The carcinogenic effect of cigarette smoke on developing breast tissue has been shown in a study in which the risk of breast cancer was 1.69 times higher, compared with age matched controls, in women who had ever been pregnant and who started to smoke within five years of menarche. Women who had never been pregnant—and so who never achieved complete differentiation of breast cells—and who smoked heavily showed a sevenfold increased risk of breast cancer (Lancet 2002;360:1044-9)

The Canadian study investigated all women under the age of 75 years in whom breast cancer was diagnosed between …