Dr. David Jenkins and Dr. Paul Dorian, both of St. Michael's Hospital and the University of Toronto, did a systematic review of trials where patients with implantable cardioverter defibrillators used fish oil supplements and found significant differences among the trials, indicating fish oil may be beneficial to some patients, but have a negative impact on others.

Multiple large-scale population studies and randomized controlled trials found intake of recommended amounts of DHA and EPA in the form of dietary fish or fish oil supplements can reduce the risk of death, heart attack and dangerous abnormal heart rhythms in people with known cardiovascular disease, as well as potentially slow hardening of the arteries and lower blood pressure. However, there is evidence also shows high doses of fish body oils can have harmful effects, such as an increased risk of bleeding.

The study, published in the Canadian Medical Association Journal, said the data suggests that caution should be exercised when analyzing data from certain subgroups, such as men with stable angina or for patients with implantable cardioverter defibrillators with a history of ventricular tachycardia and not taking antiarrhythmic medications.