For decades, the theory that lowering cholesterol is always beneficial has been a core principle of cardiology. It has been accepted by doctors and used by drug makers to win quick approval for new medicines to reduce cholesterol.

But now some prominent cardiologists say the results of two recent clinical trials have raised serious questions about that theory  and the value of two widely used cholesterol-lowering medicines, Zetia and its sister drug, Vytorin. Other new cholesterol-fighting drugs, including one that Merck hopes to begin selling this year, may also require closer scrutiny, they say.

“The idea that you’re just going to lower LDL and people are going to get better, that’s too simplistic, much too simplistic,” said Dr. Eric J. Topol, a cardiologist and director of the Scripps Translational Science Institute in La Jolla, Calif. LDL, or low-density lipoprotein, is the so-called bad cholesterol, in contrast to high-density lipoprotein, or HDL.

For patients and drug companies, the stakes are enormous. Led by best sellers like Lipitor from Pfizer, cholesterol-lowering medicines, taken by tens of millions of patients daily, are the largest drug category worldwide, with annual sales of $40 billion.