Study: Smoking puffs up breast cancer risk

Researchers have found yet another toll years of smoking can take on your health: an increased risk of developing breast cancer.

Researchers found that older women who have smoked for the equivalent of 11 or more "pack years" - the lifetime equivalent of 20 cigarettes per day for 11 years - have as much as a 40% increased risk of developing breast cancer.

"Breast cancer may be an additional risk to add to the long list of adverse outcomes related to smoking," wrote Dr. Christopher Li and colleagues in the journal Cancer Causes and Control.

In one of the first studies to look at the long-term effects of smoking on older women, Li and his team compared data from 975 women between the ages of 65 and 79 who had been diagnosed with breast cancer and 1007 women of the same age range who had no history of breast cancer. All participants were interviewed about their smoking habits and history and were questioned about a variety of factors known to play a role in breast cancer development, including postmenopausal use of hormone replacement therapy; menstrual, contraceptive and reproductive history; family history; and alcohol use.

Overall, women who had a history of smoking - whether in the past or at the time of breast cancer diagnosis - had a 30% higher risk of developing breast cancer than women who had never smoked. After a closer analysis of the numbers, the researchers found the following:

• Women who were current smokers had a 40% higher risk of breast cancer, while the risk for former smokers was 20% higher than that of women who never smoked.
• Women who smoked for 40 years or longer had a 40% higher breast cancer risk than women who never smoked.
• Women who smoked for 11 or more "pack years" had a 30-40% greater risk.
• Women who started smoking before giving birth had a 30% higher risk of developing breast cancer, while women who started smoking after giving birth had a 10% greater risk.
• In former smokers, breast cancer risk decreased as the number of years since quitting increased.

Though the effect was not found to be statistically significant, the researchers also found that smoking and using hormone replacement therapy may interact to raise a women's breast cancer risk. This relationship only seemed to exist for women who used combined estrogen-progesterone therapy, not estrogen-only.

"To our knowledge, such an interaction has not been previously reported and thus requires confirmation," the researchers note.

The researchers note that previous investigations into the relationship between smoking and breast cancer have yielded conflicting results, but say that could be the result of different study designs as well as a variation in the age groups involved.