Scientists may have found a way to test for and possibly avoid the most serious side effect of cholesterol-lowering statin drugs, one of the top-selling medicines in the world.

In rare cases, statins can cause muscle pain and weakness. Researchers have identified a genetic variation that seems to predict more than half of these cases. People on statins who have the variant were about five to 17 times as likely as others to develop muscle problems, a serious side effect that can lead to muscle breakdown, kidney failure and death.

The finding raises hope that a test could be developed to screen heart patients to find out who is at greatest risk. Normally, muscle weakness caused by statins affects 1 out of 10,000 patients a year.

"It could become a very simple check," said Rory Collins of the University of Oxford, who co-authored the study published in today's New England Journal of Medicine.

But doctors say having this knowledge doesn't mean the millions taking statins should be tested, especially those who are having no problem.

Statins are mostly prescribed to prevent heart attacks in people with clogged arteries and work by dramatically lowering LDL, or "bad cholesterol." Last year, global sales for statins topped $14.8 billion, according to the health-care research firm IMS Health.

Doctors are increasingly prescribing higher statin doses to drive down bad cholesterol. The risk of muscle problems increases when people take statins at higher doses or with other medications.

The genetic analyses drew from two previous large studies that were funded in part by Merck & Co., which makes the former cholesterol blockbuster drug Zocor, now available as the low-cost generic simvastatin.