Supplement Studies  /  evidence, scored
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25 supplements · 104 claims scored · 18 more in review

Evidence File 028 · Omega-3 · Method v0.14 of 4 claims researched

Omega-3EPA / DHA, fish or algal oil · 4 claims

Benefit and harm show up in the same analysis, and the formulation that helps most is also the one that raises the arrhythmia signal highest.

4Claims scored
5Sources
Varies widelyStudied dose
Jul 26Reviewed

Claim 01

Reduces cardiovascular events
7.0/10
Moderate·Multiple endpoints significant in pooled analysis

A 2025 pooled analysis in patients with established cardiovascular disease or at high risk found significant reductions in cardiovascular mortality (p=0.02), cardiovascular disease (p=0.03), coronary heart disease (p=0.007), myocardial infarction (p=0.008), fatal MI (p=0.0004) and revascularisation (p=0.003).

Subgroup analysis found EPA monotherapy significantly outperformed EPA+DHA combination therapy across cardiovascular mortality, CVD events, MACE, coronary heart disease, MI, fatal MI and revascularisation. Most retail fish oil is a combination product.

This is a treatment effect in a high-risk population. It is not evidence for taking fish oil as general cardiovascular insurance if you are healthy.

The working

8Quantity
6Quality
6Consistency
9Directness

(8 × 0.25) + (6 × 0.35) + (6 × 0.25) + (9 × 0.15) = 7.0
Directness scores 9 because the endpoints are hard clinical events, not markers. Consistency held at 6 because effect depends heavily on formulation.

Claim 02

Increases atrial fibrillation risk
6.2/10
Contested·Replicated across analyses · now challenged

A 2021 analysis of randomised cardiovascular outcome trials found AF risk increased by 25% with omega-3. A pooled analysis of 83,112 individuals across eight large trials found a 24% increased relative risk (p=0.0002). The 2025 cardiovascular meta-analysis above also found a significant increase (p=0.01), concentrated in EPA monotherapy.

The challenge is substantial. A 2025 reanalysis of 34 trials and 114,326 individuals argues prior estimates were inflated by inconsistent AF reporting, informative censoring — treated patients survived longer and so had more time to develop AF — and discrepancies in how AF cases were identified. It incorporated trial registries and unpublished datasets, and included trials reporting zero AF events.

Flag: that reanalysis is a preprint and has not completed peer review. Separately, a biomarker meta-analysis of 17 prospective cohorts found higher circulating marine omega-3 associated with lower AF incidence — suggesting dose may determine direction.

The working

7Quantity
6Quality
4Consistency
9Directness

(7 × 0.25) + (6 × 0.35) + (4 × 0.25) + (9 × 0.15) = 6.2
Consistency scores 4. Several independent analyses agree on the direction, and a large 2025 reanalysis argues the earlier ones systematically overestimated it. Wide band.

Claim 03

EPA alone outperforms EPA+DHA combinations
5.7/10
Emerging·Consistent across endpoints · single analysis

Within the 2025 meta-analysis, EPA monotherapy beat EPA+DHA combination therapy on cardiovascular mortality (p=0.01), CVD events, MACE, coronary heart disease and MI (all p<0.00001), fatal MI (p=0.004) and revascularisation (p<0.0001).

It is a clean, consistent pattern, and it comes from subgroup comparisons within a single synthesis rather than head-to-head trials. The authors recommend future meta-analyses stratify the same way.

The trade-off is the point. EPA monotherapy is also where the atrial fibrillation increase concentrated. Whatever mechanism is producing the cardiovascular benefit appears to be producing the arrhythmia signal too.

The working

5Quantity
5Quality
6Consistency
8Directness

(5 × 0.25) + (5 × 0.35) + (6 × 0.25) + (8 × 0.15) = 5.7
The pattern is consistent across seven separate endpoints within one analysis, which is suggestive but not the same as independent replication.

Claim 04

Causes gastrointestinal side effects
6.6/10
Supported·Significant increase versus control

The same pooled analysis that found cardiovascular benefit also found a significant increase in gastrointestinal adverse events (p=0.02).

Not dangerous, and worth stating plainly because it is the most common reason people stop taking fish oil. A separate systematic review has also examined bleeding risk in patients receiving omega-3.

The working

6Quantity
6Quality
7Consistency
8Directness

(6 × 0.25) + (6 × 0.35) + (7 × 0.25) + (8 × 0.15) = 6.6
Scored as support for the side-effect claim. Modest evidence base, consistent direction, and it matches what people actually report.

Dosage as studied

Most-studied regimen
Varies widely by trial and formulation

EPA monotherapy and EPA+DHA combinations behave differently on both benefit and harm, so the dose question cannot be separated from the form question.

EPA monotherapy
Outperformed EPA+DHA combinations on cardiovascular mortality, MACE, coronary heart disease, MI and revascularisation.
The trade-off
EPA monotherapy also carried the significant increase in atrial fibrillation risk.
Gastrointestinal effects
Significantly increased GI adverse events versus control (p=0.02).
Dietary comparison
A biomarker meta-analysis of 17 prospective cohorts found higher circulating omega-3 associated with lower AF risk — the opposite direction to the supplement trials.