Understanding the Disadvantage of Fish Oil and Potential Side Effects

December 15, 2025 •

3 min read

According to the National Institutes of Health, fish oil is one of the most commonly consumed dietary supplements in the United States. However, despite its popularity, understanding the potential disadvantages of fish oil is crucial before adding it to your regimen.

Quick Summary

Despite its benefits, fish oil can cause side effects including gastrointestinal issues, increased bleeding risk, and potential heart rhythm changes at high doses. Understanding these risks is essential for safe supplementation.

Key Points

Gastrointestinal Distress: Mild side effects like fishy burps, nausea, and diarrhea are common, particularly with higher doses or when taken on an empty stomach.
Increased Bleeding Risk: High doses of fish oil possess blood-thinning properties, which can raise the risk of bleeding, especially for those on anticoagulant medication.
Atrial Fibrillation (A-Fib): Research indicates a possible link between high-dose fish oil supplementation and an increased risk of A-fib, especially in individuals with pre-existing heart conditions.
Heavy Metal Contamination: While most high-quality supplements are purified to remove mercury, poor quality control in unregulated products remains a potential concern.
Drug Interactions: Fish oil can interact with medications for blood pressure and blood thinning, and certain supplements, requiring medical supervision.
Vitamin A Toxicity: Fish liver oil products, like cod liver oil, can contain high levels of Vitamin A, posing a toxicity risk if taken in large amounts over time.

Common Gastrointestinal Side Effects

One of the most frequently reported complaints from fish oil users is the onset of gastrointestinal issues. These are typically mild and often linked to the high fat content of the supplement.

Fishy Burps and Bad Breath: Many users report an unpleasant fishy aftertaste or belching. This can be mitigated by taking the supplement with meals or trying an enteric-coated variety.
Nausea, Heartburn, and Indigestion: Fish oil can stimulate acid production in the stomach, leading to discomfort, especially when taken on an empty stomach.
Diarrhea and Loose Stools: Taking high doses of fish oil (over 3 grams daily) is particularly associated with diarrhea, likely due to its laxative properties.

Increased Risk of Bleeding

High-dose fish oil has anti-platelet effects, which can inhibit blood clotting. While this property is sometimes beneficial for heart health, it also poses a risk, particularly for certain individuals.

Doses exceeding 3 grams per day may increase bleeding risk.
Individuals on blood-thinning medications (anticoagulants) or NSAIDs must consult a doctor, as fish oil can amplify their effects.
Patients scheduled for surgery are often advised to stop taking fish oil beforehand to prevent excessive bleeding.

Potential Cardiovascular Risks

While often promoted for heart health, some studies have raised concerns about high-dose fish oil and specific cardiovascular risks.

The Link to Atrial Fibrillation (A-Fib)

High doses (over 4 grams daily) of fish oil have been associated with an increased risk of atrial fibrillation, an irregular heart rhythm.
This risk appears to be more significant in people with existing cardiovascular conditions.

Contaminant Concerns: Heavy Metals

Another concern, particularly with less reputable brands, is the potential for contamination from heavy metals like mercury. Mercury accumulates in fish, and some worry this carries over into supplements. However, studies show that most commercial fish oil supplements contain negligible or no mercury. Reputable manufacturers use purification processes to remove environmental toxins. To ensure safety, look for third-party tested products with quality verification seals.

Who Should Be Cautious About Fish Oil?

Certain individuals should exercise caution or avoid fish oil supplements entirely without medical supervision.

Allergy sufferers: Anyone with fish or shellfish allergies is at risk of an allergic reaction to fish oil.
Individuals with bleeding disorders: People with pre-existing bleeding conditions should avoid fish oil due to its blood-thinning properties.
Pregnant or breastfeeding women: While omega-3s are vital, high-dose supplements, particularly cod liver oil with its high Vitamin A content, can be harmful. Always consult a doctor.
Diabetics and those with low blood pressure: Fish oil can affect blood sugar and lower blood pressure, requiring careful monitoring.

Supplementation vs. Dietary Intake: A Comparison

To highlight the difference between sources, consider this comparison:


Feature	Fish Oil Supplements	Dietary Fish (Baked/Broiled)
Mercury Content	Typically low to non-existent in purified, high-quality supplements.	Varies based on the type, size, and age of the fish.
Nutrient Profile	High in concentrated EPA and DHA omega-3s.	Offers a wider range of nutrients, including lean protein and minerals.
Side Effect Risk	Increased risk of specific side effects like fishy burps, bleeding, and A-fib at high doses.	Fewer gastrointestinal side effects; benefits often outweigh risks when consumed in recommended amounts.
Form	Concentrated capsules or liquid.	Natural, whole food form.

Conclusion: A Balanced Perspective

While fish oil offers significant health benefits, it is not without potential disadvantages, particularly when consumed in high doses or without medical guidance. Mild gastrointestinal issues, increased bleeding risk, and potential cardiovascular concerns like atrial fibrillation are noteworthy drawbacks. For most, sourcing omega-3s from a diet rich in fatty fish is the optimal strategy. If supplementation is necessary, it is vital to discuss it with a healthcare professional to determine the correct dosage and ensure it is safe for your health status. Being an informed consumer and choosing third-party tested products can help mitigate risks associated with quality control. For further information, consult reliable medical resources like the National Center for Complementary and Integrative Health (NCCIH).

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a healthcare professional before starting any new supplement.

Frequently Asked Questions

Yes, high doses of fish oil have an anti-platelet effect that can reduce blood clotting and increase the risk of bleeding. This is particularly relevant for individuals taking blood-thinning medications or preparing for surgery.

Yes, common gastrointestinal side effects include fishy burps, nausea, diarrhea, and heartburn, often due to the supplement's high fat content. Taking it with food or choosing an enteric-coated option can help.

Mercury contamination is typically not a concern with high-quality fish oil supplements. Reputable brands use purification processes to remove heavy metals. Looking for products with third-party testing ensures safety.

People with fish or shellfish allergies, those on blood thinners, anyone with existing bleeding disorders, and those with certain heart conditions or about to undergo surgery should avoid fish oil or consult a doctor first.

Yes, excessive intake (typically over 3-5 grams per day) can increase the risk of adverse effects like bleeding, vitamin A toxicity (with liver oil), and potentially heart rhythm issues.

Yes, some studies suggest that high-dose fish oil (over 4 grams per day) can increase the risk of atrial fibrillation, an irregular heartbeat, particularly in individuals with pre-existing heart conditions.

To minimize fishy burps, try taking your fish oil with a meal, splitting your dose throughout the day, or opting for enteric-coated capsules, which delay digestion until the supplement reaches your small intestine.