Background: Limited data exists about the effects of opioid use in breast cancer survivors post-treatment. Thus, we examined the risk of adverse events associated with opioid use among breast cancers survivors after active cancer treatment. Methods: We assembled a cohort of women diagnosed with in situ, localized, and regional breast cancer 2009-2019 within a large integrated healthcare system in southern California. Data were captured from the tumor registry, electronic health records, and pharmacy database. We examined opioids use in women with breast cancer after one-year post survival and followed them through December 2021. Opioid use after the first prescription was considered. We examined the risk of incident cardiovascular events (stroke, acute myocardial infarction, arrhythmia); lung problems (COPD, pneumonia); fractures; and falls requiring medical attention. Follow-up was censored at membership disenrollment, death, or study’s end (12/31/2021). We calculated the maximum daily morphine milligrams equivalent (MME) dose during follow-up. Incidence rates (per 1000 person-years, PY) of adverse events were calculated, and multivariable analyses were conducted to estimate hazard ratios (HR) associated with time-dependent opioid use based on Cox proportional hazards models. We adjusted for age; breast cancer surgery; adjuvant cancer therapies; BMI; comorbidity; demographics; and psychiatric drugs. Results: The cohort included 33,989 breast cancer survivors (mean age: 62 years, range: 22-106), followed a maximum of 12 years (median: 5.6 years). The cohort was diverse: 12% African American/Black; 14% Asian/Pacific Islander; 22% Latine; 51% White, and 1% other/mixed. A total of 20,311 women (60%) used opioids during follow-up. The median MME during follow-up was 43.0 (IQR: 30.0-75.0). Falls were the most common event, with a dramatically higher rate for opioid user (47.0/1,000 PY) than non-use (21.8/1,000 PY), corresponding to nearly a doubled risk of falls (adjusted HR = 1.8, 95% CI:1.6-1.9). Similarly, adjusted HRs (95% CIs) for lung problems (HR = 1.6, 1.4-1.7); fractures (HR = 2.1, 2.0-2.3); and CV events (HR = 1.6, 1.3-2.0) were all higher for opioid use. Conclusions: Over 60% of breast cancer survivors used opioids post active treatment. We found statistically significant 1.6-2.1-fold increased risks of adverse events (falls, lung problems, fractures and CV events) associated with opioid use. Clinicians must weigh the risk and benefits of treating pain with opioids post-treatment.