Does fish oil cause diarrhea, gas or an upset stomach?

Yes, diarrhea is among the mild gastrointestinal effects the NIH Office of Dietary Supplements lists for omega-3 supplements, alongside heartburn, nausea, and general stomach discomfort. In trial data specifically, burping and a fishy aftertaste are the most consistently reported effects; we haven't found a trial that isolated constipation as a reported effect.

What the safety data says

The NIH Office of Dietary Supplements' consumer fact sheet lists an unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhea, headache and smelly sweat as mild side effects reported from omega-3 supplements, and notes these are usually mild at typical intakes. Trial-level data tells a narrower story: a 2018 meta-analysis pooling 21 randomized trials of prescription-strength omega-3 (24,460 participants, doses well above a standard softgel) found more treatment-related burping (eructation) and nausea specifically with EPA+DHA combination products compared with placebo, plus more fishy aftertaste and skin reactions across the omega-3 group generally. That review didn't isolate diarrhea or constipation as a separately reported outcome, and it studied prescription-strength doses — well above the 600 mg/day Electric Tiger Omega-3 directs at two softgels a day.

Does fish oil cause constipation?

We haven't found trial evidence linking fish oil to constipation specifically. If anything, the reported gastrointestinal pattern in the literature trends toward looser stool, stomach discomfort and burping rather than constipation. If you're experiencing constipation and taking fish oil, it's worth looking at other possible causes rather than assuming a direct link the data doesn't support.

What tends to help

Taking a softgel alongside a meal, rather than on an empty stomach, is the most common practical suggestion, and it lines up with absorption research showing fish oil absorbs better with dietary fat — see when is the best time to take fish oil. Splitting a two-softgel daily amount into one with breakfast and one with dinner, rather than both at once, is another common approach people use, though we haven't found a controlled trial testing that specifically against taking both together.

Questions people ask

Does fish oil give you gas?

Gas and stomach discomfort are among the mild gastrointestinal effects reported for omega-3 supplements generally (NIH Office of Dietary Supplements). Taking it with food is the most common way people reduce it.

Can fish oil make you nauseous?

Yes — nausea is one of the more consistently reported effects in trial data, particularly with EPA+DHA combination products at higher doses, and it's also listed among the mild effects reported at smaller intakes.

Can I take fish oil on an empty stomach?

You can, but it's more likely to produce a fishy aftertaste, burping or stomach discomfort than taking it with a meal. Our label directs one softgel twice daily with meals for that reason.

Can omega-3 cause heartburn or acid reflux?

Heartburn is listed among the mild side effects reported for omega-3 supplements (NIH Office of Dietary Supplements). If it's persistent or severe, that's worth discussing with a doctor rather than adjusting the supplement on your own.

Can fish oil cause bleeding?

Fish oil has a mild blood-thinning effect on its own, more relevant if you take a blood-thinning medication or have an upcoming surgery — see who should not take fish oil for the full picture. That's a different mechanism from the gastrointestinal effects on this page.

Sources

• Chang CH, et al. Safety and tolerability of prescription omega-3 fatty acids: a systematic review and meta-analysis of randomized controlled trials. PMID 29482765.
• NIH Office of Dietary Supplements, Omega-3 Fatty Acids fact sheet.

Related reading: fish oil side effects, when is the best time to take fish oil, and who should not take fish oil.