Compared with statin therapy alone, adding eicosapentaenoic acid (EPA) reduced the rate of non-fatal major coronary events by 19 per cent in people with total cholesterol levels over 6.5mmol/l.
The study involved 18,645 people with hypercholesterolaemia who were randomly assigned to receive 1.8g of EPA daily in combination with a statin or a statin only.
The majority of patients took 5mg simvastatin or 10mg pravastatin daily.
Over an average follow-up of 4.6 years, 2.8 per cent of the EPA group and 3.5 per cent of controls had a major coronary event.
Among the 3,664 patients with a history of coronary artery disease (CAD), adding EPA cut the risk of non-fatal MI, unstable angina and coronary revascularisation by 19 per cent.
In those with no history of CAD, a similar, but non-significant, 18 per cent reduction in major coronary events was seen in those given EPA.
Rates of sudden cardiac death and coronary death were similar between the groups.
The benefits of fish oil seem independent of its effect on LDL-cholesterol, because this was reduced by 25 per cent from a baseline level of 4.7mmol/l in both groups.
Instead, EPA could help reduce the risk of coronary events through its effects on attenuation on thrombosis, inflammation and arrhythmia, say the researchers.