In a heartbeat.

Fish oil supplements failed to reduce--and may have even boosted--the risk of irregular heartbeats in people who have implanted defibrillators. The devices monitor the heart rate and automatically administer a tiny electrical jolt when needed to stabilize the heart rhythm.

Researchers studied some 200 patients with implanted defibrillators who had recent episodes of arrythmia--either tachycardia (the heart beats abnormally fast) or fibrillation (the heart flutters, rather than beats, so it pumps little or no blood) in the ventricle, or lower heart chamber. Both types of arrythmia can lead to "sudden death" heart attacks, which kill half a million Americans every year.

Each patient was randomly assigned to take either a placebo (olive oil) or fish oil (1.8 grams a day, including 0.8 grams of eicosapentaenoic acid, or EPA, and 0.5 grams of docosahexaenoic acid, or DHA) for roughly two years.

While the fish oil appeared to have no impact on patients who initially had fibrillation, it seemed to make tachycardia patients worse. Within the first six months, the implanted defibrillators had to stabilize potentially fatal abnormal heart rhythms in 61 percent of tachycardia patients taking fish oil, but only 37 percent of tachycardia patients taking the placebo. By two years, the debrillators had to stabilize the hears of 79 percent of patients taking fish oil versus 65 percent of those on the placebo.

What to do: It's not clear why these results differ from earlier studies that found a lower risk of sudden death in people who consume fish or fish oil pills. But to play it safe, the authors recommend that people with arrythmias (tachycardia or fibrillation) who have an implanted defibrillator not take fish oil pills.

J. American Medical Association 293: 2884, 2005.