Smoking may slightly increase the risk of breast cancer, writes Liz Szabo.
In addition to causing lung cancer, heart attacks and strokes, smoking may slightly increase the risk of breast cancer, a new study suggests.
Scientists have known for decades that tobacco causes lung cancer and at least nine other types of tumour, according to the National Cancer Institute.
But smoking's relationship to breast cancer has been less clear, and research has showed mixed results.
The new study, in Monday's Archives of Internal Medicine, is probably the largest ever to address the question. It draws on the records of the Nurses' Health Study and includes more than 111,000 women who were followed for 30 years.
Any history of smoking increased the women's chance of breast cancer by 6 per cent, the study says. Smoking one pack a day before menopause increased a woman's risk of breast cancer from one in eight to about one in 7.5, says co-author Walter Willett, Harvard School of Public Health professor.
Smoking poses a much clearer danger to the lungs, says Michael Thun of the American Cancer Society, who was not involved in the study. The cancer society says 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," Willett says.
Tobacco appears to affect the breast differently depending on a woman's age, the study shows.
For example, women who started smoking young had a higher risk of breast cancer.
That's significant, because virtually all smokers take up the habit when young, usually as teens, says Stanton Glantz, director of the Center for Tobacco Control Research and Education at the University of California-San Francisco, who was not involved in the study. He says breast tissue appears to be most vulnerable to carcinogens in the years before a young woman has children.
Yet the relationship between smoking and breast tumour is complex. Smoking after menopause actually lowered the risk of breast cancer slightly, perhaps because tobacco works against oestrogen, which fuels most breast tumours.
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.Do you have an idea for a story?
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