Colorectal cancer survivors who smoke cigarettes were at more than twice the risk of death than nonsmoking survivors, adding to existing evidence that cigarette smoking is associated with higher all-cause and colorectal cancer-specific mortality.

The findings come from a new study by American Cancer Society researchers in Atlanta, Georgia. It is one of the largest studies of smoking and colorectal cancer survival and the first study to prospectively collect both pre- and postdiagnosis smoking information. The study was published in the Journal of Clinical Oncology (2015; doi: 10.1200/JCO.2014.58.3831).

Existing evidence links smoking with higher chances of developing colorectal cancer, but its association with survival after colorectal cancer diagnosis is unclear.

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To investigate the association of smoking, before and after diagnosis, with all-cause and colorectal cancer-specific mortality among colorectal cancer survivors, researchers led by Peter Campbell, PhD, identified 2,548 people with newly diagnosed invasive, nonmetastatic colorectal cancer from among 184,000 adults in the American Cancer Society’s Cancer Prevention Study II. Among the 2,548 colorectal cancer survivors, 1,074 died during an average of 7.5 years of follow-up, including 453 as a result of colorectal cancer.

The researchers found those who were smokers before diagnosis had more than twice the risk of death from all causes (relative risk [RR] = 2.12) as well as from risk of dying of colorectal cancer (RR = 2.14). Former smoking before diagnosis was associated with higher all-cause mortality (RR = 1.18) but not with colorectal cancer-specific mortality. Smoking after diagnosis was also associated with more than double the risk of overall mortality (RR = 2.22) over the course of the study, and was associated with nearly twice the risk of colorectal cancer-specific mortality (RR = 1.92).

The authors said a plausible explanation is that smokers have pathologically more aggressive tumors, or that smoking may decrease the efficacy of colorectal cancer treatment.

“Further research is needed to understand mechanisms whereby smoking may increase colorectal cancer-specific mortality and determine if quitting smoking after diagnosis lowers the risk of colorectal cancer-specific mortality,” the authors concluded.