Supplementation with the omega-3 EPA (4g/day) reduced cardiovascular-related death by 25% in people with high triglycerides on statins compared to those taking a placebo (randomized, double-blind, placebo-controlled trial in 8,179 people from around the world).
The supplement used in this study was a highly pure EPA and the placebo was mineral oil. There has been some concern that the mineral oil may have had adverse effects on lipids since the placebo was associated with a 6% increase in LDL; however, that increase would only translate to a 4% increased heart attack risk. Furthermore, a similar study in Japan using 4g/day of EPA showed a 19% reduction in cardiovascular-related death but there was no placebo control.
Additionally, the effects of the highly purified EPA might be independent of lipid-lowering effects. EPA also decreases inflammation, has effects on blood thinning and cell membrane fluidity…any of these might affect sudden cardiac death.