Prescriptions for the cholesterol-lowering drugs Zetia and Vytorin are written for almost 800,000 Americans every week, at a cost this year of about $4 billion. Yet it still is not clear how well the drugs work.

Nearly two years after the medicines' makers, Merck and Schering-Plough, completed a clinical trial of the drugs, they still have not released the findings. The delay has led to a growing chorus of complaints from cardiologists. And yesterday, the companies responded by promising to publish a portion of the results next March — but not the entire set of data.