The top two causes of cancer generally are tobacco use and diet. Despite this fact, there is little evidence to indicate that smoking increases risk of breast cancer. Taking up the habit while very young usually increases cancer risk from smoking although, until one recent study, few had examined the effect of an early age of exposure to smoking on risk of breast cancer. There are several theories about the potential role of tobacco smoke exposure in the development of breast cancer.
Breast cancer is an oestrogen-dependent cancer.
The exact mechanism for this effect is not understood. Among women who take oral oestrogens, smokers have lower circulating levels of oestrogen than non-smokers, although endogenous oestradiol and oestrone levels are not lower. BRCA2 carriers indicate that future understanding of the interaction may assist the development of interventions in high risk gene carriers. In brief, most studies appear to have shown little direct evidence that smoking is associated with an increased risk of breast cancer. Their case control study found increased risks of breast cancer in women exposed to smoking before 12 years of age.