Level 2: Mid-level evidence
Previous data from the Nurses Health Study has suggested that aspirin use has no effect on breast cancer incidence (J Natl Cancer Inst. 1996;88:988-993). However, a new study provides evidence that aspirin may be beneficial after a breast cancer diagnosis. A cohort of 4,164 women, also from the Nurses Health Study, who were diagnosed with stage I, II, or III breast cancer from 1976 to 2002 was followed until 2006. Outcomes were analyzed based on frequency of aspirin use, which was assessed by questionnaire. During follow-up, there were 400 distant recurrences (9.6%) and 341 breast cancer deaths (8.2%). Aspirin use was associated with risk reduction for both of these outcomes. Compared with no use, taking aspirin 2-5 days per week was associated with reduced breast cancer mortality (relative risk [RR] 0.29, 95% CI 0.16-0.52) and reduced distant recurrence (RR 0.4, 95% CI 0.24-0.65). Taking aspirin six to seven days per week was also associated with reduced risk (RR 0.36, 95% CI 0.24-0.54 for breast cancer mortality; RR 0.57, 95% CI 0.39-0.82 for distant recurrence). Aspirin use at least two days per week was also associated with reduced all-cause mortality. Once-weekly aspirin use conferred no significant benefits on mortality or recurrence. While the study lacked data on specific aspirin doses, the authors presumed that most frequent users were taking low-dose aspirin for cardiovascular disease prevention (J Clin Oncol. 2010;28:1467-1472).