Smokers have much higher odds of developing bladder cancer than previously believed, and the changing makeup of cigarettes may be a factor, new research shows.

While cigarettes are more typically associated with lung cancer, researchers have known for years that smoking also raises the risk of bladder cancer among both men and women. Previous studies based on people who smoked prior to the 1990s had put the risk for smokers at about three times the risk seen among nonsmokers.

But in a new study, published in The Journal of the American Medical Association, a team of scientists at the National Cancer Institute used a larger population group and more recent data, following a half million people between 1995 and 2006. They found that current smokers were four times as likely to develop bladder cancer compared with people who never smoked, and former smokers had 2.2 times the risk.

The researchers speculate that the new, larger risk may be a reflection of the more toxic chemicals added to cigarettes nowadays -- some of which may specifically have a link to bladder cancer. Bowing to public pressure, many cigarette makers have cut back on tar and nicotine in recent years, but replaced them with other toxic substances like beta-napthylamine, which is known to raise the risk of bladder cancer.

That could explain, at least in part, why bladder cancer rates have remained steady over the years, even as the number of people smoking has dropped sharply since 1965. About 70,000 cases of bladder cancer will be diagnosed this year, health officials say.

''The prevalence of smoking has decreased in the United States, thankfully,'' said Dr. Neal D. Freedman, an investigator with the National Cancer Institute and an author of the study. ''But a large proportion of the U.S. still smokes, and rates of bladder cancer have stayed the same over the past 30 years. That's different from lung cancer, where the rates have decreased over the past 30 years.''

Dr. Freedman said the stable rates of bladder cancer could potentially be owed to greater awareness and other unknown changes in risk factors. Other than beta-naphthylamine, it's not clear what chemicals in cigarettes may also be playing a role. Dr. Freedman said more study was needed.

''A real explanation remains elusive,'' he said.
