In a new JAMA report, investigators found that fish oil may increase the risk of cardiac arrhythmias in some patients. The 2-year study at Oregon Health and Science University included 200 patients with ventricular tachycardia or ventricular fibrillation who were receiving implantable cardioverter defibrillators. Participants were randomly assigned to receive capsules containing fish oil (1.8 grams/day) or olive oil placebo.

Among those initially diagnosed with ventricular tachycardia, 61% experienced arrhythmias treated by implantable defibrillator within 6 months, compared to 37% receiving placebo. At 12 and 24 months, the figures were 66% and 79%, respectively, for the fish oil group, compared to 43% and 65% for the placebo group (P = 0.007). A similar trend was apparent when all participants were included, although the increased risk was significant only for those with ventricular tachycardia.

Previous studies have had mixed results. Some have suggested a benefit of fish oils for some patients; others have shown increased arrhythmia risk. The Oregon investigators recommend against the routine use of fish oils in patients with ventricular arrhythmias.

Raitt MH, Connor WE, Morris C, et al. Fish oil supplementation and risk of ventricular tachycardia and ventricular fibrillation in patients with implantable defibrillators: a randomized controlled trial. JAMA. 2005;293:2884-91.

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