Smoking is already known to significantly increase a person's risk of developing particular cancers, such as lung and kidney cancer. Now a new study has shown that people who smoked prior to their first cancer diagnosis are also much more likely to go on to develop a second smoking-associated cancer.

US researchers examined data from five major studies involving over 15,000 people with lung, head and neck, bladder and kidney cancer. They focused on the development of second primary cancers.

The first time a person is diagnosed with cancer, it is referred to as a first primary cancer. A second primary cancer is a second cancer that has been diagnosed that is not considered metastases (when the original cancer has spread), but is in fact a distinct, new cancer.

The researchers found that people who smoked at least 20 cigarettes per day prior to their first diagnosis of cancer had a five-fold increased risk of going on to develop a second smoking-associated cancer, compared to people who had survived the same cancers but had never smoked.

"As survival improves for a number of smoking-related cancers, patients are living longer, however, smoking may increase the risk of developing a second smoking-related cancer among these survivors. Our study demonstrates that healthcare providers should emphasise the importance of smoking cessation to all their patients, including cancer survivors," they said.

Almost 900 second primary smoking-associated cancers were diagnosed among the participants. Across the four different cancers assessed, those who smoked at least 20 cigarettes per day before their first cancer diagnosis were more likely to develop a second smoking-associated cancer. The risks were as follows:

-With stage one lung cancer, smokers were 3.3 times more likely to develop a second smoking-associated cancer
-With head and neck cancer, they were 4.5 times more likely
-With kidney cancer, they were 5.3 times more likely
-With bladder cancer, they were 3.7 times more likely.

The study also found that people who smoked fewer than 20 cigarettes per day, and former smokers, also had an increased risk of developing a second smoking-associated cancer compared to never smokers, however the risks fell in line with the number of years since they had quit the habit.

Details of these findings are published in the Journal of Clinical Oncology.