Background Omega-3 supplements are popular for cardiovascular disease (CVD) prevention. We aimed to assess the association between dose-specific omega-3 supplementation and CVD outcomes.

Design We included double-blind randomised clinical trials with duration ≥1 year assessing omega-3 supplementation and estimated the relative risk (RR) for all-cause mortality, cardiac death, sudden death, myocardial infarction and stroke. Primary analysis was a stratified random-effects meta-analysis by omega-3 dose in 4 a priori defined categories (<1, 1, 2, ≥3 of 1 g capsules/day). Complementary approaches were trial sequential analysis and sensitivity analyses for triglycerides, prevention setting, intention-to-treat analysis, eicosapentaenoic acid, sample size, statin use, study duration.

Results Seventeen studies (n=83 617) were included. Omega-3 supplementation as ≤1 capsule/day was not associated with any outcome under study; futility boundaries were crossed for all-cause mortality and cardiac death. For two capsules/day, we observed a statistically significant reduction of cardiac death (n=3, RR 0.55, 95% CI 0.33 to 0.90, I²=0%); for ≥3 capsules/day we observed a statistically significant reduction of cardiac death (n=3, RR 0.82, 95% CI 0.68 to 0.99, I²=0%), sudden death (n=1, RR 0.70, 95% CI 0.51 to 0.97) and stroke (n=2, RR 0.74, 95% CI 0.57 to 0.95, I²=0%).

Conclusion Omega-3 supplementation at <2 1 g capsules/day showed no association with CVD outcomes; this seems unlikely to change from future research. Compared with the robust scientific evidence available for low doses, the evidence for higher doses (2–4 1 g capsules/day) is weak. The emerging postulated benefit from high-dose supplementation needs replication and further evaluation as to the precise formulation and indication.