact with vitamins A or D, such as:

• Retinoid medications (isotretinoin, acitretin) – increased risk of vitamin A toxicity

• Tetracycline antibiotics – potential risk of increased intracranial pressure when combined with vitamin A

• Thiazide diuretics – may increase risk of hypercalcaemia with vitamin D

• Digoxin – hypercalcaemia from vitamin D may affect cardiac function

• Orlistat or cholestyramine – may reduce absorption of fat-soluble vitamins

If you fall into any of these categories, consult your GP or a registered pharmacist before starting cod liver oil supplementation.

Safe Dosage and How to Minimise Side Effects

Adhering to recommended dosages and following practical guidance can significantly reduce the risk of side effects whilst allowing individuals to benefit from cod liver oil supplementation.

Recommended dosages:

The NHS advises that adults should not consume more than 1.5 mg of vitamin A per day from food and supplements combined. Vitamin A content in cod liver oil varies considerably between products, so it is essential to check the label of your specific supplement. For vitamin D, the UK government recommends 10 micrograms (400 IU) daily for most adults, particularly during autumn and winter months.

Most manufacturers recommend one teaspoon (5 ml) of liquid or 1–2 capsules daily. It is crucial to read product labels carefully, as vitamin content varies between brands. Never exceed the stated dose without medical advice.

Strategies to minimise side effects:

• Take with food: Consuming cod liver oil with meals, particularly those containing other fats, improves absorption and reduces gastrointestinal upset

• Start with a lower dose: Begin with half the recommended dose and gradually increase over 1–2 weeks to allow your digestive system to adapt

• Choose quality products: Select supplements from reputable manufacturers that comply with UK food supplement regulations and provide clear labelling of vitamin content

• Refrigerate after opening: This helps preserve freshness and may reduce fishy aftertaste

• Consider capsule formulations: Enteric-coated or flavoured capsules can minimise taste-related issues

• Time your dose: Some people find taking cod liver oil in the evening reduces awareness of any fishy aftertaste

Monitoring and review:

If you take cod liver oil regularly, particularly alongside other supplements or medications, periodic review with your GP is advisable. Blood tests may be warranted if you have risk factors for vitamin toxicity or are taking high doses. Keep a record of all supplements you take and inform healthcare professionals during consultations, as this information is essential for safe prescribing and avoiding interactions.

If side effects persist despite these measures, discontinue use and seek advice from a pharmacist or GP, who may suggest alternative omega-3 sources such as standard fish oil supplements (which contain lower levels of vitamins A and D) or plant-based options for those who prefer non-animal sources.

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

Frequently Asked Questions