The researchers managed to adjust for other colorectal cancer risk factors, such as not getting screened, obesity, physical activity and eating a lot of red or processed meats. The issue is tricky because people who smoke are already more likely to engage in these types of behavior.
"When they took all of those other things out, smoking was still a small, elevated risk," said Dr. Michael John Hall, director of the gastrointestinal risk assessment program at Fox Chase Cancer Center in Philadelphia.
"We already know that smoking is bad. That doesn't change. A positive thing that comes out of this is that if you can stop smoking earlier, you eliminate your risk later on, but the more you smoke, the risk is higher."
This large prospective study, which followed almost 200,000 people over 13 years, found that current smokers had a 27 percent increased risk of colorectal cancer and former smokers a 23 percent increased risk compared with people who had never smoked.
People who had smoked for at least half a century had the highest risk -- 38 percent higher than never smokers -- of developing colorectal cancer
The good news is that people who tossed their cigarettes before the age of 40 or who had not smoked for 31 or more years had no increased risk.
Two other studies focused on the risk of secondhand smoke, or passive smoking. In one, children exposed to secondhand smoke had a higher risk of developing lung cancer as adults, researchers from institutions including the U.S. National Cancer Institute found. In another, California researchers found that adult non-smoking women who had spent long periods of time in smoking environments upped their odds of developing postmenopausal breast cancer.
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