Fish oil supplements are among the most popular dietary supplements worldwide, and for good reason. They’re packed with omega-3 fatty acids that support heart health, reduce inflammation, and may help with everything from joint pain to brain function.
But here’s the thing: more isn’t always better. Taking excessive amounts of fish oil can lead to unwanted side effects that range from mildly annoying to potentially serious.
Here are eight side effects you should know about before reaching for that extra capsule.
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Available on the App Store1. Digestive issues
Stomach problems are by far the most common complaint among fish oil users. The National Institutes of Health lists “gastrointestinal discomfort, diarrhea, heartburn, nausea, and odoriferous sweat” as typical side effects.1
Suggested read: 11 Ways to Improve Digestion Naturally (Evidence-Based)
Diarrhea tends to get worse at higher doses. A 2023 meta-analysis of 90 randomized controlled trials found that omega-3 supplementation significantly increased the odds of diarrhea compared to placebo.2
If your stomach rebels after taking fish oil, try these fixes:
- Take supplements with food rather than on an empty stomach
- Split your dose into smaller amounts throughout the day
- Switch to enteric-coated capsules designed to dissolve in your intestine instead of your stomach
2. Acid reflux and heartburn
That burning sensation in your chest after taking fish oil? You’re not alone. Fish oil is high in fat, and fat is a known trigger for heartburn and acid reflux.
The belching that often accompanies fish oil supplements (sometimes with an unpleasant fishy aftertaste) can also push stomach acid up into your esophagus.
Sticking to a moderate dose and taking your supplements with meals usually helps. If heartburn persists, acid reflux supplements containing ingredients like ginger or licorice root may provide relief.
3. Blood sugar changes in diabetics
If you have type 2 diabetes, pay attention to this one. Research on fish oil and blood sugar has produced mixed results, and the answer may depend on your dosage and background.
A 2020 meta-analysis looking at fish oil supplementation in people with type 2 diabetes found no significant effect on fasting blood glucose overall. However, there was an interesting twist: among Asian participants specifically, fasting glucose increased by 0.42 mmol/L.3
Other studies show neutral or even beneficial effects on blood sugar control. The takeaway? If you have diabetes, monitor your blood glucose when starting fish oil and discuss supplementation with your doctor.
Suggested read: 10 Supplements to Help Lower Blood Sugar Effectively
4. Potential bleeding concerns
Fish oil has antiplatelet effects, meaning it can reduce your blood’s ability to clot. For years, this led doctors to recommend stopping fish oil before surgery.
But recent research tells a more nuanced story. A 2024 systematic review and meta-analysis in the Journal of the American Heart Association found that omega-3 supplements were not associated with increased bleeding risk in most people.4
A separate 2017 systematic review reached a similar conclusion: “This systematic review does not support the need for discontinuation of fish oil supplements prior to surgery or other invasive procedures.”5
That said, high-dose purified EPA supplements may carry a slightly elevated bleeding risk. And if you’re taking blood thinners like warfarin or antiplatelet drugs like aspirin, talk to your doctor before adding fish oil to your routine.
5. Low blood pressure
Fish oil’s ability to lower blood pressure is actually one of its benefits for most people. But if your blood pressure is already low, this effect could become a problem.
Studies show omega-3s can reduce both systolic and diastolic blood pressure, particularly in people with hypertension or high cholesterol.
If you have low blood pressure or take blood pressure medications, coordinate with your healthcare provider about fish oil dosing. You may need to monitor your blood pressure more closely or adjust your medication.
6. Vitamin A toxicity (from cod liver oil)
This one applies specifically to cod liver oil, not regular fish oil. Cod liver oil is extracted from the livers of cod fish, where vitamin A concentrates.
Just one tablespoon of cod liver oil can provide over 250% of your daily vitamin A needs. That’s fine occasionally, but chronic overconsumption can lead to vitamin A toxicity with symptoms including:
- Dizziness and nausea
- Joint pain
- Skin irritation
- In severe cases, liver damage
If you prefer cod liver oil, check the vitamin A content on the label and make sure you’re not exceeding the tolerable upper intake level of 3,000 mcg (10,000 IU) daily from all sources combined. Standard fish oil supplements don’t contain significant vitamin A.
7. Sleep disturbances
This side effect is less well-documented but worth mentioning. Some people report insomnia or disrupted sleep when taking high doses of fish oil, particularly in the evening.
The evidence here is limited to case reports rather than controlled studies. In fact, moderate doses of omega-3s have been shown to improve sleep quality in some research.
If you notice sleep problems after starting fish oil, try taking your supplement in the morning instead of at night to see if that helps.
8. Increased atrial fibrillation risk at high doses
This is the most serious potential concern, particularly for people already at cardiovascular risk.
A 2025 meta-analysis examining 34 trials with over 114,000 participants found that high-dose omega-3 supplementation (above 1,500 mg/day of EPA+DHA) was associated with a 48% increased risk of atrial fibrillation.6
The absolute risk increase was modest (0.8%), and this effect was most pronounced in people already at high cardiovascular risk taking pharmaceutical-grade omega-3 preparations. Standard over-the-counter supplements at typical doses don’t appear to carry this risk.
Still, if you have a history of heart arrhythmias, discuss fish oil supplementation with your cardiologist.
How much fish oil is safe?
Most health organizations recommend 250–500 mg of combined EPA and DHA daily for general health. People with specific conditions like high triglycerides may benefit from higher doses under medical supervision.
The European Food Safety Authority and U.S. FDA have concluded that up to 5,000 mg of omega-3s daily appears safe for long-term use.1
For detailed guidance on optimal amounts, see our fish oil dosage guide.
A typical 1,000 mg fish oil softgel contains about 250–300 mg of combined EPA and DHA. So you’d need to take quite a few capsules to reach problematic levels.
The bottom line
Fish oil is one of the most beneficial supplements you can take, with strong evidence supporting its effects on heart health, inflammation, and more.
But like any supplement, more isn’t automatically better. Most side effects occur at high doses and resolve when you cut back.
If you experience digestive issues, try taking fish oil with food or switching brands. If you have diabetes, monitor your blood sugar. If you take blood thinners or have heart rhythm issues, consult your doctor.
And consider getting your omega-3s from whole food sources like fatty fish, which provide the same benefits without the supplement-specific side effects.
National Institutes of Health Office of Dietary Supplements. Omega-3 Fatty Acids: Fact Sheet for Health Professionals. 2024. NIH ↩︎ ↩︎
Xu J, et al. Safety of Supplementation of Omega-3 Polyunsaturated Fatty Acids: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Adv Nutr. 2024;15(1):100144. PubMed ↩︎
Gao C, et al. Effects of fish oil supplementation on glucose control and lipid levels among patients with type 2 diabetes mellitus: a Meta-analysis of randomized controlled trials. Lipids Health Dis. 2020;19(1):87. PubMed ↩︎
Javaid M, et al. Bleeding Risk in Patients Receiving Omega-3 Polyunsaturated Fatty Acids: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. J Am Heart Assoc. 2024;13(10):e032390. PubMed ↩︎
Begtrup KM, et al. No impact of fish oil supplements on bleeding risk: a systematic review. Dan Med J. 2017;64(5):A5366. PubMed ↩︎
Shi Z, et al. Effects of Omega-3 Fatty Acid Treatment on Risk for Atrial Fibrillation: An Updated Meta-Analysis of 34 Trials including 114,326 Individuals. medRxiv. 2025. medRxiv ↩︎
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