Omega-3 fish oil supplements are widely used in the UK for their potential cardiovascular and anti-inflammatory properties, but like all supplements, they can cause side effects. Whilst most people tolerate these products well, common issues include fishy aftertaste, gastrointestinal discomfort, and mild effects on bleeding tendency. Understanding omega-3 fish oil side effects helps you make informed decisions about supplementation and recognise when medical advice is needed. This article examines the range of side effects associated with fish oil supplements, from minor inconveniences to rare but serious complications, and provides practical guidance on safe use in line with UK clinical recommendations.

What Are Omega-3 Fish Oil Supplements?

Omega-3 fish oil supplements are dietary products derived from the tissues of oily fish, such as mackerel, salmon, sardines, and anchovies. These supplements contain two primary long-chain polyunsaturated fatty acids: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These omega-3 fatty acids are not technically essential, as the body can produce them in small amounts from alpha-linolenic acid (ALA), which is the essential omega-3 fatty acid. However, this conversion is inefficient, making dietary intake or supplementation important for optimal health.

These fatty acids play roles in cardiovascular health, brain function, and inflammatory regulation. EPA and DHA are incorporated into cell membranes throughout the body, where they influence membrane fluidity and cellular signalling pathways. They also serve as precursors to compounds that may help resolve inflammation, though clinical evidence for these mechanisms translating to health benefits remains limited.

In the UK, omega-3 fish oil supplements are widely available over the counter as food supplements. There are also prescription-only medicinal products such as Omacor (omega-3-acid ethyl esters), which is licensed for treating hypertriglyceridaemia, and Vazkepa (icosapent ethyl), which may be used in specific high-risk patients according to NICE guidance. The Medicines and Healthcare products Regulatory Agency (MHRA) regulates medicinal fish oil products, whilst food supplements fall under Food Standards Agency regulations.

Typical supplement doses range from 250 mg to 1,000 mg of combined EPA and DHA daily, though therapeutic doses for specific conditions may be higher (2-4 g daily) and should only be taken under medical supervision. It's worth noting that current NICE guidance does not recommend omega-3 compounds for the prevention of cardiovascular disease. The NHS advises eating two portions of fish per week, one of which should be oily, as part of a balanced diet.

Common Side Effects of Omega-3 Fish Oil

Most people tolerate omega-3 fish oil supplements well, but several common side effects may occur, particularly at higher doses. These effects are generally mild and often resolve with continued use or dose adjustment.

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

• Fishy aftertaste and belching – Many users experience a characteristic fishy taste or odour in their breath, which can be unpleasant but is not medically concerning

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

• Diarrhoea or loose stools – Higher doses can have a laxative effect due to the fat content

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

Altered bleeding tendency is another recognised effect, though significant bleeding complications are uncommon at standard supplemental doses. Omega-3 fatty acids have mild antiplatelet properties, which may prolong bleeding time slightly. This effect becomes more clinically relevant at higher doses or when combined with anticoagulant or antiplatelet medications.

Mild skin reactions, including occasional rashes or itching, are relatively uncommon. A small proportion of people notice changes in their sense of taste beyond the fishy aftertaste.

Headaches have been reported in clinical trials, though the incidence is similar to placebo groups, suggesting there is no definitive causal link. Similarly, some individuals report feeling more tired when initiating supplementation, but this effect is not consistently demonstrated in research.

Other potential effects include a possible increase in LDL-cholesterol levels and mild, asymptomatic elevations in liver enzymes, particularly with higher medicinal doses. These effects are generally monitored in patients taking prescription omega-3 products.

These common side effects rarely necessitate discontinuation of supplementation and can often be managed through simple strategies such as taking capsules with meals or switching to enteric-coated formulations.

Serious Side Effects and When to Seek Medical Advice

Whilst serious adverse effects from omega-3 fish oil supplements are uncommon, certain situations warrant immediate medical attention or consultation with a healthcare professional.

Significant bleeding or bruising represents an important potential complication. Although omega-3 fatty acids have antiplatelet effects, clinically significant bleeding is rare at recommended doses. However, you should call 999 or go to A&E if you experience severe bleeding, or contact your GP or NHS 111 if you notice:

• Unexplained or excessive bruising

• Prolonged bleeding from minor cuts

• Blood in urine or stools (which may appear black or tarry)

• Unusual nosebleeds or bleeding gums

• Vomiting blood or material resembling coffee grounds

These symptoms may indicate an increased bleeding tendency, particularly if you are taking blood-thinning medications concurrently.

Allergic reactions to fish oil supplements, whilst uncommon, can occur. Call 999 or go to A&E immediately if you develop:

• Difficulty breathing or swallowing

• Swelling of the face, lips, tongue, or throat

• Widespread rash or hives

• Dizziness or feeling faint

These symptoms may indicate anaphylaxis, a medical emergency requiring urgent treatment.

Atrial fibrillation has been identified in recent large-scale trials as a potential concern with high-dose omega-3 supplementation (typically 4 grams daily or more, particularly with icosapent ethyl). If you experience palpitations, irregular heartbeat, chest discomfort, or unexplained breathlessness, contact your GP promptly for assessment.

Persistent or severe gastrointestinal symptoms that do not improve with dose adjustment or formulation changes should be discussed with your healthcare provider, as they may indicate intolerance or an underlying condition requiring investigation.

Signs of liver problems such as yellowing of the skin or eyes (jaundice), dark urine, severe itching, or right upper abdominal pain should prompt you to stop taking the supplement and seek medical advice.

Patients with diabetes should continue their usual monitoring when taking fish oil supplements. While significant effects on blood glucose are uncommon, any unexplained changes in blood sugar control warrant medical review.

If you suspect you've experienced a side effect from an omega-3 supplement, you can report it through the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk.

Who Should Avoid or Use Omega-3 Fish Oil with Caution?

Certain individuals should exercise caution or avoid omega-3 fish oil supplements altogether due to increased risk of adverse effects or potential drug interactions.

People with fish or seafood allergies should generally avoid fish oil supplements, as allergic reactions are possible. However, highly purified fish oil preparations may contain minimal allergenic protein. If you have a known fish allergy, consult your GP or allergist before considering supplementation. Alternative omega-3 sources, such as algal oil (which provides DHA and EPA from marine algae), may be suitable alternatives. Also check product excipients, as some contain other potential allergens such as soya.

Patients taking anticoagulant or antiplatelet medications require careful consideration before starting fish oil supplements. These medications include:

• Warfarin, apixaban, rivaroxaban, or other anticoagulants

• Aspirin, clopidogrel, or other antiplatelet agents

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

Whilst the combination is not absolutely contraindicated, the additive effect on bleeding tendency may increase risk. Always inform your GP or anticoagulation clinic before starting fish oil supplements, and they may recommend additional monitoring.

Individuals with bleeding disorders (such as haemophilia or von Willebrand disease) should consult their haematologist before using omega-3 supplements due to the theoretical increased bleeding risk.

People scheduled for surgery should discuss omega-3 supplements with their surgical team during pre-operative assessment. Many clinicians advise stopping supplements about a week before elective procedures, though this should be individualised based on your specific situation and bleeding risk.

Patients with diabetes can generally use omega-3 supplements safely. While high doses have shown minimal impact on blood glucose in most studies, continue your usual monitoring as per standard diabetes care.

Individuals with liver disease should seek medical advice before taking fish oil supplements. Liver enzyme elevations can occur with high-dose medicinal omega-3 products, and monitoring may be recommended for those with hepatic impairment.

Pregnant and breastfeeding women should consult their midwife or GP before taking fish oil supplements. Whilst omega-3 fatty acids are important for foetal development, certain fish oil products may contain contaminants. Importantly, fish liver oil supplements should be avoided during pregnancy due to their vitamin A content. Choose products specifically labelled as suitable for pregnancy.

How to Minimise Side Effects When Taking Fish Oil

Several practical strategies can help reduce the likelihood and severity of side effects associated with omega-3 fish oil supplementation, improving tolerability and adherence.

Timing and administration techniques can significantly impact gastrointestinal side effects:

• Take supplements with meals – Consuming fish oil capsules with food, particularly meals containing some fat, enhances absorption and reduces stomach upset

• Divide the daily dose – Rather than taking the entire dose at once, split it between two or three meals throughout the day

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

• Take supplements at bedtime – Some people find that taking fish oil before sleep minimises awareness of any fishy aftertaste

Choosing the right formulation can make a substantial difference:

• Enteric-coated capsules are designed to dissolve in the intestine rather than the stomach, significantly reducing fishy burps and nausea

• High-quality, purified products from reputable UK suppliers are less likely to cause side effects and should meet FSA regulations for contaminant levels

• Flavoured or deodorised formulations may be better tolerated by those sensitive to fishy taste

• Liquid formulations with added flavourings (such as lemon or orange) provide an alternative to capsules

• Algal oil supplements offer a fish-free alternative source of EPA and DHA for those who cannot tolerate fish products

Dose optimisation is crucial for minimising side effects:

• Start with a lower dose and gradually increase over several weeks to allow your digestive system to adapt

• Do not exceed the manufacturer's recommended dose – higher doses (2-4 g daily) should only be taken under medical supervision

• Consider your total omega-3 intake from both diet and supplements to avoid excessive consumption

Storage and quality considerations help maintain supplement integrity:

• Store fish oil supplements in a cool, dark place or refrigerate them to prevent oxidation

• Check expiration dates and discard supplements that smell rancid or unusually strong

• Purchase products from reputable UK suppliers that meet quality standards

If side effects persist despite these measures, consult your GP or pharmacist, who may recommend alternative formulations or investigate whether the symptoms might be unrelated to the supplement.

Frequently Asked Questions