PURPOSE: Polypharmacy is prevalent in older adults starting cancer treatment and associated with potentially inappropriate medications (PIM), potential drug-drug interactions (DDI), and drug-cancer treatment interactions (DCI). For a large cohort of vulnerable older adults with advanced cancer starting treatment, we describe patterns of prescription and nonprescription medication usage, the prevalence of PIM, and the prevalence, severity, and type of DDI/DCI. METHODS: This secondary analysis used baseline data from a randomized study enrolling patients aged ≥70 years with advanced cancer starting a new systemic cancer treatment (University of Rochester Cancer Center [URCC] 13059; PI: Mohile). PIM were categorized using 2019 Beers criteria and Screening Tool of Older Persons' Prescriptions. Potential DDI/DCI were evaluated using Lexi-Interact Online. Medication classification followed the World Health Organization Anatomical Therapeutic Chemical system. Bivariate associations were evaluated between sociodemographic and geriatric assessment (GA) measures and medication measures. Chord diagrams and network analysis were used to understand and describe DDI/DCI. RESULTS: Among 718 patients (mean age 77.6 years), polypharmacy (≥5 medications), excessive polypharmacy (≥10 medications), and ≥1 PIM were identified in 61.3%,14.5%, and 67.1%, respectively. Cardiovascular medications were the most prevalent (47%), and nonprescription medications accounted for 26% of total medications and 40% of PIM. One-quarter of patients had ≥1 potential major DDI not involving cancer treatment, and 5.4% had ≥1 potential major DCI. Each additional medication increased the odds of a potential major DDI and DCI by 39% and 12%, respectively. Polypharmacy and PIM are associated with multiple GA domains. CONCLUSION: In a cohort of vulnerable older adults with advanced cancer starting treatment, polypharmacy, PIM, and potential DDI/DCI are very common. Nonprescription medications are frequently PIMs and/or involved in potential DDI/DCI.