Many people continue to smoke after being diagnosed with cancer, according to a new analysis published on Monday. 

Harvard researchers found that 39 percent of lung cancer patients and 14 percent of colorectal cancer patients were smoking at the time of diagnosis, and 14 percent of lung patients and nine percent of colorectal cancer patients were still smoking five months after being diagnosed.

Researchers found factors and characteristics that predicted continued smoking differed by cancer type. 

Colorectal cancer patients are less likely than lung cancer patients to quit smoking following diagnosis. 

On average, lung cancer patients who continued smoking were on Medicare or other public health insurance, had a lower body mass index, had low emotional support, had not received chemotherapy, had not had surgery, had prior heart disease and smoked a high number of cigarettes daily at some point in their lives.

Colorectal cancer patients who continued to smoke were usually male, completed less education, uninsured, had not had surgery, and had once smoked a high number of cigarettes per day.

Researchers said that cancer patients must stop smoking after diagnosis because smoking can negatively affect patients’ responses to treatments, increase cancer risk and decrease their survival chances.

"Most clinicians acknowledge the importance of addressing tobacco cessation in their patients; however, few do it," Carolyn Dressler of the Arkansas Department of Health in Little Rock wrote in an accompanying editorial, according to a statement released on Monday.

"We know enough now to implement effective cessation programs to identify and help cancer patients quit at the time of diagnosis and support them to prevent relapse. By doing so, we maximize patients' response to therapy, their quality of life, and their longevity," Dressler added.

The study is published online in CANCER, a journal of the American Cancer Society.