But smoking's relationship to breast cancer have been less clear, with studies showing mixed results, according to a study in today's Archives of Internal Medicine.
The new study is believed to be the largest ever to address the question, drawing on the records of the long-running Nurses' Health Study, including more than 111,000 women followed from 1976 to 2006.
Any history of smoking increased the women's chance of breast cancer by 6%, the study says. Smoking one pack a day before menopause increases a woman's risk of breast cancer from 1 in 8 to about 1 in 7.5, says co-author Walter Willett, a professor at the Harvard School of Public Health.
Exposure to secondhand smoke didn't appear to increase the women's risk, the study says. But tobacco appears to affect the breast differently, depending on a woman's age, the study says. For example, women who started smoking young — before they have children — had an 18% greater risk of breast cancer.
Yet the relationship between smoking and breast tumors is complex. Smoking after menopause actually slightly lowered the risk of breast cancer, perhaps because tobacco works against estrogen, a hormone that fuels most breast tumors, the study says.
That doesn't mean that doctors recommend cigarettes. Older women who smoke are especially vulnerable to heart attacks, strokes and other potentially fatal health problems.
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