Millions of Americans take statins, which are a common class of medication that lowers cholesterol.

While statins might be effective at that, new research says they can also trigger a rare but serious autoimmune muscle disease in a small number of the 30 million Americans who take them.

Statins, researchers say, can sometimes cause the body to produce antibodies against its own proteins, creating a condition that gets progressively worse -- not better -- even after the medication is discontinued.

The painful and debilitating disorder is uncommon and can be treated with steroids and other immune-suppressing drugs, so the researchers caution that people who must be on statins to reduce serious risk of heart disease and stroke should not avoid the drugs.

"We have long known that there must be environmental triggers to the development of autoimmune disorders," said Andrew L. Mammen, assistant professor of neurology and medicine at Johns Hopkins University. "Now we have evidence that this medication is just such a trigger and, under certain circumstances, provokes a sustained autoimmune disease."

Statins are a class of drug that lowers cholesterol levels by inhibiting the enzyme HMG-CoA reductase, which plays a key role in producing cholesterol in the liver. Some of the best-selling statins are Lipitor, which racked up sales of $12.4 billion in 2008, and Crestor, Zocor, and Vytorin.

Details of the study, published online in the journal Arthritis & Rheumatism, could lead to lab tests that identify early autoimmune muscle disease, guide treatment before symptoms escalate and, possibly, predict who is at risk before statins are prescribed.

Mammen cautions that the research describes a rare side effect, noting that statins are a "fantastic medication" with proven value.

"No one who needs statins should be afraid to take them because of the slim risk of developing this autoimmune disease," he said. "Statins save a huge number of lives. They dramatically reduce the risk of strokes and heart attacks."

The researchers said their ultimate goal is to determine before patients start taking statins who might be sensitive to the medication and who might be susceptible to its potentially toxic effects on the muscle.

"We want to prevent this autoimmune disease," Mammen said.