11149 Background: The United States experienced cisplatin (CP) and carboplatin (CB) shortages in 2023, leading to potential rationing or switching to alternative therapies in multiple cancers. Using a US nationwide, real-world oncology dataset, we assessed changes in the use of CP and CB in 7 cancers during the shortage period. Methods: All patients (pts) from the Flatiron Health electronic health record-derived de-identified database (~280 cancer clinics, ~800 sites of care) with a machine learning-extracted metastatic (met) diagnosis of a cancer of interest between March 2022 and June 2023 and evidence of first-line (1L) therapy, were selected. Pts with multiple primaries were excluded. Platinum of interest was identified based on frequency of pre-period 1L use for a given cancer. The pre-period was June 2022 through first month of reported shortage (February 2023 for CP; April 2023 for CB); data cutoff was June 2023. Clinical characteristics of platinum-treated pts stratified by pre- vs post-period by disease were assessed. 1L platinum utilization rates (UR), calculated as the proportion of 1L-treated pts initiating CB/CP within 30 days of met diagnosis, were plotted by month. Cohort-level impact was evaluated with an interrupted time series analysis—overall and stratified by practice type, practice size, and socioeconomic status (SES). Multiple testing was controlled using the Benjamini-Hochberg procedure. Results: 10,983 pts received 1L therapy in the study period. Monthly platinum UR were stable across diseases in the pre-period. However, there was a significant post-period decrease in CB usage for non-small cell lung (NSCLC) and endometrial cancers (EC), and a decreasing trend in UR for CB in bladder (BC), ovarian (OC), and small cell lung cancers (SCLC). We also observed a decreasing trend in CP usage for cholangiocarcinoma (CC). Post-period monthly UR were stable for CB and CP in head and neck cancer (HNC), and for CP in BC. Odds ratios (OR) representing the month-over-month change in the odds of receiving platinum therapy during both pre- and post-periods are shown (table). OR trends varied by practice type and disease; no notable trends were seen by practice size or SES. UR with additional follow-up times will be presented. Conclusions: We observed decreased platinum use across multiple cancers during the shortage period, most notably for met NSCLC and EC, suggesting shortages led oncologists to seek alternatives to standard 1L platinum-based regimens. Further study of how treatment changes due to platinum shortages impacted pt outcomes is warranted. [Table: see text]