Adolescent girls who smoke are at a greater risk of developing breast cancer later in life, according to Canadian researchers.

Prior studies linking smoking with breast cancer have been inconclusive. In this study reported in The Lancet, researchers aimed to assess the effects of smoking on the risk of developing breast cancer.

A questionnaire was sent to 1,431 women under age 75 who had breast cancer. A questionnaire was also sent to 1,502 women of the same age randomly selected from a list of voters to serve as the control group.

Researchers obtained information on all known and suspected risk factors for breast cancer, and on lifetime smoking, consumption of alcohol, and job history. The effect of smoking was assessed separately for premenopausal and postmenopausal women. A total of 318 premenopausal women and 340 controls answered the questionnaire.

Results of the study showed that the risk of breast cancer was significantly higher in women who had been pregnant and who started to smoke within five years of the start of menstruation, and in women who had never given birth who smoked 20 cigarettes a day or more.

Postmenopausal women whose body mass index increased from age 18 to current and who started to smoke after a first full-term pregnancy had a significantly reduced risk of breast cancer, according to the study.

"Our results suggest that cigarette smoke exerts a dual action on the breast, with different effects in premenopausal and postmenopausal women," wrote the authors.

"Our observations reinforce the importance of smoking prevention, especially in early adolescence," they concluded.

Other sources: The Lancet