Although previous studies have looked at the relationship between breast cancer and smoking, the scientific community has remained divided about the issue. Part of the reason is that, while many studies have found a slight increase in risk overall, they have not found that smoking more cigarettes per day or smoking for more years increases the risk further.

In addition, findings about alcohol have complicated many of these studies. This is because alcohol is a known risk factor for breast cancer, and women who smoke are also more likely to drink alcohol.

In the new study, published in the Journal of the National Cancer Institute, researchers from the American Cancer Society analysed data from 73,388 women in the organisation’€™s Cancer Prevention Study II (CPS-II) Nutrition Cohort. During more than 13 years of follow-up, researchers counted 3 721 cases of invasive breast cancer. The rate of new cases was 24 percent higher in smokers than in non-smokers and 13 percent higher in former smokers than in non-smokers.

The risk of invasive breast cancer was highest in women who began smoking at an earlier age. When compared to women who never smoked, those who started smoking before their first menstrual cycle had a 61 percent higher risk, while those who started smoking after their first cycle, but 11 or more years before having a child, had a 45 percent higher risk.

The researchers also found that these results were supported by the findings of earlier cohort studies. When combining the results of nine studies, they found a 12 percent increase in breast cancer risk among women who started smoking at a younger age, and a 21 percent increase in risk among women who started before the birth of their first child.

Mia Gaudet, PhD, American Cancer Society director of genetic epidemiology, said breast tissue is not fully developed until after a woman has her first child, and that makes it more sensitive to the harmful effects of tobacco.

Gaudet said, ‘€œThe key message from this study should be additional motivation to young women to not start smoking.’€

Source: American Cancer Society