Women may have yet another reason not to smoke

In addition to causing lung cancer, heart attacks and strokes, a new study suggests that smoking also may slightly increase the risk of breast cancer.

Scientists have known for decades that tobacco causes lung cancer and at least nine other types of tumors, according to the National Center Institute. Worldwide, smoking kills 5.7 million people a year, according to the World Health Organization.

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.

Smoking poses a much clearer danger to the lungs, says Michael Thun of the American Cancer Society (ACS), who was not involved in the new study. According to the ACS, smoking increases a woman's risk of lung cancer by 13 times.

On its own, the impact of smoking on breast cancer is not major, but this adds to the many other damaging effects of tobacco. Exposure to secondhand smoke didn't appear to increase the women's risk.

But tobacco appears to affect the breast differently, depending on a woman's age. For example, women who started smoking young — before they have children — had an 18% greater risk of breast cancer.

That's significant, since virtually all smokers — male or female — take up the habit when young, usually as teens. Breast tissue appears to be most vulnerable to carcinogens during the years before a young woman has children.

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.

"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. Two of the biggest influences on breast cancer risk, for example, are the age at which girls enter puberty and the age at which they have their first child. Breastfeeding and having children before age 30 also reduce the risk. Women also can reduce their risk by avoiding alcohol and weight gain, both of which increase the risk of breast cancer. One alcoholic drink per day increases a woman's risk of breast cancer by 10% to 12%."