A very high dose of purified EPA fish oil (4g/day) for 5 years reduced heart attacks and strokes by 25% compared to placebo in a multinational randomized double-blind placebo-controlled trial involving over 8,000 patients with high triglycerides (called the REDUCE-IT trial).

There are several differences between this very large randomized placebo-controlled trial and other failed fish oil trials including a very high dose of fish oil (4g/day); the formulation is purified EPA instead of a mixture of DHA +EPA; patients had hypertriglyceridemia.

More reading on the REDUCE-IT trial: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5396348/