Fish oil supplements, rich in omega-3 fatty acids EPA and DHA, are widely used for cardiovascular and general health support. Whilst generally well tolerated, fish oil side effects can occur, ranging from mild gastrointestinal discomfort to rare but serious complications. Understanding these potential adverse effects, recognising warning signs, and knowing how to minimise symptoms are essential for safe supplementation. This article examines common and serious side effects, identifies individuals who should exercise caution, explores medication interactions, and provides practical strategies to improve tolerability. Always consult your GP or pharmacist before starting fish oil supplements, particularly if you have existing health conditions or take prescribed medications.

What Are the Common Side Effects of Fish Oil?

Fish oil supplements, which contain omega-3 fatty acids (primarily eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]), are generally well tolerated by most individuals. However, several common side effects may occur, particularly when starting supplementation or taking higher doses.

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

• Fishy aftertaste or burping (eructation)

• Nausea and mild stomach discomfort

• Diarrhoea or loose stools

• Indigestion and bloating

These symptoms typically occur due to reflux or regurgitation of the oil. The characteristic fishy taste often persists for several hours after ingestion, which many people find unpleasant.

Less common effects may include headaches and, rarely, skin reactions such as mild rashes, though evidence for these is limited.

The severity and frequency of these side effects often correlate with dosage. Standard supplementation (typically 250–500 mg combined EPA and DHA daily) tends to produce fewer adverse effects than higher doses. Most common side effects are mild, self-limiting, and do not require discontinuation of supplementation. However, if symptoms persist or worsen, it is advisable to consult with a GP or pharmacist to discuss alternative formulations or dosing strategies.

It's important to note that high-dose omega-3 therapy (2–4 grams daily) is typically provided through licensed medicines prescribed for specific conditions under medical supervision, rather than through standard supplements.

Serious Side Effects and When to Seek Medical Advice

Whilst serious adverse effects from fish oil supplementation are uncommon, certain symptoms warrant medical attention. Understanding these warning signs is essential for patient safety.

Bleeding complications represent a potential concern, though clinically significant bleeding is uncommon at typical supplement doses. Omega-3 fatty acids possess mild antiplatelet properties that may affect blood clotting. Seek medical advice if you experience:

• Unusual or excessive bruising

• Blood in urine or stools

• Frequent nosebleeds or bleeding gums

• Prolonged bleeding from cuts

Allergic reactions, though rare, can occur, particularly in individuals with fish or shellfish allergies. Symptoms may include urticaria (hives), facial or throat swelling (angioedema), difficulty breathing, or anaphylaxis. Any signs of a severe allergic reaction constitute a medical emergency requiring immediate attention via 999.

Cardiovascular concerns: Some studies suggest that high-dose prescription omega-3 products may increase the risk of atrial fibrillation in certain populations. This risk is primarily associated with prescription doses rather than standard supplements. If you experience palpitations, irregular heartbeat, chest pain, or unexplained breathlessness, contact your GP promptly.

Other symptoms requiring medical review include severe or persistent abdominal pain or significant unexplained symptoms after starting fish oil supplements.

The MHRA advises reporting any suspected adverse reactions to medicines and supplements through the Yellow Card Scheme to contribute to ongoing safety monitoring.

Who Should Avoid Taking Fish Oil Supplements?

Certain individuals should exercise caution or avoid fish oil supplementation altogether due to increased risk of adverse effects or potential health complications. A thorough medical assessment is essential before commencing supplementation in these groups.

Individuals with bleeding disorders or those taking anticoagulant medications should use fish oil with caution and only under healthcare provider supervision. This includes people with haemophilia, von Willebrand disease, or thrombocytopenia. The mild antiplatelet effects of omega-3 fatty acids may require additional monitoring in these conditions.

People with fish or seafood allergies should approach fish oil supplements with considerable caution. Whilst highly purified fish oils may contain minimal allergenic proteins, cross-reactivity remains possible. Alternative omega-3 sources, such as algal oil supplements (which provide DHA from marine algae), may be more appropriate for this population. Always consult an allergist before considering fish oil if you have a documented seafood allergy.

Pregnant and breastfeeding women should seek medical advice before taking fish oil supplements. Standard fish oil supplements (not fish liver oil, which contains vitamin A) are generally considered acceptable during pregnancy. The NHS recommends that pregnant women obtain omega-3s primarily through dietary sources of low-mercury fish, following specific NHS guidance on fish consumption during pregnancy.

Additional caution is warranted for:

• Individuals scheduled for surgery (inform your surgical team about all supplements and follow their specific advice)

• People with diabetes (monitor blood glucose when starting supplements)

• Those with a history of atrial fibrillation

Always disclose fish oil supplementation to healthcare professionals before any medical procedures or when prescribed new medications.

How to Reduce Side Effects When Taking Fish Oil

Several evidence-based strategies can significantly minimise the common side effects associated with fish oil supplementation, improving tolerability and adherence.

Timing and administration techniques play a crucial role in reducing gastrointestinal symptoms. Taking fish oil capsules with meals, particularly those containing fat, enhances absorption and reduces stomach upset. The presence of food helps dilute the oil, minimising digestive discomfort. Dividing the daily dose into two or three smaller doses taken throughout the day, rather than a single large dose, often improves tolerance substantially.

Choosing the right formulation can make a considerable difference. Consider these options:

• Enteric-coated capsules: These have a special coating that prevents dissolution until reaching the small intestine, which may reduce fishy burps and aftertaste

• High-quality products from reputable UK brands: Choose supplements that meet quality standards

• Liquid formulations with flavouring: Some individuals tolerate flavoured liquid fish oil better than capsules

• Smaller capsule sizes: Taking multiple smaller capsules may be easier to digest than large, high-dose capsules

Storage and freshness are important factors. Fish oil is susceptible to oxidation, which can increase the likelihood of gastrointestinal upset and unpleasant taste. Follow the manufacturer's storage instructions, ensure bottles are tightly sealed, and check expiration dates carefully. Oxidised fish oil has a distinctly rancid smell—discard any supplements with this characteristic.

Gradual dose escalation allows the digestive system to adapt. Begin with a lower dose (such as 500 mg daily) and increase gradually over 1–2 weeks to the target dose. This approach can reduce the incidence of gastrointestinal side effects.

If side effects persist despite these measures, consult a pharmacist or GP about alternative omega-3 sources, such as algal oil, which may offer better tolerability for some individuals.

Fish Oil Interactions with Medications

Fish oil supplements can interact with various medications, potentially altering their efficacy or increasing the risk of adverse effects. Understanding these interactions is essential for safe supplementation, and patients should always inform healthcare professionals about fish oil use.

Anticoagulant and antiplatelet medications represent the most significant interaction. Fish oil's mild antiplatelet properties may interact with:

• Warfarin: May affect clotting measures; monitoring is advised when starting or changing fish oil doses

• Direct oral anticoagulants (DOACs): Such as apixaban, rivaroxaban, and dabigatran—combined use requires caution

• Antiplatelet agents: Including aspirin, clopidogrel, and dipyridamole—additive effects on platelet function are possible

• Non-steroidal anti-inflammatory drugs (NSAIDs): Such as ibuprofen and naproxen—theoretical increased bleeding risk

Patients taking these medications should discuss fish oil use with their healthcare provider.

Antihypertensive medications may interact with fish oil, as omega-3 fatty acids can modestly reduce blood pressure. Whilst this effect is generally beneficial, combining fish oil with antihypertensive drugs (such as ACE inhibitors, angiotensin receptor blockers, or calcium channel blockers) may occasionally result in additional blood pressure reduction. Regular blood pressure monitoring is advisable when initiating fish oil supplementation in patients on antihypertensive therapy.

Diabetes medications: Fish oil generally has neutral effects on blood glucose control. However, patients taking insulin or oral hypoglycaemic agents may wish to monitor blood glucose levels when starting fish oil supplementation and report any changes to their diabetes care team.

Other potential interactions include:

• Orlistat (weight loss medication): May theoretically reduce fish oil absorption, though evidence is limited

Always consult a GP or pharmacist before combining fish oil with prescription medications, and report fish oil use before any surgical procedures. The NHS recommends maintaining an up-to-date medication list that includes all supplements to facilitate safe prescribing and reduce interaction risks.

Frequently Asked Questions