▎ WuXi AppTec content team editor
Smoking, including exposure to secondhand smoke, is the leading modifiable risk factor for cancer development and death worldwide, with about 35% of new cancers associated with smoking, particularly lung cancer.
Although it is known that quitting smoking reduces the risk of cancer, for example, the risk of lung cancer decreases by 39% within 5 years of quitting smoking. But for many "smokers", it is difficult to quit smoking; there are also some "smokers" who, even if they are successful in quitting smoking, will still be tempted to start smoking again after a period of time. Studies have found that 51%-67% of people will smoke again after 1 year of quitting; even after 30 years of quitting, 10% of people will smoke again.
How will the cancer risk change for those who smoke after quitting? According to a study published in Cancer, an authoritative journal of the American Cancer Society (ACS), smoking cessation and smoking less help reduce cancer risk; smoking after quitting may be more likely to develop cancer than people who do not quit smoking.
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The researchers analyzed data from South Korea's National Health Insurance Service (NHIS) and included 893582 participants with an average age of 51. All participants were current smokers, 94.5 percent of whom were men.
During an average follow-up period of more than 6 years, the researchers followed all participants 3 times to count basic health information, lifestyle information, history of treatment and family history of chronic diseases such as hypertension, and information on the incidence of different types of cancers, including smoking-related cancers such as lung cancer, oral cancer, pharyngeal cancer, esophageal cancer, stomach cancer, colorectal cancer, liver cancer, pancreatic cancer, laryngeal cancer, kidney cancer, bladder cancer, cervical cancer, and acute myeloid leukemia.
In addition, the researchers also collected information on changes in the participants' smoking status and smoking volume. Based on the changes in participants' smoking status and smoking volume between the 1st and 2nd follow-up, the researchers divided the participants into 4 groups:
Smoking cessation group, which has stopped smoking;
A small number of reduced groups, daily smoking decreased by 20%-50%;
In the large reduction group, the daily smoking volume was reduced by at least 50%, but there was no cessation;
In the group of persistent smoking, there was no change in daily smoking, or the daily smoking reduction < 20%, or the daily smoking increased by < 20%;
Increase the number of groups with at least 20% increase in daily smoking.
The researchers found that 45.7% of the participants belonged to the group of continuous smoking; 20.6% of the participants achieved smoking cessation; 7.3% of the participants smoked a small amount; 11.6% of the participants smoked a lot less; and the remaining 14.8% of the participants belonged to the group of increased smoking.
Participants in a small reduction group tended to be older than participants in other groups, more likely to be female, and more likely to be heavy smokers with a long smoking age (≥ 20 cigarettes/day).
During the follow-up period, a total of 50,869 participants developed cancer, of which 81.0% were smoking-related cancers and 23.3% were lung cancers.
After removing the influence of factors such as alcohol consumption and physical activity, the researchers found that quitting smoking or reducing smoking was associated with a reduced risk of cancer, while increasing smoking was associated with an increased risk of cancer.
Compared with the participants in the persistent smoking group, the participants in the small reduction group, the large reduction group and the smoking cessation group reduced their overall cancer risk by 4%, 7% and 6%, respectively, the risk of smoking-related cancers by 5%, 8% and 9%, and the risk of lung cancer by 17%, 16% and 21%, respectively. Participants in the increased group were associated with an increased risk of 4%, 3%, and 15%, respectively.
In addition, the researchers also found that smoking after quitting smoking, even if the amount of smoking decreased, increased the risk of cancer.
During the 2nd to 3rd follow-up period, 36.5% of the participants in the ex-smoker group started smoking again. Compared with participants who continued to quit smoking, participants who quit smoking were associated with an increased overall risk of cancer, 19%, and 48%, respectively, if their smoking volume returned to less than 50% before, and if they smoked to previous levels, they were associated with an overall increased risk of cancer, smoking-related cancer and lung cancer by 15%, 23%, and 38%, respectively.
Since the study was observational, it only showed changes in smoking volume and smoking status, which were associated with cancer risk, and did not show a causal relationship. The study also had some limitations, such as relying on participant recalls of smoking amounts, which may also have an impact on the findings.
However, the study concludes by highlighting that a growing body of research proves that "quitting smoking reduces the risk of all cancers, smoking-related cancers, and lung cancers," that even reducing smoking can have a beneficial effect on cancer risk, and that the sooner you quit, the better. But for people who quit smoking, the harm of re-smoking will be greater. Therefore, in addition to encouraging people to quit smoking, people who quit smoking should be monitored continuously to ensure that they will not start smoking again.