Cod liver oil is a popular dietary supplement rich in omega-3 fatty acids and vitamins A and D, traditionally used to support general health. Whilst many people tolerate it well, cod liver oil side effects can occur, ranging from mild gastrointestinal discomfort to more serious complications in certain individuals. Understanding these potential adverse effects, who should avoid the supplement, and how to minimise risks is essential for safe use. This article examines the common and serious side effects of cod liver oil, provides guidance on when to seek medical advice, and offers practical strategies to reduce unwanted symptoms whilst supplementing safely.

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). It is a rich natural source of omega-3 fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), as well as fat-soluble vitamins A and D. This distinguishes it from standard fish oil supplements, which contain omega-3s but not significant amounts of these vitamins. Historically, cod liver oil has been used for centuries in Northern European countries to support general health, particularly during winter months when sunlight exposure is limited.

The therapeutic effects of cod liver oil are primarily attributed to its omega-3 fatty acid content. EPA and DHA play crucial roles in cellular membrane structure and function throughout the body. These polyunsaturated fatty acids exert anti-inflammatory effects by modulating the production of inflammatory mediators called eicosanoids. They compete with arachidonic acid in metabolic pathways, leading to the production of less inflammatory prostaglandins and leukotrienes. This mechanism may contribute to cardiovascular benefits, including modest reductions in triglyceride levels and potential improvements in endothelial function.

Vitamin D in cod liver oil supports calcium absorption and bone health, whilst vitamin A is essential for vision, immune function, and cellular differentiation. However, the concentrations of these vitamins can vary significantly between products. Adults should not exceed 1.5 mg (1,500 µg) of vitamin A per day from all sources, and the UK reference nutrient intake for vitamin D is 10 µg daily, with an upper safe level of 100 µg daily. The UK Scientific Advisory Committee on Nutrition (SACN) recognises the potential benefits of omega-3 fatty acids for cardiovascular health, though evidence remains mixed for other conditions.

The NHS recommends eating at least two portions of fish per week, including one portion of oily fish, as part of a balanced diet. Supplements are not a substitute for healthy eating.

Cod liver oil is available over-the-counter in various formulations, including liquid preparations and capsules. It is classified as a food supplement rather than a medicine in the UK, meaning it is not subject to the same rigorous regulatory oversight as pharmaceutical products. Consumers should be aware that quality, purity, and potency can vary considerably between manufacturers.

Common Side Effects of Cod Liver Oil

Most people tolerate cod liver oil well when taken at recommended doses, but several common side effects may occur, particularly during the initial weeks of supplementation. These effects are generally mild and often resolve as the body adjusts to the supplement.

Gastrointestinal disturbances are the most frequently reported side effects. These include:

• Fishy aftertaste and belching – Many users experience a persistent fishy taste or fish-flavoured reflux, which can be unpleasant

• Nausea and indigestion – Mild stomach upset may occur, especially when taken on an empty stomach

• Diarrhoea or loose stools – Higher doses of omega-3 fatty acids can have a laxative effect

• Abdominal discomfort or bloating – Some individuals report feelings of fullness or mild cramping

Other common effects include bad breath with a fishy odour. Some users may occasionally experience mild headaches, though this is uncommon. Some users notice their stools become softer or slightly oily in appearance, which reflects the fat content of the supplement.

These gastrointestinal effects are dose-dependent and typically more pronounced with liquid formulations compared to capsules. The fishy aftertaste, whilst not medically concerning, is often cited as the primary reason for discontinuation. It's worth noting that these common side effects do not indicate an allergic reaction or serious adverse event.

The frequency and severity of these symptoms vary considerably between individuals. Factors influencing tolerability include the dose taken, whether the supplement is consumed with food, the quality and freshness of the product, and individual digestive sensitivity. Most common side effects are considered nuisance symptoms rather than clinically significant adverse events and do not require medical intervention.

If gastrointestinal symptoms are troublesome or persist despite trying the administration strategies outlined later in this article, contact your pharmacist or GP for advice.

Serious Side Effects and When to Seek Medical Advice

Whilst serious adverse effects from cod liver oil are uncommon, certain complications warrant medical attention. Understanding these potential risks is essential for safe supplementation.

Bleeding complications are a potential concern, though clinically significant bleeding is uncommon at typical supplement doses. Omega-3 fatty acids possess mild antiplatelet properties, which may affect bleeding time. This risk may increase with higher doses or when combined with anticoagulants or antiplatelet medications. Warning signs include:

• Unexplained bruising or purpura (purple spots on skin)

• Prolonged bleeding from minor cuts

• Blood in urine or stools

• Nosebleeds that are difficult to stop

• Unusual bleeding from gums

If you experience any unexplained bleeding, contact your GP promptly. For severe or uncontrolled bleeding, call 999 or attend A&E immediately.

Allergic reactions can occur in individuals with fish allergies (note that fish and shellfish allergies are distinct). Seek immediate medical help if you develop:

• Difficulty breathing or wheezing

• Swelling of face, lips, tongue, or throat (angioedema)

• Widespread urticarial rash (hives)

• Dizziness or feeling faint

Call 999 immediately if you experience symptoms of anaphylaxis (severe allergic reaction).

Vitamin toxicity is a potential concern with long-term, high-dose cod liver oil consumption. Excessive vitamin A (hypervitaminosis A) can cause headaches, blurred vision, nausea, dizziness, and in severe cases, liver damage. Adults should not exceed 1.5 mg (1,500 µg) of vitamin A daily from all sources. Vitamin D toxicity (hypervitaminosis D) may lead to hypercalcaemia, presenting with excessive thirst, frequent urination, confusion, or kidney problems. The adult upper safe level for vitamin D is 100 µg daily.

Other reasons to seek medical advice include persistent severe abdominal pain, unexplained fatigue, jaundice (yellowing of skin or eyes), or significant changes in mood. If you develop any concerning symptoms whilst taking cod liver oil, discontinue use and consult your GP. Never ignore symptoms that are severe, persistent, or worsening.

You can report suspected side effects from supplements to the Medicines and Healthcare products Regulatory Agency (MHRA) through the Yellow Card Scheme at yellowcard.mhra.gov.uk.

Who Should Avoid Cod Liver Oil?

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

People with bleeding disorders or those taking anticoagulant or antiplatelet medications should seek medical advice before taking cod liver oil. This includes individuals on:

• Warfarin or other vitamin K antagonists

• Direct oral anticoagulants (DOACs) such as apixaban, rivaroxaban, or dabigatran

• Antiplatelet agents including aspirin, clopidogrel, or prasugrel

• Non-steroidal anti-inflammatory drugs (NSAIDs) taken regularly

The combination may increase bleeding risk, and supplementation should be discussed with your GP or pharmacist.

Individuals with fish allergies should avoid cod liver oil due to the risk of allergic reactions, which can range from mild skin reactions to life-threatening anaphylaxis. Shellfish allergy is distinct from fish allergy, but you should always check product allergen information for potential cross-contamination risks.

Pregnant women should avoid cod liver oil due to its high vitamin A content, which poses teratogenic risks (may cause birth defects). The NHS specifically advises pregnant women to avoid cod liver oil and vitamin A supplements. Breastfeeding women should consult their GP, midwife or health visitor before taking cod liver oil, as high doses of vitamin A should be avoided during lactation.

People with liver disease should seek medical advice, as both the supplement's metabolism and the risk of vitamin A accumulation may be affected by hepatic impairment. Similarly, those with diabetes should discuss with their healthcare provider, as some evidence suggests high-dose omega-3 supplements might affect glycaemic control in some individuals, though evidence is inconsistent.

Individuals scheduled for surgery should inform their surgical and anaesthetic team about all supplements they take, including cod liver oil. They may advise stopping the supplement 1-2 weeks before surgery based on individual risk assessment and the type of procedure.

How to Minimise Side Effects When Taking Cod Liver Oil

Several practical strategies can significantly reduce the likelihood and severity of side effects associated with cod liver oil supplementation.

Timing and administration techniques are crucial for minimising gastrointestinal symptoms:

• Take with meals – Consuming cod liver oil with food, particularly meals containing other fats, improves absorption and reduces stomach upset

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

• Divide the daily dose – If taking multiple capsules, split them between meals rather than taking all at once

• Freeze capsules – Storing capsules in the freezer can reduce fishy aftertaste and reflux, as the oil remains solid longer in the stomach

Product selection matters considerably. Choose high-quality supplements that have been tested for purity and are free from contaminants such as heavy metals (mercury, lead) and persistent organic pollutants. Look for products certified by independent testing organisations. Enteric-coated capsules dissolve in the intestine rather than the stomach, which can substantially reduce fishy burps and nausea.

Dosage considerations are important for safety. Always follow the product label instructions. For general health, the NHS recommends obtaining omega-3s through diet (at least one portion of oily fish weekly). The European Food Safety Authority (EFSA) notes that 250 mg daily of combined EPA and DHA contributes to normal heart function. Higher intakes should be supervised by a healthcare professional. Always check the vitamin A and D content on the label and ensure you don't exceed 1.5 mg (1,500 µg) of vitamin A daily or 100 µg of vitamin D daily from all sources.

Monitor for interactions by maintaining a list of all medications and supplements you take. Inform your GP and pharmacist about cod liver oil use, particularly before starting new medications. Regular review of your supplement regimen ensures continued appropriateness.

If side effects persist despite these measures, consider alternative omega-3 sources such as algal oil supplements (suitable for vegetarians) or increasing dietary intake of oily fish like salmon, mackerel, or sardines, which provide omega-3s without the concentrated vitamins found in cod liver oil.

Frequently Asked Questions