People who take cholesterol-lowering statins for at least one to two years appear to be less likely to develop gallstones, a study of nearly two million Danish residents shows.

Among those receiving at least five prescriptions for the drugs, the risk of developing gallstones fell by 11 to 24 percent - and the more prescriptions, the larger the decrease.

People who had just started taking statins, meanwhile, had a higher risk of developing gallstones, which form when bile stored in the gallbladder hardens into pieces of stone-like material.

That makes sense, study author Dr. Rune Erichsen of Aarhus University Hospital in Denmark told Reuters Health. People are prescribed statins to lower their cholesterol, which is one of the ingredients in gallstones. What’s more, High cholesterol is tied to obesity, poor diet, and other factors that can increase the risk of gallstones.

“This is the reason that we find an increased gallstone disease risk in patients (who) just started taking statins,” Erichsen said. But after a while on statins, those risks appeared to diminish, which squares with earlier research.

Some gallstones are tiny, while others can be as large as a golf ball. Often they cause no symptoms, but if they become lodged in the wrong place - blocking the outlet from the gallbladder or pancreas, for instance - they can cause inflammation and severe pain. Depending on the country, between six and 50 percent of adults in the Western world eventually develop some form of gallstone disease.

Statins, too, are common in the Western world. They are prescribed more than any other class of medications in the U.S. and used by about one in five adults. Their cost ranges widely, from $11 to more than $200 per month, and they have been shown to increase the rates of liver dysfunction, kidney failure, muscle weakness and cataracts in some populations.

To investigate further how statins impact gallstones, Erichsen and colleagues reviewed nationwide data collected from 1.7 million people living in Northern Denmark. Nearly 33,000 people developed gallstones between 1996 and 2008.

Reporting in the American Journal of Epidemiology, the authors found that more five percent of people who developed gallstones also took statins - slightly more than among those without gallstones.

But after accounting for diseases tied to gallstones, such as liver and heart problems, people with at least five statin prescriptions had lower rates of gallstones than people who didn’t take the drug.

Those with 20 or more prescriptions, for instance, had a 24-percent decrease in their chances of developing gallstones compared with non-users.

In an e-mail, Erichsen noted that the study cannot prove that statins directly caused the decrease.

“However, in the specific issue of statins and gallstone disease, there is compelling evidence that long-term statin use reduces the risk of gallstone disease,” the researcher concluded. “Statins reduce the synthesis of cholesterol, so less cholesterol is excreted in the liver, and the risk of gallstone disease eventually seems to decrease.”

SOURCE: American Journal of Epidemiology, published November 17, 2010.