The study of more than 4,000 nurses showed that those who took aspirin — usually to prevent heart disease — had a 50 percent lower risk of dying from breast cancer and a 50 percent lower risk that the cancer would spread.
"This is the first study to find that aspirin can significantly reduce the risk of cancer spread and death for women who have been treated for early stage breast cancer, " said Dr. Michelle Holmes of Harvard Medical School, who led the study published in the Journal of Clinical Oncology.
"If these findings are confirmed in other clinical trials, taking aspirin may become another simple, low-cost and relatively safe tool to help women with breast cancer live longer, healthier lives," Holmes added in a statement.
Holmes and her team studied 4,164 female registered nurses taking part in the Nurses' Health Study, an ongoing analysis of a wide range of health issues.
They started in 1976, looking at who took aspirin, watching for breast cancer and all causes of death until 2006.
Over this time, 341 of the nurses died of breast cancer.
Women who took aspirin two to five days a week had a 60 percent reduced risk of their cancer spreading and a 71 percent lower risk of breast cancer death. Six to seven aspirins a week lowered the risk of spread by 43 percent and the risk of breast cancer death by 64 percent.
Most of the women were taking low-dose aspirin to prevent heart attacks and stroke.
Other drugs in the same class as aspirin also apparently lowered the risks, too. These drugs, called non-steroidal inflammatory drugs or NSAIDs, include ibuprofen and naproxen but not acetaminophen, also known as paracetamol.
But there was not enough data on these drugs to give a clear answer.
The researchers said they are not sure how aspirin and other NSAIDS may affect tumors but it could be by lowering inflammation. Other studies have shown that aspirin and ibuprofen can lower colon cancer risk, for instance.
"Aspirin has relatively benign adverse effects compared with cancer chemotherapeutic drugs and may also prevent colon cancer, cardiovascular disease, and stroke," the researchers wrote. It affected both estrogen-positive tumors and those not fueled by the hormone.
Holmes' team stressed that patients should not take aspirin while undergoing radiation or chemotherapy because of the risk of side effects.
And aspirin can cause stomach bleeding so it should not be taken without a doctor's supervision.