WASHINGTON - A drug that boosts people's good cholesterol did not go on to prevent heart attacks or strokes, leading U.S. officials to abruptly halt a major study Thursday.

The disappointing findings involve superstrength niacin, a type of B vitamin that many doctors prescribe as potential heart protection. "This sends us a bit back to the drawing board," said Susan Shurin, cardiovascular chief at the National Institutes of Health.

The bad kind of cholesterol, called LDL, is the main source of artery clogs. Popular statin drugs, sold under such names as Zocor and Lipitor, plus generic forms, are mainstays in lowering LDL. Yet many statin users still have heart attacks, because LDL isn't the whole story.

HDL cholesterol, the good kind, helps fight artery buildup by carrying fats to the liver to be disposed of. That's one reason that people with too little HDL also are at risk of heart disease. So scientists are testing whether giving HDL-boosting drugs in addition to statins could protect more heart patients.

The newest study tested Abbott Laboratories' Niaspan, an extended-release form of niacin that is a far higher dose than is found in dietary supplements. Previous studies have shown it does boost HDL levels. But no one knew if that translated into fewer heart attacks.

Researchers enrolled more than 3,400 statin users in the United States and Canada who had stable heart disease and well-controlled LDL but were at risk because of low HDL levels and too much of a different bad fat, triglycerides. They were given either Niaspan or a dummy pill to add to their daily medicine.

As expected, the Niaspan users saw their HDL levels rise and their levels of risky triglycerides drop more than people who took a statin alone. But the combination treatment did not reduce heart attacks, strokes, or the need for artery-clearing procedures such as angioplasty, the NIH said.

That finding "is unexpected and a striking contrast to the results of previous trials," said Jeffrey Probstfield of the University of Washington, who helped lead the study.

But it led the NIH to stop the study 18 months ahead of schedule.

Adding to the decision was a small rise in strokes in the high-dose niacin users - 28 among those 1,718 people given Niaspan, compared with 12 among the 1,696 placebo users. The NIH said it wasn't clear if that small difference was merely a coincidence; previous studies have shown no stroke risk from niacin.

What's the message for heart patients? Statin users with very low LDL levels, like those in this study, do not need an extra prescription for niacin, said University of Colorado cardiologist Robert Eckel, who was not involved with the study.