Breast cancer is the most frequently diagnosed cancer in women and it is the second most common cause of cancer mortality. Breast cancer incidence rates have continued to rise since 1980, while mortality rates have declined. As a result, more women are at risk for breast cancer recurrences. Recurrences have important negative consequences requiring a variety of palliative treatments and often lead to death. Numerous commonly used medications such as such as, statins, antihypertensives, antidepressants, and antibiotics have been hypothesized to alter breast cancer risk, in many cases, there is strong biologic evidence to support these hypotheses. However, no studies have examined how these commonly used medications are associated with risk of breast cancer recurrence. We propose to test the hypotheses that use of statins and ACE inhibitors will be associated with a reduced risk of recurrence, and use of antidepressants, antibiotics, calcium channel blockers, and diuretics will be associated with an increased risk of recurrence. To test these hypotheses, we will conduct a retrospective cohort study among approximately 4,550 women diagnosed between 1990 and 2008 with incident early stage invasive breast carcinoma, while enrolled at Group Health Cooperative, a large Health Maintenance Organization in western Washington State. We will use medical records and automated administrative data to follow women an average of 10 years to assess risk of breast cancer recurrence and to collect information on medication use and indication for use, primary therapy, and co-morbidities. Determining whether use of these drugs impacts breast cancer recurrence is of considerable public health importance given the widespread and increasing use of the medications under study and the burden of breast cancer in our nation. The hypotheses we are seeking to test are in line with the National Cancer Institute's 2015 goals of eliminating the suffering and death due to cancer. Learning more about whether there are factors that influence recurrence will help us with the secondary prevention of breast cancer and will lead to a reduction in breast cancer morbidity and mortality.