Cigarette smoking appears to increase the risk of breast cancer, especially
when women start smoking early in life, new research indicates.
For years, experts have questioned whether cigarette smoking is directly
linked with breast cancer risk or whether the association is complicated by the
fact that many women who smoke also drink alcohol, which has also been tied to
breast cancer risk.
Studies have produced conflicting results. When the US Surgeon General last
reviewed the issue in 2004, the report concluded that there was no
cause-and-effect relationship between smoking and breast cancer risk.
Now, however, researchers who took another look, analysing data from more
than 73 000 women, have found strong evidence for a link between cigarette
smoking and breast cancer.
Smoking by itself is a risk
"It's not just a relationship between alcohol and breast cancer, but in fact
smoking by itself is related to breast cancer," said Mia Gaudet, director of
genetic epidemiology at the American Cancer Society. She led the study, which
was published online in the Journal of the National Cancer
The timing of the smoking appears to affect the degree of risk, she said. "It
seems that women who start smoking before their first birth are at greatest risk
of breast cancer," Gaudet said.
The researchers looked at data from women enrolled in a large, long-term
cancer society study involving lifestyle factors and prevention. Over the
follow-up of nearly 14 years, more than 3 700 cases of invasive breast cancer
When the women entered the study in 1992, they were aged 50 to 74. They
supplied information on smoking habits, past and present. At the start, about 8% smoked, about 36% had quit and about 56% never smoked.
The incidence of invasive breast cancer was 24% higher in current smokers and
13% higher in former smokers, compared to never smokers, the researchers
The researchers next focused on the timing of the smoking. "Women who started
smoking before their first menstrual period were 61% more likely [to get breast
cancer than nonsmokers]," Gaudet said. Women who took up the smoking habit after
their period had started but 11 or more years before giving birth were at a 45%
higher risk, compared to nonsmokers.
What the research found
To analyse whether the cigarette smoking by itself - not the combination of
drinking and smoking - affects breast cancer risk, Gaudet first looked at both
smoking and drinking in the same model. ''It showed that the relationship still
existed between smoking and breast cancer," she said.
Next, she looked separately at the groups of drinkers - never, former and
current - and analysed their risk of breast cancer. For the never drinkers,
smoking now or in the past was not linked with breast cancer risk. Ideally, she
said, all three groups would have a similar risk to prove smoking by itself is a
risk factor. However, she acknowledged, "We did not see that."
Gaudet said she is not sure why that was, but that there may be something
about the combination of smoking and drinking that affects breast cancer risk.
"Or the numbers [of women] may be too small to show an accurate result."
Even so, she said, the new research suggests that smoking by itself drives up
breast cancer risk. The researchers found a link or association, but cannot
prove cause and effect.
One cancer expert praised the study.
"This paper is another important step toward the conclusion that smoking is a
risk factor [for breast cancer] on its own," said James Lacey, an associate
professor of cancer etiology at the City of Hope Comprehensive Cancer Center in
Duarte, Calif. He was not involved with the research.
One interesting finding, he said, is that the window before a woman has
children seems to be the period of most concentrated increased risk. "It should
allow our lab colleagues to look more closely at this window," Lacey said. Among
the questions to be answered, he said, is this: "Is the smoking making the
tissue more susceptible to other cancer-causing agents or is it starting the
cancer in the breast?"
Gaudet said that experts think breast tissue is more susceptible to toxic
exposures before a woman gives birth the first time compared to after.
The study finding, Gaudet said, ''provides additional motivation for young
women who are thinking of starting to smoke not to." Those who smoked while
young and gave up the habit still had a higher risk of breast cancer, she said,
than never smokers.
To learn more about smoking and cancer, visit the American