Compared to hormone receptor-positive (HR+) cancers, "the prognosis tends to be worse for HR negative breast cancers," says lead author Jonas Manjer, MD, a Physician and Researcher in the department of community medicine at Malmö University Hospital in Sweden. "You can't use the most effective drugs, such as tamoxifen, that work by blocking estrogen," he says.

Manjer used Swedish medical records to track more than 10,000 women, including 268 with breast cancer, over 12 years. About 35% of the women smoked. A pathologist who examined tissue samples from all of the breast cancers to determine hormone receptor status found that current smokers and former smokers were about twice as likely to have HR- cancers as women who had never smoked.

What Manjer doesn't know is whether women with the more aggressive HR- breast cancer are more likely to die of their disease than women with HR+ cancers. Previous studies have found that women who are smokers at the time their breast cancer is discovered tend to have a worse prognosis than breast cancer patients who don't smoke, he says. However, this study did not include information on survival or even on how far the cancer had spread at the time of diagnosis. Manjer is now studying long-term survival in women with HR- breast cancers compared to those with HR+ tumors.

"Women with HR positive breast cancers generally have a better prognosis, but there are a lot of exceptions," says Carmen Rodriguez, MD, Senior Epidemiologist for the American Cancer Society (ACS). "The survival rate for all types of breast cancer is very high when you find it early. Women who smoke need to be very concerned about several types of cancer -- lung, stomach, throat, mouth, and more -- but we don't believe that smoking increases the risk of breast cancer overall."

Women fear breast cancer as a major killer, says Debbie Saslow, PhD, ACS Director of Breast and Cervical Cancer, but heart disease is actually the leading cause of death among women. Breast cancer is the second most common cancer in women after skin cancer, and, like skin cancer, can be treated easily and successfully if diagnosed early, she says.

"It can't hurt to let women know that smoking may give them a worse prognosis if they do get breast cancer," Saslow says. "At the least, that might motivate some people to quit. The problem is that people who tend to start smoking are teenagers and most teens don't really think about breast cancer."

About 192,200 women will be diagnosed with breast cancer this year, according to ACS projections.