New population-based research published in JAMA Network Open highlights the long-term benefits of quitting smoking, indicating that individuals who maintain smoking cessation for a decade are significantly less likely to develop various types of cancer, especially lung cancer. The study also found that quitting smoking for a long time greatly lowers the risk of developing cancer, even if it includes periods of relapse.
It is well-known that smoking is linked to a higher risk of cancer, and those who smoke can lower their risk by quitting. However, it remains unclear exactly how many years of continued smoking cessation are needed to see a substantial risk reduction.
The new research was conducted by Eunjung Park, Ph.D., and colleagues at the National Cancer Center in Korea. Their retrospective cohort study included more than 2 million participants in Korea and confirms that quitting smoking can greatly reduce the risk of cancer, especially after 10 years of cessation. After 15 years of quitting, the risk of cancer was reduced by about 50% compared to those who continued to smoke. Lung cancer risk decreased faster than other types of cancer after quitting.
To investigate the time course of cancer risk after smoking cessation, the researchers evaluated health insurance claims data from a national database. The study analyzed data from 2,974,820 participants, 58.1% of which were men, with an average age of 43.1 years. Over 13.4 years, 196,829 cases of cancer were confirmed.
The researchers compared the risk of cancer among continuous smokers and complete quitters. They found that complete quitters had a lower risk of cancer compared to continuous smokers. The hazard ratios (HRs) were calculated for various cancer sites. Complete quitters had HRs of 0.83 for all cancer sites, 0.58 for lung cancer, 0.73 for liver cancer, 0.86 for stomach cancer, and 0.80 for colorectal cancer.
Compared to people who continued smoking, the researchers observed that those who quit smoking even temporarily had a lower risk of developing cancers in the lung, liver, stomach, and colorectal areas. The findings confirm a steady decline in cancer risk from continuous smokers to relapsed or transient quitters, to complete quitters. Those who quit completely experienced the greatest decrease in lung cancer risk.
The study also found that the risk of cancer was slightly higher for the first 10 years after quitting compared to continued smoking. However, the risk decreased over time and reached 50% of the risk associated with continued smoking after 15 or more years. The risk of lung cancer decreased 3 years earlier than the risk of other types of cancer, and the reduction in risk was larger relative to other cancer types.
Quitting smoking at any age was associated with a decreased risk of developing cancer. Quitting before the age of 50 was associated with a greater reduction in the risk of lung cancer compared to quitting at age 50 or later. The hazard ratio for quitting before the age of 50 was 0.43, while the hazard ratio for quitting at age 50 or later was 0.61.
Limitations of the study include possible selection bias, limited statistical power due to a smaller sample size of women, and an insufficient follow-up period for age-related associations. Other limitations were the use of estimations for smoking cessation duration and the relatively healthy study population.