The survey from 1976 to 2006 of more than 4,000 registered nurses showed that women who took aspirin regularly (two to five days a week) cut their risk of having the cancer spread by 60 percent and reduced their risk of dying from breast cancer by 71 percent.
Most of the women were taking the aspirin to reduce their risk of stroke or cardiac problems.
Researchers theorize that aspirin may help control cancer by fighting inflammation. Breast cancers produce more inflammatory chemicals than normal breast cells. Lab tests show that aspirin keeps breast tumor cells from growing and invading other tissue.
A study in August also found that aspirin offered a potential benefit against colon cancer.
Researchers warn however that more study is needed and they caution that aspirin therapy can thin blood, a problem for those undergoing chemotherapy or other cancer treatments and can cause gastric bleeding, even in otherwise healthy women.