Smoking as teenagers significantly increases women's risk of going on to develop breast cancer, researchers have found.

Canadian researchers found women who began smoking within five years of starting their periods were around 70% more likely to develop cancer later in life than non-smokers.

Our observations reinforce the importance of smoking prevention, especially in early adolescence

Dr Pierre Band British Columbia Cancer Agency

They say their findings, published in the Lancet, reinforce the importance of informing teenagers of the health risks of smoking.

Previous research looking at the link between smoking and breast cancer has been inconclusive.

One in nine women in the UK have a risk of developing breast cancer at some point during their life.

Menopause

The researchers surveyed 2,000 women under 75, half of whom had had breast cancer.

They looked at their smoking history and also whether they had known risk factors for breast cancer, such as whether they had taken hormone replacement therapy, so those factors could be eliminated from their analysis.

The researchers looked at whether smoking affected pre and postmenopausal women differently.

They found that women who had started smoking as teenagers, and had been pregnant at some point, had an increased risk of developing breast cancer.

It's a very plausible theory, but you really need to see more data

Dr Lesley Walker, Cancer Research UK

The team say this could be because developing breast tissue is particularly sensitive to the effect of cancer-causing chemicals.

Women who have never had children, and who smoked 20 or more cigarettes a day were also at an increased risk of breast cancer.

But in postmenopausal women, smoking was associated with a decreased risk of breast cancer.

However, the researchers say the number in this group was very small, and much more research is needed to confirm that finding.

'Chance findings'

Dr Pierre Band of the British Columbia Cancer Agency in Vancouver, Canada, said: "Our observations reinforce the importance of smoking prevention, especially in early adolescence."

He adds: "British Columbian women have a premenopausal breast cancer risk of 1 in 55. An increased risk off 70% would lead to an additional 1000 premenopausal breast cancer cases out of 100 000 teenage smokers."

Dr Lesley Walker, of Cancer Research UK, said: "This is a very interesting study which raises a lot of questions, but we would like to see more data from bigger studies .

"It is quite a sophisticated study, but when you get down to the sub-groups of women, i.e. women who had taken up smoking during their teenage years and had children, or who had smoked for 20 years and never had children, you begin to talk about small groups of women.

"The idea of a protective effect [for postmenopausal women] is interesting, and there have certainly been precedents for thinking that in the past.

"But a much more crucial issue is whether or not smoking is harmful for young women.

"It's a very plausible theory, but you really need to see more data."

Dr Michelle Barclay, of Breakthrough Breast Cancer, said: "Breakthrough would strongly advise all women and men, especially young to avoid or stop smoking as this is important for a healthy lifestyle."