Do the antiestrogenic effects of smoking lower the risk of breast cancer, despite the known carcinogenic components of cigarettes? To help answer that question, researchers analyzed data from a prospective cohort study of more than 111,000 women.

The researchers drew on data from the Nurses' Health Study (1976 to 2006). Female nurses between 30 and 55 years of age were asked to report on their health and lifestyle in questionnaires mailed every other year. Smoking history, including important factors such as current smoking status, age at which they began smoking, smoking duration, and the amount smoked, was elicited from the start of the study; in 1982 questions on exposure to passive smoking were added.

Over the 30-year time span (yielding approximately 3 million person-years of follow-up), 8,772 incident cases of breast cancer were diagnosed. After adjustment for potential confounding factors such as age, family history of breast cancer, and alcohol use, the incidence of breast cancer was slightly higher in women who smoked (or had smoked) a pack or more of cigarettes a day, began smoking before or at 17 years of age, smoked for longer than 20 years, or had quit in the previous 10 years. Additionally, smoking before menopause was associated with a small increase in breast cancer risk, with the most notable association seen in women who hadn't yet given birth to a child. Passive smoking exposure, however, wasn't found to have an effect on breast cancer incidence.

Because a small decrease in breast cancer incidence was observed in postmenopausal women who smoked, possibly explained by a further reduction of the naturally declining estrogen levels caused by smoking, the researchers advise that additional large-scale studies be conducted to further clarify the relationship between smoking and breast cancer.-AK

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