What Are Possible Fish Oil Side Effects?

Medically Reviewed By Jerlyn Jones, MS MPA RDN LD CLT

Fish oil is used clinically to treat high triglycerides and may have many health benefits. However, taking too much fish oil may have some adverse effects. Fish oil supplements come from fatty fish such as herring, sardines, anchovies, and salmon. They contain long-chain omega-3 fatty acids, eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and sometimes other nutrients such as vitamins A and D.  

While numerous benefits Trusted Source PubMed Central Highly respected database from the National Institutes of Health Go to source are associated with consuming omega-3 fatty acids, there may be potential side effects and risks from taking fish oil supplements, including bleeding, gastrointestinal distress, heartburn, and diarrhea.

A National Institutes of Health (NIH) study estimated that 7.8% Trusted Source National Cancer Institute Governmental authority Go to source of people in the United States take fish oil supplements. Most experience no serious side effects.

This article discusses possible side effects of fish oil, how to manage them, recommended dosage, when to contact a doctor, and more.

Blood thinning

there are some fish oil pills on a table
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Fish oil has blood thinning effects, so it can prevent blood clots from forming. Taking high doses of fish oil supplements may inhibit platelet aggregation and increase the risk of bleeding. This can manifest as bleeding gums, nosebleeds, or possibly even stroke.

Taking fish oil with certain anti-coagulants, such as warfarin, may increase bleeding risk.

An older study found that participants who took 640 milligrams (mg) of fish oil daily over 4 weeks had decreased blood clotting.

Surgeries and certain invasive procedures for patients taking fish oil tend to be delayed because of concern for bleeding risk associated with the potential antiplatelet effect of fish oil.

However, study results are mixed. A 2018 randomized control trial Trusted Source AHA/ASA Journals Peer reviewed journal Go to source of 1,516 patients found that 8–10 grams (g) of fish oil for 2–5 days before surgery did not lead to increased bleeding.

How to manage blood thinning

If you are taking anti-coagulant medications or have bleeding disorders, check with your doctor before trying fish oil supplements. They may recommend avoiding fish oil supplements altogether or taking a lower dosage.

Learn about blood thinners for heart attack prevention.


Diarrhea is one of the most common Trusted Source PubMed Central Highly respected database from the National Institutes of Health Go to source adverse effects associated with taking fish oil. It is most commonly reported when a person takes more than 3 g of fish oil daily. This may be due to the intrinsic laxative properties of omega-3 fatty acids.

Other types of omega-3 supplements may also cause diarrhea, such as flaxseed or algae oil.

How to manage diarrhea

Taking fish oil with food or lowering your dose may help mitigate this side effect. If you continue to have diarrhea after trying these modifications, contact a healthcare professional.

Learn more ways to deal with diarrhea.


Fish oil is a concentrated source of fat, so it can trigger Trusted Source National Institutes of Health Office of Dietary Supplements Governmental authority Go to source heartburn and other types of gastrointestinal discomfort. If you are prone to acid reflux, you may be more susceptible to experiencing heartburn as a side effect after taking fish oil.

Other gastrointestinal symptoms that can occur include belching, nausea, and bloating. These are common side effects largely due to fish oil’s high fat content.  

How to manage heartburn

Taking fish oil with food can help reduce the risk of heartburn and other gastrointestinal distress. If you have gastroesophageal reflux disease, you should talk with your healthcare professional before starting a fish oil supplement regimen.

Learn treatment options for acid reflux.

Atrial fibrillation

Atrial fibrillation is an arrhythmia. It refers to an irregular and extremely fast heartbeat that can lead to poor blood circulation and blood clots in the heart. Atrial fibrillation increases the risk of stroke, cardiac arrest, and other heart-related complications.

A 2022 meta-analysis Trusted Source AHA/ASA Journals Peer reviewed journal Go to source that analyzed seven randomized control trials found that fish oil supplementation was associated with an increased risk of atrial fibrillation. The risk appeared to be greater in studies that tested more than 1 g of fish oil daily.

How to manage atrial fibrillation

Atrial fibrillation is a serious complication that can be life threatening.

If you are at risk for arrhythmia or notice any irregular and rapid heartbeat, seek immediate medical attention.

Learn about treatment options for atrial fibrillation.

Is it possible to take too much fish oil?

Some fish oil supplements, such as cod liver oil, contain vitamins A and D in varying amounts. Vitamins A and D can be harmful in excessive amounts. High doses of vitamin A can cause Trusted Source PubMed Central Highly respected database from the National Institutes of Health Go to source liver damage. Too much vitamin D can cause Trusted Source PubMed Central Highly respected database from the National Institutes of Health Go to source hypercalcemia, which is excess calcium in the blood.

The Food and Drug Administration (FDA) Trusted Source National Institutes of Health Office of Dietary Supplements Governmental authority Go to source has prohibited the labels of dietary supplements from including daily intake recommendations higher than 2 g. It also has advised against consuming more than 3 g a day.

Any supplement has the potential to be harmful when taken in excess.

Learn more about when to take omega-3s.

When to see a doctor for fish oil side effects

If you see bleeding that cannot be stopped or suspect internal hemorrhage, seek emergency medical help. Anyone who develops serious complications, such as the following, should stop taking fish oil and seek emergency medical help:

Contact a doctor if you are experiencing any other symptoms that concern you.

Frequently asked questions

Here are some commonly asked questions about fish oil.

Is it good to take fish oil every day?

Some people may benefit from taking fish oil every day, while others may not. Contact your healthcare professional for more personalized advice if you are interested in starting a fish oil supplement regimen.

Who should not take fish oil?

People at risk for arrhythmia or have bleeding, clotting disorders, or shellfish allergies may receive advice from their doctor not to take fish oil.

You should talk with your doctor before trying fish oil supplements if you are taking anticoagulants or blood pressure medication.

When is the best time to take fish oil, morning or night?

No research suggests that certain times of the day are better than others for taking fish oil. However, it may be best to take two or more smaller doses in the morning and evening with meals to reduce acid reflux, diarrhea, belching, and nausea.

Taking fish oil with a meal containing dietary fat has been shown to increase the bioavailability of omega-3 fatty acids.

Learn more about when it is best to take fish oil.


Fish oil supplements are widely used for their many documented health benefits. However, there may be potential side effects or even serious complications from taking fish oil, such as atrial fibrillation, gastrointestinal (GI) distress, and increased bleeding.

The most common side effects are GI-related, such as heartburn, diarrhea, and acid reflux. These symptoms can be managed by modifying the dosage or taking them at separate times with meals.

If you are concerned about potential adverse effects from fish oil, talk with your doctor. They can help you understand potential risks and benefits based on your situation.

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Medical Reviewer: Jerlyn Jones, MS MPA RDN LD CLT
Last Review Date: 2022 Dec 21
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