e13027 Background: Despite the ubiquitous prescribing of pain medications (PMs) in cancer clinical trials, the impact of such prescribing patterns and reporting on the experience of pain is not often investigated. We examined patient-reported pain before initiation of PM reporting and at the next available pain assessment. Our aim was to understand change in patient-reported pain. Methods: We pooled data from 7 phase 3 randomized, controlled, registration trials of CDKI with endocrine therapy in patients with hormone receptor positive, human epidermal growth factor receptor-2 negative MBC. We restricted our analyses to patients who started therapy with no PM reported and looked at patients who had NSAID or opioid medication documented. We calculated change between 2 assessments in patient-reported pain before and after PM using the pain occurrence item (Q9) on the EORTC Quality of Life questionnaire (QLQ-C30). Results: Of the 4200 patients who received at least 1 dose of CDKI/placebo, 1488 started with no documented PM, with 48% reporting none at all when asked about pain at baseline. Subsequently, 185 patients had documented NSAID and 43 an opioid and had a pain PRO assessment before and after. NSAIDs documentation occurred on average 11 weeks into trial and opioids 5. Before documentation of NSAIDs, 45% of patients reported no pain compared to 23% of patients with an opioid. Patients who had documented NSAIDs, 29% experienced an improvement in their self-reported pain, whereas 32% of patients with documented opioids improved. On average the time between the 2 pain assessments was around 58 days for both PMs. Conclusions: In this analysis in patients who had a pain assessment before and after documentation of a PM, there is a small group whose pain improved. It is important to note that patients’ response to the pain item was not provided to the clinical care team, which may explain why there may have been suboptimal pain control. Further study is needed to examine how pain management can be achieved in patients with advanced breast cancer. Future analysis should be performed with patients whose PRO pain results are communicated with the clinical care team in real-time. [Table: see text]