Almost 4,000 men and 627 women were diagnosed with bladder cancer during the study.
The researchers found that current smokers now had four times the risk of developing bladder cancer compared to people who never smoked.
Overall, smoking was attributed to the development of about half of all bladder cancers.
And, the risk of bladder cancer tends to rise with the number of cigarettes smoked, according to Freedman. Even people who smoke one to 10 cigarettes a day have a significantly higher risk of bladder cancer than people who don't smoke, according to the study.
He also noted that while the risk of bladder cancer remains elevated for former smokers, it does go down the longer someone stays off cigarettes.
Freedman said the authors don't know exactly why the risk of bladder cancer has increased for smokers, but they suspect changes in the composition of cigarette smoke. Cigarettes contain less tar and nicotine now, but cigarette smoke is likely have more cancer-causing chemicals such as beta-napthylamine, which is known to cause bladder cancer, suggest the authors.
"Cigarettes may be even more toxic now than they were years ago," said Dr. Len Lichtenfeld, deputy chief medical officer for the American Cancer Society.
"People who consider themselves casual smokers may think they're not getting as many of the bad effects of smoking, but in this study, the impact of smoking increases the risk of bladder cancer dramatically," he pointed out.
Lichtenfeld advised that anyone who smokes should stop, though he acknowledged that doing so isn't easy. "Cigarettes are a very powerful addiction that's difficult to quit."
But, both experts agreed that for preventing bladder and other cancers, as well as for heart health, quitting smoking -- or never starting to smoke -- is one of the best things you can do.
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