A large new prospective study using data from the Nurses' Health Study has reported that cigarette smoking increases the risk of breast cancer modestly by 6% overall. However, women who smoked at least one pack per day for 30 years or more had a 28 percent higher risk than nonsmokers. Tobacco smoke contains carcinogens, some of which have been reported to increase breast cancer risk. On the other hand, cigarette smoking also reduces the level of circulating estrogen, which might serve to reduce the risk of breast cancer. The study included 111,140 women for purposes of investigating active smoking (from 1976 to 2006) and 36,017 for passive smoking (1982 to 2006). A total of 8,772 new cases of invasive breast cancer were diagnosed among study participants through 2006.

Increased risk of breast cancer was found to be associated with a higher amount of current and past smoking, as well as beginning to smoke at a young age, greater number of years of smoking, and more pack-years of smoking. Premenopausal smoking was found to be associated with an 11% increase in risk of breast cancer for every increase of 20 pack-years; the risk increased to 18% if smoking began before the birth of a woman's first child. However, smoking appeared to lower risk of breast cancer slightly in postmenopausal women. Passive smoking in childhood or adulthood was not found to be associated with breast cancer risk. In a separate interview, lead author Karin Michels, an associate professor at Harvard Medical School, commented, "I would not put smoking on the list of important risk factors, but when you look at the subgroup of heavy-duty smokers who start early and smoke for a long time, that's more serious."

Please see our article on how to protect our daughters from breast cancer for more information on factors affecting young women that appear to increase subsequent risk of breast cancer.