Fish Oil Side Effects and How to Avoid Them

Fish oil is one of the most widely used supplements in the world, and most people tolerate it without any problems. The side effects that do occur are usually mild and manageable, things like fishy burps or minor stomach discomfort that resolve with simple adjustments.

There are also a few less common risks worth understanding, particularly at higher doses. Knowing the difference between side effects caused by the omega-3 itself, side effects caused by poor product quality, and actual health risks helps you make better decisions about whether and how to take fish oil.

Common Fish Oil Side Effects

The side effects most people experience are digestive, mild, and almost always temporary.

Fishy burps and aftertaste. This is the single most common complaint. The taste of fish repeating hours after taking a capsule is unpleasant but not harmful. This is more often a sign of poor-quality or oxidized oil than a reaction to omega-3 itself.

Nausea and stomach discomfort. More common at higher doses or when fish oil is taken on an empty stomach. The fat content in fish oil can cause queasiness if there is no other food in the digestive tract to buffer absorption.

Diarrhea or loose stools. Usually dose-dependent. Taking a large dose all at once is more likely to cause GI issues than splitting the same amount across two meals. Reducing the dose for 1-2 weeks and gradually increasing typically resolves this.

Bad breath or fishy body odor. Rare but possible, particularly at doses above 2 grams of EPA and DHA per day. This tends to resolve by lowering the dose or switching products.

Most of these side effects improve within 1 to 2 weeks as the body adjusts. If they persist beyond that window, the issue is more likely the product quality or the dose rather than fish oil itself.

Why Your Fish Oil Might Be Making You Sick: The Rancidity Problem

Not all fish oil side effects are caused by omega-3 fatty acids. Many are caused by oxidized, rancid oil, and this distinction matters more than most people realize.

A 2023 study in the Journal of Dietary Supplements tested 72 of the most popular fish oil brands and found that 45% tested positive for rancidity. Many of these products included added flavoring that masked the oxidized taste, meaning consumers had no way to detect the problem.

Oxidized fish oil does not just lose its effectiveness. Research suggests it may actively promote inflammation and oxidative stress, which is the opposite of what omega-3 supplementation is supposed to do. If you are taking fish oil to reduce inflammation and your product is rancid, you may be making things worse.

Signs your fish oil may be rancid include a strong fishy smell when you open the bottle, a bitter or unusual taste if you bite into a capsule, visibly discolored or cloudy softgels, and storage past the expiration date or in warm conditions.

Switching to a high-quality triglyceride-form product with verified freshness testing may eliminate the side effects that people blame on “fish oil” as a category. For product recommendations that prioritize freshness and third-party testing, see our best fish oil supplement picks.

Fish Oil and Heart Rhythm: The Atrial Fibrillation Question

This is the most discussed serious risk in recent fish oil research, and it requires careful context.

A 2024 prospective study using UK Biobank data from 415,737 participants found that regular fish oil supplement use was associated with a 13% increased relative risk of atrial fibrillation in people without existing cardiovascular disease (Chen et al., 2024). However, for people who already had cardiovascular disease, fish oil use was associated with beneficial outcomes, including reduced progression from atrial fibrillation to major adverse cardiovascular events.

To put this in perspective, the 13% figure is a relative risk increase. The baseline rate of atrial fibrillation in healthy adults is low, so a 13% relative increase translates to a very small absolute difference. The study was also observational and did not control for supplement dose or quality, meaning participants taking 500 mg of standard fish oil and those taking 2,000 mg of pharmaceutical-grade products were grouped together.

A separate meta-analysis of 38 randomized controlled trials also noted a small increase in atrial fibrillation risk alongside reduced cardiovascular mortality (Khan et al., 2021). This means the same supplement can carry both a modest risk and a meaningful benefit, depending on your baseline health status.

For most healthy adults taking standard doses (up to 2 grams of EPA and DHA per day), the atrial fibrillation risk appears to be very small in absolute terms. If you have a personal or family history of heart rhythm issues, discuss fish oil use with your healthcare provider before starting.

Fish Oil and Drug Interactions

Fish oil is generally safe alongside most medications, but there are a few combinations that warrant attention.

Blood thinners (warfarin, aspirin, clopidogrel). Fish oil has mild anticoagulant properties, meaning it can slightly reduce blood clotting. At standard doses this effect is minimal, but at higher doses (above 3 grams of EPA and DHA per day) it may increase bleeding risk. If you are on anticoagulant therapy and want to take fish oil, your doctor can monitor your INR levels to ensure the combination remains safe. This is a manageable interaction, not an absolute contraindication.

Blood pressure medications. Fish oil may modestly lower blood pressure on its own. Combined with blood pressure drugs, this could create an additive effect that drops pressure lower than intended. This is rarely dangerous, but worth monitoring if you are already on BP medication and adding fish oil at doses above 2 grams per day.

Orlistat (Xenical, Alli). This weight-loss drug reduces fat absorption in the gut, which can also reduce absorption of the fat-soluble omega-3s in fish oil. If you take both, separate them by at least 2 hours.

Contraceptive drugs. Some oral contraceptives may reduce the triglyceride-lowering effect of fish oil. This does not make either medication dangerous, but it may reduce one of fish oil’s primary benefits.

If you take any prescription medication and are considering fish oil supplementation, a brief conversation with your doctor is the most practical step. Most interactions are manageable with dose adjustments or monitoring rather than complete avoidance.

Can Fish Oil Raise Your Cholesterol?

Fish oil reliably lowers triglycerides, which is its best-established cardiovascular benefit. However, research also shows that fish oil can slightly increase LDL cholesterol, sometimes called “bad” cholesterol.

This creates a paradox that confuses many people: the same supplement improves one lipid marker while potentially worsening another. The triglyceride reduction is typically much larger (15-30% at therapeutic doses) than the LDL increase, which tends to be modest and clinically insignificant for most people.

For individuals already managing high LDL or taking statin therapy, this small increase is worth knowing about and discussing with a healthcare provider. It does not mean fish oil is harmful for cardiovascular health overall, but it does mean a lipid panel check after 2-3 months of supplementation is a reasonable precaution.

For the full picture on how fish oil affects cholesterol markers, see our article on whether fish oil can lower cholesterol. For a broader overview of the research-backed benefits that may outweigh these side effects, see our guide to fish oil benefits.

How to Minimize Fish Oil Side Effects

Most fish oil side effects are avoidable with a few practical adjustments.

Take it with a fat-containing meal. This is the single most effective step. Fish oil is fat-soluble and absorbs significantly better with food. Taking it on an empty stomach is the primary cause of nausea and fishy burps.

Start low and build up. Begin with 500 mg of EPA and DHA per day for the first 1-2 weeks, then increase to your target dose. This gives your digestive system time to adjust and helps you identify your tolerance threshold.

Split the dose. Instead of taking 2 grams of EPA and DHA all at once, take 1 gram with breakfast and 1 gram with dinner. Splitting reduces the GI load per serving and improves overall absorption.

Choose triglyceride form over ethyl ester. A scoping review found that triglyceride-form supplements have higher bioavailability than ethyl ester forms (Walker et al., 2023). Better absorption means fewer undigested fatty acids in the gut, which translates to fewer digestive side effects.

Check for rancidity before blaming fish oil. Buy from brands that publish third-party oxidation testing results (look for TOTOX values), store capsules in a cool dark place, and always check expiration dates. If your current product smells or tastes off, replace it before concluding that fish oil does not agree with you.

Try enteric-coated capsules. These are designed to dissolve in the intestine rather than the stomach, which significantly reduces fishy burps and stomach discomfort.

If side effects persist after 2-3 weeks despite all of these adjustments, the issue may genuinely be individual intolerance. But in most cases, switching products resolves the problem entirely. For specific dosing guidance to find the right amount for your goals, see our fish oil dosage guide.