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Background Prior epidemiologic studies have shown an inverse relationship of aspirin and NSAID use with breast cancer (BC) risk in the general population. Women with benign breast disease (BBD) are at an increased BC risk compared with women in the general population, and we have shown that BBD biopsies contain increased numbers of immune cells compared with normal breast tissues donated for research in the Komen Tissue Bank. We hypothesized that NSAID use among women with BBD is associated with reduced risk of BC. Methods We evaluated BC risk in relation to aspirin and NSAID use in a single institution retrospective cohort of 4,498 women who received a biopsy diagnosis of BBD between 1/1/1992 and 12/31/2001. Information on usual medication use, including average number of days used per month (0, 1-7, 8-28 and >28), was collected via questionnaires mailed after identification of participants. NSAIDs were categorized into three classes: 1) Aspirin, 2) Ibuprofen (plus other prostaglandin synthesis inhibitors), and 3) COX inhibitors. BC events were captured via medical record, questionnaires, and Tumor Registry records. Medication use was evaluated for associations with clinical and pathology features. Hazard ratios (HR) and 95% confidence intervals (CI) examining associations between medication use and BC risk were estimated using Cox regression analyses, with adjustment for age and BMI at biopsy, years between biopsy and questionnaire, and pathologic features related to BC risk (severity of BBD and degree of involution in background lobules). Tests for trend examining frequency of use were calculated from ordered values (lowest to highest) in Cox models with a one degree-of-freedom linear term. We assessed BC associations with any type of NSAID use, as well as the three subclasses. Results Information on NSAID use was available for 3,089 (69%) of the participants in the cohort; these women had a median age of 49 and did not differ in BC risk from women who did not provide data on NSAID use (p=0.46). Of the 3,089 respondents, 313 (10.1%) subsequently developed BC with median follow-up of 17 years. Age was significantly associated with use of all types of NSAIDs, with increasing use in older women. BMI at the time of biopsy, severity of BBD findings, and involution in background lobules were also associated with NSAID use; thus these factors and age were adjusted for in risk association analyses. Ever users of NSAIDs (any type) had a significantly lower risk of BC than never users (HR 0.76, CI 0.59-0.98, p=0.03), and increasing frequency of NSAID use was significantly associated with lower BC risk (HRs 0.81, 0.79, and 0.70 for regular use 1-7, 8-28, and >28 days/month), trend p=0.02. Analyses of drug classes revealed that ever use of ibuprofen (HR 0.61, CI 0.42-0.89, p=0.01) and ever use of COX inhibitors (HR 0.59, CI 0.39-0.89, p=0.01) were associated with decreased BC risk. COX inhibitors in particular showed a strong dose response effect with increasing use (HRs 0.97, 0.52, and 0.47 for regular use 1-7, 8-28, and >28 days/month), trend p Citation Format: Amy Degnim, Robert Vierkant, Stacey Winham, Marlene Frost, Daniel Visscher, Jodi Carter, Suneetha Kaggal, Julie Cunningham, Teresa Allers, Tanya Hoskin, Ethan Heinzen, Celine Vachon, Keith Knutson, Keith Knutson, Derek Radisky, Mark Sherman. NSAID use and breast cancer risk in a retrospective cohort of women with benign breast disease [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-08-06.