Fish oil has a reputation as one of the “safe” supplements, which is exactly why a few of its documented side effects catch people off guard: a modestly increased risk of an irregular heart rhythm at high doses, a small but real increase in depression risk found in one large trial, and digestive symptoms that have little to do with the supplement’s actual safety.
None of these findings mean fish oil is dangerous for most people, but they’re worth knowing, especially if you’re managing a chronic condition or already prone to worrying about your health.
Fish Oil, Briefly
Fish oil supplements contain omega-3 fatty acids, primarily EPA and DHA, nutrients associated with heart, brain, and joint health benefits across a large body of research. It’s one of the most commonly used dietary supplements in the world, taken by tens of millions of people, which is part of why even a small percentage-based side effect can translate into a meaningful number of real cases.
Most people who take fish oil at standard doses experience no problems at all. The effects below tend to show up at higher doses, in specific health situations, or in a small subset of users, not universally.
An Increased Risk of Irregular Heart Rhythm
This is probably the most surprising finding for anyone who thinks of fish oil purely as heart-protective, and it deserves a non-alarming explanation. Multiple recent studies have found a connection between fish oil supplementation and an increased risk of atrial fibrillation (AFib), an irregular and sometimes rapid heart rhythm.
A large 2024 study published in BMJ Medicine followed a general population over 12 years and found that people taking fish oil supplements had an increased likelihood of developing AFib and stroke compared to non-users, with the researchers noting that risk appeared to climb at higher doses specifically.
Because this was an observational study, the researchers were upfront that they couldn’t account for every variable, such as which specific fish oil products people took or other factors influencing heart health.
The risk isn’t uniform across everyone. The same body of research suggests this elevated risk appears primarily in the general population without existing heart conditions, while fish oil may still offer benefits for people who already have cardiovascular disease.
In other words, this isn’t a reason for someone managing heart disease under a doctor’s care to panic about a prescribed or recommended fish oil regimen; it’s a reason for anyone taking high-dose fish oil without a specific medical reason to have an informed conversation with their doctor about whether that dose still makes sense for them.
More than 20 clinical trials have examined this relationship, with variable results, reflecting an actively evolving area of research rather than a single settled answer. If you have any history of heart rhythm problems, or a family history of AFib, mentioning your fish oil use and its dose to your cardiologist or primary care doctor is a reasonable, low-effort step.
Reassuring News on Bleeding Risk
This next one goes against what a lot of people have heard. It’s a case where the unexpected finding is actually good news.
Fish oil has long carried a reputation for thinning the blood and raising bleeding risk, particularly around surgery or for people on blood-thinning medication. A large, recent meta-analysis suggests that reputation is largely overstated.
A 2024 systematic review and meta-analysis published in the Journal of the American Heart Association pooled data from 120,643 patients across 11 randomized clinical trials and found no increased overall bleeding risk from omega-3 supplementation. The only exception was a modest 0.6% increase observed specifically with high-dose EPA taken alone, and even then, the researchers found no evidence of increased serious bleeding, such as intracranial bleeding or hemorrhagic stroke.
The same analysis found no meaningful relationship between bleeding events and background use of antiplatelet medication among people taking omega-3 supplements, suggesting fish oil can generally be used alongside common blood-thinning treatments without the bleeding concern many people have been told to worry about.
Earlier research on perioperative use reached a similar conclusion, finding that omega-3s don’t meaningfully increase bleeding risk during or after surgery.
None of this means you should make medication or surgical planning decisions on your own. If you’re on a blood thinner like warfarin or have a procedure scheduled, your doctor’s specific guidance on your fish oil use still matters and should be followed, since individual circumstances vary. But the underlying evidence base is considerably more reassuring than the common assumption that fish oil and blood thinners are a risky combination.
A Small but Real Increase in Depression Risk
This finding surprised researchers as much as it surprises most people who hear about it, since fish oil is frequently discussed as a potential mood booster. The VITAL-DEP trial, an ancillary study of the large VITAL trial, followed 18,353 adults aged 50 and older who did not have depression at the start of the study, tracking them for a median of 5.3 years.
The results, published in a 2021 randomized clinical trial, found a small but statistically significant increase in depression risk in the omega-3 group: 13.9 depression events per 1,000 person-years compared to 12.3 per 1,000 person-years in the placebo group. To be clear about what that means in practical terms: This is a modest difference, not a dramatic one, and the researchers found no significant difference in overall mood scores between the two groups across the study.
Researchers say there’s no reason for adults without depression to take fish oil supplements specifically to prevent depression or improve mood. This doesn’t mean fish oil causes depression in any straightforward way, and it doesn’t mean people already taking fish oil for other reasons, like heart or joint health, need to stop.
It does mean that “fish oil helps your mood” isn’t a claim well supported by this large trial, and if you’re taking fish oil primarily for that reason, it may be worth discussing with your doctor whether that’s still the right goal for this particular supplement.
Digestive Side Effects: Common, Annoying, and Manageable
This is the side effect most people actually notice day-to-day. Fish oil is well known for causing a lingering fishy aftertaste or “fish burps,” along with mild digestive upset, such as nausea, bloating, or loose stools, in some people.
These symptoms are generally linked to how and when the supplement is taken, as well as product quality, rather than being a sign of a deeper problem.
Fish oil that has started to oxidize or go rancid tends to produce a stronger fishy taste and can be more likely to cause stomach upset, which is part of why proper storage, typically refrigeration after opening, and choosing a reputable, appropriately packaged product matter more than people often realize.
Taking fish oil with meals, splitting a larger dose into smaller portions throughout the day, or trying an enteric-coated capsule, which is designed to dissolve further along in digestion rather than in the stomach, are commonly recommended strategies that many people find meaningfully reduce these symptoms.
This is an uncomfortable but low-stakes side effect that’s usually solvable with simple adjustments, not a sign that fish oil isn’t right for you.
Interactions With Other Supplements and Medications
Beyond blood thinners, a few other combinations deserve a mention, particularly for anyone managing multiple health conditions and taking several supplements or medications at once.
Fish oil taken alongside blood pressure medication can, in some people, have an additive effect on lowering blood pressure. This is generally a mild and manageable interaction, but it’s worth flagging to a doctor if you’re also on antihypertensive medication, especially if you notice dizziness or lightheadedness after starting fish oil.
High-dose fish oil supplements, particularly the prescription-strength versions sometimes used for very high triglyceride levels, are typically monitored more closely by a doctor because the doses are far higher than those in an over-the-counter bottle. If you’ve been prescribed a high-dose omega-3 medication rather than taking an over-the-counter supplement, the research discussed in this article, most of which studied more moderate supplemental doses, may not translate directly to your situation, and your prescribing doctor’s specific monitoring plan is the more relevant guide.
Fish oil supplements vary considerably in their actual EPA and DHA content from bottle to bottle, even when doses look similar on the label. This variability is part of why comparing your own experience to a friend’s or to what you read online doesn’t always tell you much: the product itself may be meaningfully different, even under a similar-sounding name or dose.
What This Means If You Have a Chronic Illness
If you’re managing a chronic condition, whether that’s heart disease, diabetes, an autoimmune condition, or something else, fish oil occupies an unusually nuanced position: The same supplement can have different implications depending on your specific diagnosis. This isn’t a reason for alarm; it’s a reason for personalization rather than a one-size-fits-all approach.
For people with existing heart disease, current research suggests fish oil may still offer benefits despite the AFib findings discussed above, since that population appears to respond differently than the general population in some observational studies.
For people managing blood sugar or diabetes, the evidence on fish oil’s effect on glucose control is meaningfully mixed across different trials, meaning it’s neither clearly helpful nor clearly harmful for that specific concern, and it shouldn’t be assumed to change blood sugar management one way or the other without your doctor’s input.
For anyone on regular anticoagulant medication, the newer, larger bleeding-risk research is reassuring, but your prescribing doctor still has the fuller picture of your individual health history that a general research summary can’t account for.
The throughline here isn’t that fish oil is risky for people with chronic illness. It’s that “should I take this, and at what dose?” is a question best answered by your own care team, who knows your specific diagnoses, medications, and lab values, rather than by a general list of research findings alone.
Why “Natural” Doesn’t Mean Risk-Free
Part of what makes findings like these catch people off guard is a common, understandable assumption that a supplement made from a food source, in this case fish, must be inherently gentler or safer than a pharmaceutical medication. That assumption doesn’t hold up well under closer examination.
Fish oil at supplemental doses delivers concentrations of EPA and DHA far beyond what a typical diet provides, which means it behaves physiologically much more like a medication than simply eating more salmon.
This isn’t a reason to view supplements with suspicion across the board. It’s a reason to treat any substance capable of producing a measurable biological effect, whether it’s sold over the counter or by prescription, with the same basic questions: what dose am I taking, why am I taking it, and does my current health situation change how that dose should be approached? Fish oil holds up well under that kind of scrutiny for most people. It simply isn’t automatically exempt from having real, documented effects worth understanding.
A Comforting Note for Anyone Feeling Anxious About This
If reading through a list of fish oil’s side effects has left you feeling more unsettled than informed, that reaction makes complete sense, especially if you’re already managing a chronic illness or tend to worry about your health. A few things are worth holding onto.
- Every finding described in this article comes from research studying specific populations, specific doses, and specific outcomes, not a blanket statement that fish oil is unsafe. The AFib research points to elevated risk primarily at higher doses in people without existing heart disease. The bleeding research is actually reassuring news that runs counter to a long-standing but largely unsupported worry. The depression finding describes a small statistical difference across many thousands of people over years, not a guarantee that taking fish oil will affect your mood in any noticeable, individual way.
- Millions of people take fish oil regularly without experiencing any of these effects. Reading about a possible risk doesn’t mean it applies to you, and it doesn’t mean you need to stop a supplement that’s working well for you without first talking it through with a doctor who knows your full picture.
- If anything on this list feels relevant to your specific health situation, whether that’s a heart rhythm history, a bleeding concern, or a mood change you’ve noticed since starting a supplement, that’s information to bring to your next appointment. It gives your doctor something concrete to work with, rather than leaving you to sit with worry on your own.
- Taking care of your health includes staying informed and not letting that information spiral into more fear than the data supports. You’re allowed to hold both: taking this seriously enough to ask good questions, and trusting that most people who take fish oil, including many managing chronic conditions, do so without these effects ever becoming a real problem for them.
Frequently Asked Questions
Is fish oil safe for someone with a heart condition?
Often, yes, and in some cases fish oil may specifically benefit people with existing heart disease, even though the AFib risk findings apply more to people without a prior heart condition. Because individual heart conditions vary widely, this is best discussed directly with a cardiologist who knows your specific diagnosis and medication regimen.
Can I take fish oil with blood thinners like warfarin?
Recent large-scale research has not found a significant increase in overall bleeding risk with omega-3 supplementation, even when used alongside antiplatelet medications, which is more reassuring than older assumptions suggested. That said, any change to a supplement routine while on a blood thinner should be discussed with your prescribing doctor first, since they can account for your specific situation.
Does fish oil actually help with depression?
A large, well-designed randomized trial found no meaningful mood benefit from fish oil in adults without depression, and even found a small increase in depression risk over several years of use. Fish oil shouldn’t be relied on as a standalone treatment or preventive strategy for depression; a doctor or mental health provider can help identify appropriate treatment options.
How much fish oil is considered a high dose that might raise AFib risk?
Research in this area hasn’t established a single universal cutoff, but higher supplemental doses, well above those found in a typical diet from eating fish, appear more closely linked to increased risk in the available studies. If you’re taking a high-dose fish oil supplement, it’s reasonable to review the specific dose and reason for taking it with your doctor.
Should I stop taking fish oil because of these findings?
Not necessarily, and definitely not without talking to a doctor first, especially if you’re taking it for a specific medical reason. These findings are meant to inform a conversation about dose and purpose, not to suggest that fish oil is broadly unsafe for most people.
Are fishy burps and stomach upset a sign fish oil isn’t safe for me?
No, these symptoms are common, generally harmless, and often related to product quality or timing rather than a dangerous reaction. Refrigerating your supplement, taking it with meals, or trying an enteric-coated version are simple steps that often resolve these symptoms.
Do these side effects apply to eating fish, or only to supplements?
Most of the research discussed here focused on concentrated fish oil supplements, not on fish consumed as food. Whole fish contains omega-3s at much lower, more variable concentrations alongside other nutrients, and dietary fish intake is generally associated with a different risk and benefit profile than high-dose supplementation.