What is the Downside of Taking Fish Oil Supplements?

October 7, 2025 •

2 min read

According to a 2022 study, an estimated 7.8% of people in the United States take fish oil supplements, but while often associated with health benefits, it is crucial to understand the potential downsides. The downside of taking fish oil is a topic that requires careful consideration, especially regarding individual health conditions.

Quick Summary

Fish oil supplements can cause a range of issues, from mild gastrointestinal side effects like burping and nausea to more serious risks such as increased bleeding, atrial fibrillation, and interactions with certain medications. The risks often depend on the dosage and an individual's health status.

Key Points

Gastrointestinal Distress: A primary downside is mild but frequent digestive issues, such as fishy burps, nausea, and diarrhea.
Increased Bleeding Risk: Combining fish oil with blood-thinning medications can slow blood clotting and increase the risk of bleeding.
Potential for Atrial Fibrillation: Some studies have linked regular fish oil use to an increased risk of developing atrial fibrillation, a heart rhythm disorder.
Drug Interactions: Fish oil can interact with medications for high blood pressure and blood thinners, potentially causing adverse effects.
Risk for Healthy Individuals: Research suggests that for healthy individuals without pre-existing heart disease, the risks of supplementation may outweigh the benefits.
Contaminants and Purity: The lack of FDA regulation means lower-quality supplements may contain pollutants, while freshness can be a concern for all products.

Common Gastrointestinal Side Effects

Common side effects of fish oil include fishy burps, nausea, upset stomach, diarrhea, heartburn, acid reflux, bloating, and gas. These issues can often be mitigated by taking the supplement with food or using enteric-coated capsules.

Increased Risk of Bleeding

Fish oil can act as a blood thinner, potentially increasing the risk of bleeding. This is particularly risky for those with bleeding disorders or those taking anticoagulant medications like warfarin, aspirin, or clopidogrel, as it can heighten the risk of bruising, nosebleeds, and prolonged bleeding. Individuals facing surgery are also typically advised to stop fish oil use beforehand.

Potential Cardiovascular Complications

Despite being associated with heart health, fish oil can pose risks. Research, including a 2024 study, suggests a link between regular fish oil use and an increased risk of atrial fibrillation, an irregular heart rhythm, particularly in healthy individuals and some patients with existing cardiovascular issues. The same study indicated that healthy individuals taking fish oil might have a higher risk of initial heart disease and stroke, while those with pre-existing heart conditions might see some protective effects.

Other Adverse Effects and Risks

Other potential downsides include the risk of lowered blood pressure, especially when combined with blood pressure medications, which can cause dizziness. Supplements like cod liver oil contain vitamins A and D, and excessive vitamin A intake can lead to toxicity symptoms such as nausea, joint pain, and liver damage. Fish oil might also potentially suppress the immune system. Fish oil is prone to oxidation, which can cause it to go rancid and potentially be harmful; a strong, unpleasant smell indicates rancidity. While refined fish oil is generally free of mercury, lower-quality products may contain other contaminants like PCBs and dioxins, highlighting the importance of choosing reputable, third-party-tested brands.

Comparison of Potential Risks


Risk Factor	Potential Concern
Gastrointestinal Issues	Mild to more frequent symptoms such as burping, fishy taste, and nausea are possible.
Bleeding Risk	Potential for increased bleeding and bruising, especially when combined with anticoagulant medication.
Atrial Fibrillation	Some studies suggest a potential link, particularly in healthy individuals and people with pre-existing heart conditions.
Blood Pressure	Modest to more pronounced blood pressure reduction possible, potentially dangerous for those with low blood pressure.
Vitamin A Toxicity	Risk of accumulation and toxicity, especially with prolonged use of supplements containing high levels of Vitamin A like cod liver oil.

Conclusion: Navigating the Downsides of Fish Oil

Fish oil supplements have potential downsides, including common digestive issues, increased bleeding risk, and a possible link to atrial fibrillation. For many healthy people, obtaining omega-3s from food sources like fatty fish may be a safer alternative, as the risks of supplements might outweigh the benefits. Those considering supplementation should consult a healthcare provider to discuss potential drug interactions and individual risk factors, especially if they have existing health conditions or take other medications. For further information on omega-3s, the National Institutes of Health, Office of Dietary Supplements is an authoritative source.

Frequently Asked Questions

Yes, fish oil supplements can cause gastrointestinal issues like nausea, heartburn, fishy burps, and diarrhea. Taking the supplement with food or choosing an enteric-coated variety may help reduce these side effects.

Fish oil can increase the risk of bleeding by slowing blood clotting. This is a particular concern for individuals with bleeding disorders or those taking anticoagulant medications.

Some studies suggest a link between regular fish oil supplementation and an increased risk of atrial fibrillation (AFib), especially in individuals with or without pre-existing heart conditions.

Fish oil can lower blood pressure. Taking it with blood pressure medication may cause blood pressure to drop too low. It is essential to monitor your blood pressure and consult a doctor before combining them.

Individuals with fish or shellfish allergies, those on blood-thinning medication, people with low blood pressure or liver disease, and anyone scheduled for surgery should avoid or use caution with fish oil supplements.

Higher intakes are associated with increased risks, including more pronounced bleeding effects and a higher likelihood of other adverse side effects. It is important to consult with a healthcare professional to determine appropriate intake.

High-quality, refined fish oil supplements typically undergo a purification process that removes mercury and other contaminants. The risk of mercury contamination is much lower in supplements than in eating certain types of fish.