Doctors should stop using the drugs Tambocor and Enkaid to treat patients with mildly irregular heartbeats because studies show an unexpectedly high death rate among users, federal authorities say.

The two drugs may continue to be used to treat patients in emergencies in which the heart has slipped into a life-threatening rhythm.About 200,000 heart patients take the medicines regularly, and most of these patients suffer only from mild heartbeat irregularities, officials at the National Heart, Lung and Blood Institute said this week.

The rate of deaths among patients taking the drugs was discovered only after a mortality study was conducted by the NHLBI. The study was intended to determine if heart attack patients who suffered mild heartbeat irregularities lived longer if they took medicine to modify that irregular beat, or arrhythmia.

Instead, the study found that death was almost twice as frequent among those who took the drugs as among those who took the placebo.