A summary of published research, not medical advice. See the Disclaimer.
Studies: every indexed trial and review on this topic, scored
Omega-3 fatty acids are polyunsaturated fats. The long-chain forms, EPA and DHA, come mainly from fish and fish oil. ALA comes from plants such as flax and walnuts.
Best-supported effect: Lower triglycerides in adults with high triglycerides. Helps (strong evidence).
Most-cited weak claim: Fewer major cardiovascular events (heart attack, stroke and cardiovascular death combined) in adults without heart disease. Makes little or no difference (strong evidence).
How we grade evidence · Last reviewed: October 2026
What you need to know
In adults with high triglycerides, omega-3 lowered triglycerides, with strong evidence. In statin-treated adults with raised triglycerides and heart disease or diabetes, high-dose purified EPA reduced cardiovascular events (moderate evidence). In adults without heart disease it made little or no difference to major cardiovascular events or invasive cancers, and in adults with and without heart disease it made little or no difference to deaths from any cause, all with strong evidence. Atrial fibrillation was more common, with a hazard ratio of 1.25 across 7 trials and 1.49 above 1 g a day. With high-dose purified EPA, bleeding was a little more common, and the label reports more bleeding in people also on aspirin, clopidogrel or warfarin. The primary-prevention trial used 1 g a day, and the triglyceride and purified EPA trials used 4 g a day.
| Effect | Verdict | What was measured | Studied in |
|---|---|---|---|
| Lower triglycerides | Helps (strong evidence) Cochrane review of 86 controlled trials, consistent drops in triglycerides (blood fats) |
Lab marker | Adults with triglycerides of 200–499 mg/dL or higher |
| Fewer cardiovascular events with high-dose purified EPA | Probably helps (moderate evidence) One large trial, participants did not know what they got, manufacturer-funded and not repeated |
Hard outcome | Statin-treated adults with raised triglycerides and heart disease or diabetes |
| Lower depressive symptoms in adults | Probably helps a little (moderate evidence) Pooled analysis of 26 adult trials, small benefit only with EPA-rich formulas, none with DHA-rich ones |
Self-reported | Adults with depression |
Triglycerides: an American Heart Association science advisory reviewed the trials of 4 g/day prescription omega-3. In people starting at 500 mg/dL or above, EPA plus DHA products lowered triglycerides by at least 30%. In people with triglycerides of 200–499 mg/dL, EPA plus DHA and EPA alone lowered triglycerides by similar amounts [1]. A Cochrane review of 86 RCTs with 162,796 participants found long-chain omega-3 lowered triglycerides by about 15%, with larger falls at higher doses [2]. Helps (strong evidence): many consistent RCTs measured triglycerides directly.
Purified EPA: one trial gave 4 g/day of a purified EPA ethyl ester to 8,179 statin-treated adults for a median of 4.9 years. The primary endpoint occurred in 17.2% versus 22.0% on placebo, a 25% relative reduction [7]. The manufacturer funded the trial. Probably helps (moderate evidence): one large blinded trial of clinical events, downgraded because it was manufacturer-funded and not repeated.
Depression in adults: a meta-analysis of 26 RCTs with 2,160 participants found a small benefit, limited to formulas with at least 60% EPA at up to 1 g/day of EPA. DHA-dominant formulas showed no benefit [8]. Probably helps a little (moderate evidence): downgraded because results disagreed and the benefit appeared only in subgroups defined by formula and dose.
| Effect | Verdict | What was measured | Studied in |
|---|---|---|---|
| Fewer major cardiovascular events overall | Makes little or no difference (strong evidence) 25,871 adults without heart disease at 1 g/day, no statistically clear drop |
Hard outcome | Adults without heart disease (men 50+, women 55+) |
| Lower all-cause mortality | Makes little or no difference (strong evidence) Cochrane review and 25,871-person trial found no clear reduction |
Hard outcome | Adults with and without heart disease |
| Fewer strokes | Probably makes little or no difference (moderate evidence) Cochrane review and 25,871-person trial found no lower stroke risk |
Hard outcome | Adults with or at risk of cardiovascular disease |
| Fewer invasive cancers | Makes little or no difference (strong evidence) One very large, independently funded trial in adults without cancer who did not know what they got |
Hard outcome | Adults without cancer (men 50+, women 55+) |
More on these results under Claimed but unsupported or contested.
Cancer: the same 25,871-person trial found no difference in invasive cancer (hazard ratio 1.03) [6]. Makes little or no difference (strong evidence): one very large, blinded, independently funded trial.
| Claim | Verdict | What was measured | Studied in |
|---|---|---|---|
| Fewer heart attacks and coronary deaths | May help a little (weak evidence) Pooled analyses found small reductions, but results were inconsistent |
Hard outcome | Adults with or at risk of cardiovascular disease |
| Lower depressive symptoms in children and adolescents | Unknown (very weak evidence) 5 small trials with 228 young participants, results on symptoms and full recovery differ |
Self-reported | Children and adolescents with depression |
Heart attacks and coronary deaths: a meta-analysis of 13 RCTs with 127,477 participants found about 8% lower risk of heart attack and of coronary death, even after excluding the purified EPA trial. Risk reductions appeared to rise with dose [3]. A meta-analysis of 40 trials found about 13% fewer heart attacks, with larger effects at higher doses [4]. A third meta-analysis, of 38 RCTs, found lower cardiovascular death, with larger reductions in trials of EPA alone than in trials of EPA plus DHA [5]. May help a little (weak evidence): results were inconsistent, the Cochrane review rated the coronary benefit as low-certainty evidence [2], and heart attack was only a secondary outcome in the largest primary-prevention trial [6].
Major cardiovascular events: in a trial of 25,871 adults given 1 g/day for a median of 5.3 years, the combined rate of heart attack, stroke and cardiovascular death was not significantly lower than with placebo (hazard ratio 0.92, 95% CI 0.80–1.06) [6]. The Cochrane review found little or no effect on combined cardiovascular events [2].
All-cause mortality and stroke: the Cochrane review found little or no effect on death from any cause or on stroke [2]. The 25,871-person trial also found no lower risk of stroke or of death from any cause [6].
Depression in young people: a Cochrane review of 5 small trials with 228 participants found a possible reduction in self-reported symptoms and little or no effect on remission, both at very low certainty [9]. Unknown (very weak evidence).
Much of the heart claim traces back to observational studies of fish-eating populations. The trials cited here did not test long-term effects on cognition in healthy adults or compare fish oil with eating fish.
Shown in humans
Shown only in animals or cells
Theory only
| Side effect | Verdict | How often vs placebo | Source | Studied in |
|---|---|---|---|---|
| Atrial fibrillation | Happens (strong evidence) Pooled analysis of 7 trials, higher risk, especially above 1 g/day |
Hazard ratio 1.25 across 7 trials (1.49 above 1 g/day, 1.12 at 1 g/day or less); hospitalized 3.1% vs 2.1% in the purified EPA trial | [10][7] | |
| More atrial fibrillation recurrences in people who already have it | May happen (weak evidence) Drug label reports possible link in one trial of people with the condition |
53% vs 47% (paroxysmal), 52% vs 35% (persistent); combined hazard ratio 1.25 | [12] | People with the condition |
| Bleeding with high-dose purified EPA (4 g/day) | Probably happens (moderate evidence) Trial of purified EPA (an omega-3 fat) and pooled analysis show small rise in actual risk |
Any bleeding 12% vs 10%, serious bleeding 3% vs 2% in the purified EPA trial; about 50% higher relative risk, 0.6% higher absolute risk, in a pooled analysis | [5][7][11][13] | Statin-treated adults with raised triglycerides and heart disease or diabetes |
| Bleeding with omega-3 products overall | Probably no harm (moderate evidence) 11 trials of all omega-3 products, no overall rise in bleeding |
No overall difference across 11 trials (rate ratio 1.09, 95% CI 0.91–1.31); no difference in brain or gut bleeding | [13] | |
| Bleeding around surgery | Probably no harm (moderate evidence) 1,516-person heart surgery trial plus small spinal surgery study, no increase |
No increase in a 1,516-person cardiac surgery trial; no increase in a small spinal surgery case-control study | [16][17] | |
| Higher LDL cholesterol with DHA-containing products | Probably happens (moderate evidence) Seen at triglycerides of 500 mg/dL or above, no rate reported |
Not reported as a rate; seen in people with triglycerides of 500 mg/dL or above | [1][12] | People with triglycerides of 500 mg/dL or above |
| Burping | Probably happens (moderate evidence) Drug label for EPA plus DHA (omega-3 fats), 4% vs 1% on placebo |
4% vs 1% | [12] | |
| Indigestion | Probably happens (moderate evidence) Drug label for EPA plus DHA (omega-3 fats), 3% vs 2% on placebo |
3% vs 2% | [12] | |
| Change in taste | Probably happens (moderate evidence) Drug label for EPA plus DHA (omega-3 fats), 4% vs under 1% on placebo |
4% vs under 1% | [12] | |
| Musculoskeletal pain, swelling of the legs, constipation, gout | Probably happens (moderate evidence) Drug label for purified EPA (an omega-3 fat), at least 3% and above placebo |
At least 3%, and at least 1 point above placebo, with purified EPA | [11] | |
| Joint pain, throat pain | Probably happens (moderate evidence) Drug label for purified EPA (an omega-3 fat), 12-week trials, at least 1 point above placebo |
At least 1 point above placebo with purified EPA (12-week trials) | [11] | |
| Raised liver enzyme (ALT) | Unknown (very weak evidence) Drug label for EPA plus DHA (omega-3 fats) cited, no rate vs placebo reported |
Not reported | [12] |
EPA-only products did not raise LDL in the triglyceride trials [1][11]. Across trials, EPA alone was linked to more total bleeding (rate ratio 1.49) [5]. In a pooled analysis, high-dose purified EPA raised the relative risk of bleeding by 50%, while the absolute rise was 0.6%; bleeding risk rose with EPA dose [13].
This lists what studies and labels report. A pharmacist or prescriber can say whether it applies to a specific person.
This lists what studies and labels report. A pharmacist or prescriber can say whether it applies to a specific person.
| Combined with | What happened | Verdict | Source | Studied in |
|---|---|---|---|---|
| Anticoagulants or antiplatelet drugs (aspirin, clopidogrel, warfarin), with high-dose purified EPA | The purified EPA label reports more bleeding in people also on these drugs in its trial, a subgroup of one manufacturer-funded trial; across 11 trials of all omega-3 products, the extra bleeding risk was not linked to background antiplatelet use; labels advise bleeding monitoring | May happen (weak evidence) Subgroup of one manufacturer-funded trial, no link to aspirin-type drugs across 11 trials |
[11][12][13] | |
| Warfarin plus high-dose fish oil | Case reports of serious brain bleeding after falls or head trauma in older adults | Unknown (very weak evidence) Reports of single patients only, in older adults after falls or head injury |
[14][15] | Older adults |
| Warfarin (blood levels and clotting) | Purified EPA 4 g/day did not change warfarin levels or its effect on clotting in 25 healthy adults | May not increase it (weak evidence) 25 healthy adults on purified EPA (an omega-3 fat), levels and clotting unchanged |
[11] | 25 healthy adults |
| Statins (atorvastatin, simvastatin, rosuvastatin) | 4 g/day of either product did not change statin blood levels in healthy adults | May not increase it (weak evidence) Healthy adults only, statin blood levels unchanged at 4 g/day |
[11][12] | Healthy adults |
| Omeprazole, rosiglitazone | Purified EPA 4 g/day did not change their blood levels in healthy adults | May not increase it (weak evidence) Healthy adults only, drug blood levels unchanged on purified EPA (an omega-3 fat) |
[11] | Healthy adults |
These are the amounts used in the studies. They are not recommendations.
Trials used EPA and DHA as fish oil, as prescription ethyl esters, or as purified EPA alone. Plant ALA was studied separately and showed little or no effect on mortality [2]. Doses ranged from about 0.5 g/day to more than 5 g/day [2]. The primary-prevention trial used 1 g/day [6]. The triglyceride and purified EPA trials used 4 g/day, given as 2 g twice daily in the EPA trial [1][7]. The depression trials that showed benefit used formulas with at least 60% EPA and up to 1 g/day of EPA [8]. Cardiovascular trials ran for about 1 to 7 years [2], with medians near 5 years in the largest [6][7]. The youth depression trials lasted 10 to 16 weeks [9].