Fish liver oil, commonly derived from cod, is a popular dietary supplement rich in omega-3 fatty acids and vitamins A and D. Whilst many people take fish liver oil for general health support, it can cause side effects ranging from mild gastrointestinal discomfort to serious complications such as vitamin toxicity or increased bleeding risk. Understanding fish liver oil side effects is essential for safe use, particularly for pregnant women, those taking anticoagulant medications, or individuals with liver or kidney disease. This article examines common and serious adverse effects, identifies who should avoid fish liver oil, and provides guidance on safe supplementation in line with UK health recommendations.

What Is Fish Liver Oil and How Does It Work?

Fish liver oil is a dietary supplement derived from the livers of fish, most commonly cod (cod liver oil). Unlike standard fish oil, which is extracted from the body tissue of oily fish, fish liver oil is particularly rich in fat-soluble vitamins A and D, in addition to omega-3 fatty acids (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]).

The omega-3 fatty acids in fish liver oil may incorporate into cell membranes throughout the body, where they may exert anti-inflammatory effects and influence various metabolic pathways. EPA and DHA may help reduce triglyceride levels, might modestly lower blood pressure, and may contribute to normal heart rhythm. Vitamin A supports vision, immune function, and skin health, whilst vitamin D is crucial for calcium absorption, bone health, and immune function.

In the UK, fish liver oil supplements are classified as food supplements, not medicines, and are regulated by the Food Standards Agency (FSA) rather than the Medicines and Healthcare products Regulatory Agency (MHRA). They are widely available over the counter. It's important to note that prescription-strength omega-3 medicines (such as icosapent ethyl) are distinct from supplements and are used under medical supervision.

Whilst many people take fish liver oil for general health maintenance, it is important to recognise that supplements can cause side effects and interact with medications. The concentration of vitamins A and D in fish liver oil distinguishes it from standard fish oil supplements and requires particular attention to dosing, as both vitamins can accumulate in the body and cause toxicity at excessive levels.

Common Side Effects of Fish Liver Oil

Most people tolerate fish liver oil well when taken at recommended doses, but several common side effects may occur, particularly when starting supplementation or taking higher doses. The most frequently reported adverse effects are gastrointestinal in nature and include:

• Fishy aftertaste and belching – perhaps the most common complaint, often described as persistent fishy breath or reflux

• Nausea and indigestion – particularly when taken on an empty stomach

• Diarrhoea or loose stools – due to the high fat content affecting digestive processes

• Abdominal discomfort or bloating – resulting from delayed gastric emptying

These gastrointestinal effects can often be minimised by taking fish liver oil with meals, choosing enteric-coated capsules, or refrigerating liquid formulations. Some individuals find that starting with a lower dose and gradually increasing helps the digestive system adapt.

At standard doses, mild anticoagulant properties of omega-3 fatty acids rarely cause problems in healthy individuals. However, some people may notice a slightly increased tendency to bruise or experience minor nosebleeds, particularly at higher doses or when combined with medications that affect blood clotting.

Allergic reactions are uncommon but possible, particularly in individuals with fish allergies. People with shellfish allergies should check product labels and manufacturer information, as cross-contamination may occur in some products. Symptoms of allergic reactions may include skin rash, itching, or mild swelling.

It is worth noting that fish liver oil supplements vary considerably in purity and processing methods. Choosing supplements that have been tested for heavy metals, PCBs, and other pollutants according to FSA standards can help ensure product quality.

Serious Side Effects and When to Seek Medical Advice

Whilst serious adverse effects from fish liver oil are uncommon at recommended doses, they can occur, particularly with prolonged use of high doses or in individuals with certain underlying conditions. The most significant concerns relate to vitamin A and D toxicity (hypervitaminosis), as these fat-soluble vitamins accumulate in the liver and fatty tissues.

Vitamin A toxicity may develop with chronic excessive intake and can manifest as:

• Severe headaches, dizziness, or visual disturbances

• Nausea, vomiting, and loss of appetite

• Dry, itchy skin and hair loss

• Bone and joint pain

• Liver damage (hepatotoxicity) in severe cases

Vitamin D toxicity is less common but potentially serious, causing hypercalcaemia (elevated blood calcium levels) with symptoms including:

• Excessive thirst and frequent urination

• Weakness, fatigue, and confusion

• Kidney stones or kidney damage

• Cardiac arrhythmias in severe cases

Bleeding complications represent another serious concern, particularly in individuals taking anticoagulant medications (warfarin, apixaban, rivaroxaban) or antiplatelet drugs (aspirin, clopidogrel). Omega-3 fatty acids have mild blood-thinning properties, and high doses may increase bleeding risk. Call 999 or go to A&E immediately if you experience unusual bruising, blood in urine or stools, prolonged bleeding from cuts, or any signs of internal bleeding.

You should contact your GP promptly if you develop persistent severe abdominal pain, jaundice (yellowing of skin or eyes), significant changes in vision, severe headaches, or symptoms of hypercalcaemia. Additionally, if you are scheduled for surgery, inform your surgeon that you take fish liver oil, as they may advise you to stop taking it before your procedure according to local hospital guidelines. Anyone experiencing signs of a severe allergic reaction—such as difficulty breathing, facial swelling, or widespread rash—should call 999 or go to A&E immediately.

If you suspect you've experienced a side effect from fish liver oil, you can report it through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Who Should Avoid Fish Liver Oil?

Certain groups of people should avoid fish liver oil or use it only under medical supervision due to increased risk of adverse effects or complications. Pregnant women should exercise particular caution, as excessive vitamin A intake during pregnancy is associated with teratogenic effects (birth defects), particularly affecting the developing nervous system and heart. The NHS and NICE guidance (NG201) recommend that pregnant women avoid supplements containing vitamin A, including fish liver oil, and avoid liver products. Whilst omega-3 fatty acids are beneficial during pregnancy, standard fish oil supplements (without added vitamins A and D) are generally considered safer alternatives.

Individuals with liver disease should avoid fish liver oil, as the liver is the primary storage site for vitamins A and D, and impaired hepatic function increases the risk of toxicity. Similarly, those with kidney disease should consult their doctor before use, as vitamin D metabolism is altered in renal impairment, potentially leading to dangerous calcium imbalances.

People taking anticoagulant or antiplatelet medications should discuss fish liver oil with their GP or anticoagulation clinic, as the combination may possibly increase bleeding risk. Those taking warfarin should be particularly cautious as monitoring of INR (International Normalised Ratio) may be needed. Those with fish allergies should avoid fish liver oil due to the risk of allergic reactions. People with shellfish allergies should check product labels and manufacturer information regarding potential cross-contamination.

Individuals with hypercalcaemia or hyperparathyroidism should avoid fish liver oil due to its vitamin D content, which could worsen elevated calcium levels. Finally, those scheduled for surgery should inform their surgical team about fish liver oil use and follow their specific advice about when to stop taking it before the procedure.

Safe Use and Dosage Recommendations

Safe use of fish liver oil requires attention to dosage, quality, and individual circumstances. In the UK, there is no single official recommended dose for fish liver oil, as it is classified as a food supplement rather than a medicine. However, guidance exists for the individual components.

The UK Expert Group on Vitamins and Minerals (EVM) has established a Safe Upper Level for vitamin A of 1.5 mg (5,000 IU) daily from all sources, including supplements and fortified foods. The NHS advises avoiding long-term intake above this level. For vitamin D, Public Health England recommends 10 micrograms (400 IU) daily for most adults, with an upper limit of 100 micrograms (4,000 IU) per day. Higher doses should only be taken under medical supervision.

Fish liver oil supplements vary considerably in their vitamin content. Products may provide anywhere from 800–4,000 IU of vitamin A and 200–2,000 IU of vitamin D per standard dose (typically one teaspoon or 1–2 capsules). It is essential to read product labels carefully and avoid taking multiple supplements containing these vitamins simultaneously, as this increases the risk of exceeding safe upper limits. The omega-3 content also varies between products but typically provides 200–500 mg of combined EPA and DHA per dose.

To minimise side effects and maximise safety:

• Take fish liver oil with food to reduce gastrointestinal upset

• Start with a lower dose and increase gradually if tolerated

• Choose high-quality supplements tested for purity and contaminants (look for third-party certification)

• Store according to manufacturer instructions to prevent rancidity

• Avoid exceeding the recommended dose on the product label

• Keep supplements out of reach of children, as accidental overdose can be serious

If you are taking prescribed medications, particularly anticoagulants or antiplatelet drugs, consult your GP or pharmacist before starting fish liver oil to check for potential interactions. Regular monitoring may be necessary for those on anticoagulation therapy. For most healthy adults not in the at-risk groups mentioned previously, fish liver oil at recommended doses is generally safe, but standard fish oil supplements (without added vitamins A and D) may be a more appropriate choice for long-term daily use, particularly if you already obtain adequate vitamins A and D from your diet or other supplements.

Frequently Asked Questions