Fish oil can cause gas, and it’s more common than most people expect. In large clinical trials, between 25% and 33% of participants taking fish oil reported gastrointestinal side effects including gas, bloating, and abdominal discomfort. So if you’re dealing with this, you’re far from alone, and there are concrete reasons it happens and practical ways to reduce it.
Why Fish Oil Causes Gas
Fish oil is concentrated fat, and fat is the slowest macronutrient for your body to break down. When you swallow a large capsule of it, your digestive system has to produce extra bile and enzymes to process it. If the oil isn’t fully broken down in your small intestine, it moves into your colon, where gut bacteria ferment it and produce gas as a byproduct.
The type of fish oil matters too. Most supplements come in one of two molecular forms: triglyceride or ethyl ester. The triglyceride form closely resembles the fat structure found in actual fish, so your body recognizes and digests it more easily. The ethyl ester form, which is cheaper and more common, breaks down differently in the gut and can release small amounts of ethanol during digestion. This makes it harder on the stomach and more likely to cause bloating, gas, and general discomfort.
Dose Makes a Difference
How much you take has a direct effect on whether you get gassy. Research using national dietary data found that moderate omega-3 intake, roughly 1.4 to 2.25 grams per day, was actually associated with fewer digestive issues. But once intake exceeded 2.25 grams per day, the protective effect disappeared and the risk of diarrhea and GI upset climbed. Many prescription-strength fish oil products deliver 3 to 4 grams daily, which helps explain the high side effect rates seen in clinical trials.
If you’re taking a standard over-the-counter supplement (typically 1 to 2 grams of fish oil per capsule, with a fraction of that being actual omega-3), you’re less likely to hit that threshold. But stacking multiple capsules or combining fish oil with other fat-heavy supplements can push you past it.
Rancid Oil Makes It Worse
Oxidation is a hidden culprit. Fish oil is extremely prone to going rancid because its fatty acids are chemically fragile. When the oil oxidizes, it produces volatile compounds like ketones, alcohols, and aldehydes, the same chemicals responsible for that unmistakable fishy smell and taste. These breakdown products irritate the digestive tract and can amplify gas, bloating, and nausea beyond what fresh oil would cause.
The tricky part is that many capsules mask rancidity with flavoring or thick gelatin coatings, so you may not taste anything off. A simple check: cut open a capsule and smell the oil inside. Fresh fish oil should smell mildly oceanic at most. If it smells strongly fishy or sour, the oil is oxidized and more likely to upset your stomach. Storing capsules in the refrigerator or freezer slows oxidation and extends shelf life.
Krill Oil Isn’t Necessarily Gentler
A common suggestion for people who get gas from fish oil is to switch to krill oil, since its omega-3s are bound to phospholipids rather than triglycerides. In theory, this should make it easier to absorb. In practice, the research tells a different story. A study comparing krill oil to standard fish oil found that gas, bloating, and flatulence were all significantly more common in the krill oil group. Burping and aftertaste were also worse. So switching to krill oil specifically to avoid gas is unlikely to help and may make it worse.
How to Reduce Gas From Fish Oil
The most effective strategy is choosing enteric-coated capsules. These have a coating that resists stomach acid, so the capsule doesn’t dissolve until it reaches the lower part of your intestine. A study published in the New England Journal of Medicine found that enteric-coated fish oil significantly reduced flatulence, heartburn, belching, and diarrhea compared to standard capsules. The slower breakdown and delayed release appear to spread out digestion and reduce fermentation in the gut.
Freezing your capsules before taking them works on a similar principle. The frozen capsule dissolves more slowly, giving your digestive system more time to process the oil gradually rather than all at once. National Jewish Health, a major respiratory and research hospital, recommends freezing capsules or taking them at night as a first-line fix for GI side effects.
Other practical approaches that help:
- Take fish oil with a meal. Fat digestion requires bile, and your body releases more bile when you eat a full meal, especially one that already contains some fat. Taking fish oil on an empty stomach forces your gut to ramp up bile production from scratch, which slows digestion and increases the chance of gas.
- Split your dose. If you’re taking two or more capsules a day, spreading them across meals keeps the total fat load per sitting lower and gives your gut less to process at once.
- Choose triglyceride-form supplements. Look for labels that say “rTG” (re-esterified triglyceride) or “triglyceride form.” These cost slightly more but are closer to the fat structure in whole fish and digest more easily than ethyl ester versions.
- Stay under 2 grams of omega-3 per day unless you have a specific medical reason for a higher dose. Most general health benefits are associated with 1 to 2 grams daily, and going above 2.25 grams is where digestive side effects become more common.
When Gas Signals a Bigger Issue
For most people, fish oil gas is annoying but harmless. However, if you consistently get severe bloating, cramping, or diarrhea from even small amounts of fish oil, it could point to difficulty digesting fat in general. People who’ve had their gallbladder removed, or who have conditions affecting bile production or pancreatic enzyme output, often struggle with concentrated fat supplements. In those cases, the issue isn’t the fish oil specifically but your body’s reduced capacity to handle a bolus of fat, and a liquid fish oil taken in smaller amounts with food may be better tolerated than large capsules.