Teenage girls who smoke increase their risk of developing breast cancer before they reach menopause, according to a report in the Oct. 5 issue of the journal The Lancet (Vol. 360: 1044-1049).

The authors said the risk is almost double if young women start smoking within five years of their first menstrual cycle.

Pierre R. Band, MD, and a team of Canadian scientists found that the chance of breast cancer in women by age 50 was 80% higher than if they hadn't started smoking at a young age.

Cigarette smoke contains potent cancer-causing chemicals. Scientists have been able to show that these chemicals can cause breast cancer cells in laboratory cultures to become cancerous.

But linking smoking to breast cancer has been difficult. Experts say one reason is that smoking suppresses estrogens, and that in postmenopausal women estrogen levels are partly responsible for causing breast cancer. So the cancer-causing effect of smoking is countered by its suppression of estrogens in older women.

The authors suggest this explains why studies that looked at breast cancer in postmenopausal women were unable to show an effect of smoking. This study also looked at older women with breast cancer and didn't find a connection with smoking.

But when the researchers looked at women who started smoking in their teens and developed breast cancer before menopause, they found a link. There are two reasons for this, the authors said.

First, teenage breast tissue is still developing. This makes it more susceptible to the cancer-causing effects of the chemicals in smoke.

Second, the estrogen level in premenopausal women is so high, that smoking is unlikely to affect it much. This means that the anti-estrogen effect of smoking, which is important in postmenopausal women, has no effect in these younger women.

Another factor that turned out to be important was the amount of smoking, the authors reported. More cigarettes per day and more years of smoking led to a higher chance of breast cancer.

Band concluded that this study "should reinforce the importance of smoking prevention, particularly in early adolescence."

An editorial in the same journal, by Irma H. Russo, MD, from Fox Chase Cancer Center in Philadelphia, points out that tobacco already has a devastating effect on women by causing lung cancer.

Lung cancer has overtaken breast cancer as the leading cause of cancer deaths in women, she said. Now with this Canadian study, women have even more reasons to avoid tobacco.ACS News Center stories are provided as a source of cancer-related news and are not intended to be used as press releases.