Fish Oil Side Effects, Interactions, and Who Should Be Careful

Quick answer: Fish oil is well tolerated by most people at typical doses, but it can cause digestive side effects like fishy burps, heartburn, and loose stools. At higher doses used in some clinical trials, omega-3 supplements have been linked to a modestly higher risk of atrial fibrillation. People who take blood thinners, have heart rhythm problems, or have a fish or shellfish allergy should talk with their doctor before adding fish oil to their routine.

Common Fish Oil Side Effects (and How to Reduce Them)

The majority of side effects from fish oil are gastrointestinal and mild. They tend to be more noticeable when you first start taking fish oil or if you take it on an empty stomach.

Fishy Burps and Aftertaste

This is the most frequently reported complaint. Fish oil softgels release their contents in the stomach, and the resulting fishy-smelling gas can come back up. For many people this is the single reason they stop taking fish oil, even though the oil itself is doing its job. A few strategies consistently reduce fishy burps:

• Take fish oil with food. A meal slows digestion and buffers the oil, which reduces the chance of reflux.
• Choose enteric-coated softgels. An enteric coating delays the capsule from opening until it reaches the small intestine, bypassing the stomach where burping originates.
• Store capsules in the freezer. Frozen softgels dissolve more slowly and the cold can reduce the intensity of any aftertaste.
• Split the dose. Taking one softgel at breakfast and one at dinner instead of two at once reduces the concentration of oil hitting the stomach at one time.

Me First Living Omega-3 Fish Oil uses enteric-coated softgels designed specifically to reduce fishy aftertaste and burps. Each serving of two softgels provides 800 mg EPA and 400 mg DHA from molecularly distilled fish oil. See product details here.

Heartburn and Indigestion

Some people experience mild heartburn or an unsettled stomach, especially if they take fish oil without food or take a large dose at once. Splitting the daily dose and always pairing it with a meal resolves this for most people.

Loose Stools and Digestive Upset

At doses of 3 grams per day or more, omega-3 fatty acids can have a laxative effect and may cause loose stools in some people. This is more common when starting a new supplement than after a few weeks of consistent use. Starting with a lower dose and working up gradually can help your digestive system adjust.

Nausea

Nausea is less common but does occur, particularly on an empty stomach or at higher doses. Taking fish oil in the middle of a meal rather than just before or just after tends to help.

Fish Oil and Blood Thinners: What You Need to Know

This is one of the most common concerns people have, and it is worth addressing carefully because the evidence is more nuanced than the warning labels suggest.

Omega-3 fatty acids have antiplatelet properties, meaning they may slightly reduce the tendency of platelets to clump together. In theory, this could add to the blood-thinning effect of medications like warfarin (Coumadin), clopidogrel (Plavix), aspirin, rivaroxaban (Xarelto), and apixaban (Eliquis).

In practice, several research reviews have found that fish oil at typical supplement doses (1 to 3 grams per day) does not significantly increase bleeding risk in most people taking anticoagulants, and does not meaningfully change INR (a measure of clotting time) in people on warfarin. That said, research in this area is ongoing, individual responses vary, and higher doses carry more uncertainty.

The practical approach: Always tell your doctor or pharmacist if you take fish oil alongside any blood-thinning medication. They can monitor your INR if needed and help you choose a dose that fits your situation. This is especially important before any surgical procedure, because many surgeons ask patients to stop fish oil supplements a week or two beforehand as a standard precaution.

Fish Oil and Atrial Fibrillation (AFib)

Several large randomized trials and meta-analyses have found that omega-3 supplementation, particularly at higher doses, is associated with a modestly higher risk of atrial fibrillation, an irregular heart rhythm. The signal has appeared most consistently at prescription-level doses (3 to 4 grams per day), but some analyses have observed it at lower doses as well.

Two trials worth knowing about:

• VITAL studied approximately 1 gram per day of omega-3 in a large general adult population and found no significant reduction in major cardiovascular events overall. The AFib signal in this trial was more modest than in higher-dose studies.
• REDUCE-IT studied 4 grams per day of icosapent ethyl, a purified EPA prescription drug (not a standard fish oil supplement), in people with high triglycerides already on statins. It showed a significant reduction in cardiovascular events but also a significantly higher rate of atrial fibrillation compared to placebo.

The takeaway for most people taking a standard fish oil supplement at 1 to 2 grams per day is that the absolute AFib risk appears small. But if you already have AFib or a history of it, or if you are at elevated risk for heart rhythm problems, this is a conversation to have with your cardiologist before starting or continuing fish oil.

For a broader look at how omega-3s relate to cardiovascular health, see our overview: Omega-3 and Fish Oil Benefits: What the Research Actually Says.

Fish Oil Interactions: A Quick Reference

The table below summarizes the most clinically relevant interactions and considerations. This is not a complete list. Ask your pharmacist if you take multiple medications.

Who Should Not Take Fish Oil, or Should Use Extra Caution

Fish oil is appropriate for most healthy adults, but some groups should check with a healthcare provider before starting:

• People with fish or shellfish allergies. Fish oil is made from fish tissue. Although highly refined fish oil removes most fish proteins, the risk of an allergic reaction is real. Algae-based omega-3 supplements provide DHA and EPA without any fish-derived ingredients.
• People taking anticoagulant or antiplatelet medications. As described above, coordination with your prescriber is important, especially at higher doses or before surgery.
• People with a history of atrial fibrillation. Talk to your cardiologist, particularly if you are considering doses above 1 gram per day.
• People scheduled for surgery. Follow your surgical team's guidance on stopping fish oil before the procedure.
• Pregnant or breastfeeding women. Omega-3s, particularly DHA, are important during pregnancy and breastfeeding, but specific products and doses should be guided by an obstetrician or midwife.
• People taking cod liver oil at high doses. Cod liver oil contains vitamin A in addition to EPA and DHA. Taking it alongside other vitamin A supplements or at very high doses can lead to vitamin A excess. Standard fish oil (not cod liver oil) does not carry this concern.

For help choosing a high-quality product, see our guide: How to Choose a Fish Oil Supplement: What to Look For on the Label.

How to Reduce Fish Oil Side Effects: Practical Steps

1. Always take fish oil with a full meal, not just a snack or coffee. Fat in the meal helps absorption and reduces digestive irritation.
2. Start with one softgel per day for the first week, then move to the full two-softgel serving. This lets your digestive system adjust gradually.
3. Choose enteric-coated softgels if fishy burps are a problem. The coating delays capsule opening until after the stomach.
4. Store softgels in the freezer. Frozen softgels are slower to dissolve and many people notice less aftertaste this way.
5. Check for freshness. Oxidized fish oil smells noticeably more pungent and rancid. If your softgels smell strongly fishy when you cut one open, they may be past their prime. Look for supplements that undergo third-party testing, including for oxidation levels.
6. Review your medications with your pharmacist when you start fish oil. A brief conversation can catch interactions you might not think to ask about.

For guidance on how much fish oil to take and how to read supplement labels, see: Fish Oil Dosage: How Much Omega-3 Do You Actually Need?

If you are comparing fish oil to krill oil, this guide covers the key differences: Fish Oil vs. Krill Oil: Which Is Better?

Omega-3s are sometimes paired with other nutrients that have been studied for inflammation and joint support. For context on how they may complement each other, see: Turmeric Benefits: What the Research Says and Cinnamon and Omega-3: Can You Take Them Together?

Key Takeaways

• The most common fish oil side effects are gastrointestinal: fishy burps, heartburn, and loose stools. These are usually mild and manageable.
• Taking fish oil with food, choosing enteric-coated softgels, storing capsules in the freezer, and splitting the dose across the day all help reduce side effects.
• Fish oil has antiplatelet properties. If you take warfarin, clopidogrel, or other blood thinners, tell your prescriber before starting fish oil.
• Several large trials have linked omega-3 supplementation, especially at higher doses, to a modestly elevated risk of atrial fibrillation. People with AFib should consult their cardiologist.
• Omega-3s may modestly lower blood pressure, which could add to the effect of blood pressure medications.
• People with fish or shellfish allergies should avoid fish oil; algae-based omega-3 is an alternative.
• Cod liver oil, unlike standard fish oil, contains vitamin A. Taking it at high doses or combining it with vitamin A supplements can cause vitamin A accumulation.
• At typical supplement doses of 1 to 2 grams per day, fish oil is well tolerated by most healthy adults.

This article is for educational purposes and isn't medical advice. Talk with your healthcare provider about your individual needs.

Frequently Asked Questions

Can fish oil cause bleeding?

At typical supplement doses of 1 to 2 grams per day, fish oil has not been shown to significantly increase bleeding risk in most people. Omega-3 fatty acids do have antiplatelet properties, and at higher doses they could theoretically add to the effect of blood-thinning medications. If you take warfarin, aspirin, clopidogrel, or similar drugs, tell your doctor before starting fish oil. Most surgeons also recommend pausing fish oil one to two weeks before elective procedures as a routine precaution.

Does fish oil affect atrial fibrillation?

Several large trials and meta-analyses have found that omega-3 supplementation, particularly at higher doses, is associated with a modestly higher risk of atrial fibrillation. The association has been seen most clearly at prescription-level doses (3 to 4 grams per day), but some analyses suggest a signal at lower doses as well. If you have a history of AFib or are being monitored for a heart rhythm disorder, talk with your cardiologist before taking fish oil supplements.

Who should not take fish oil?

People with fish or shellfish allergies should avoid fish oil supplements and may want to consider algae-based omega-3 instead. People taking anticoagulants or antiplatelet drugs, those with a history of atrial fibrillation, people scheduled for surgery, and pregnant or breastfeeding women should all check with a healthcare provider before starting or continuing fish oil. Most other healthy adults tolerate it well.

Why does fish oil give me fishy burps?

Fish oil softgels that dissolve in the stomach can release their contents during digestion, and the gas that forms may carry a fishy smell when you burp. The most effective solutions are taking fish oil with a full meal, choosing enteric-coated softgels (which are designed to open after the stomach), and storing capsules in the freezer. Splitting the daily dose between two meals instead of taking it all at once also helps.

Is it safe to take fish oil every day?

For most healthy adults, taking fish oil daily at doses of 1 to 3 grams is considered safe. Large-scale trials lasting years have not identified serious safety concerns at these doses in the general population. If you have specific health conditions, take prescription medications, or are pregnant, daily use should be discussed with your doctor to make sure the dose and product are appropriate for you.

Can I take fish oil with blood pressure medication?

Omega-3 fatty acids may have a modest blood-pressure-lowering effect. For most people this is a benefit, but if you already take medication to lower blood pressure, the combined effect could cause it to drop lower than intended. Let your doctor or pharmacist know you are taking fish oil so they can factor it into your care. Self-monitoring your blood pressure at home is also a reasonable step when starting any new supplement.

Does cod liver oil have different side effects than regular fish oil?

Yes, in one important way. Cod liver oil contains vitamin A in addition to EPA and DHA, while standard fish oil does not. Taking cod liver oil at high doses, or combining it with other supplements that contain vitamin A (including many multivitamins), can lead to vitamin A accumulation, which is harmful over time. Standard fish oil supplements do not carry this concern. Otherwise the digestive side effects of cod liver oil are similar to those of regular fish oil.