Background Polypharmacy raises the risk of drug-drug interactions and adverse events among patients with cancer. Most polypharmacy research has focused on adults age 65 or older enrolled in Medicare insurance. To better inform pharmacy practice and cancer care delivery, data are needed on polypharmacy among commercially insured patients with cancer and those younger than 65. Methods We performed a retrospective analysis of insurance enrollment and claims files linked to the Puget Sound Cancer Surveillance System for adults age 18 and older who were commercially insured, diagnosed with stage IV cancer, survived 30+ days after diagnosis, and did not enroll in hospice. We describe the prevalence of polypharmacy, chemotherapy use, and medication-related out-of-pocket (OOP) costs in the last month of life. Results Of 606 patients, 390 (64%) experienced polypharmacy (i.e. 5+ medications) in the last 30 days of life. Almost half (n = 297, 49%) received chemotherapy or targeted agents; chemotherapy was associated with significantly higher odds of polypharmacy (odds ratio (OR) 2.93, 95% confidence interval (CI) 2.04–4.20). The most commonly prescribed medications at end of life were opioids, benzodiazepines and anti-emetics. Among 484 patients (80%) incurring medication-related costs in the last month of life, median total OOP cost was $82 (interquartile range $30–$200). Seven patients (1%) had total costs above $5,000. The median chemotherapy-related OOP cost was $446 (IQR $150–$1896); 32 patients (7%) had chemotherapy-related OOP costs between $1,000 and $5,000. Conclusion Most patients with advanced cancer experienced polypharmacy at end of life, although most medications observed herein are commonly used for supportive care. Patients receiving chemotherapy had higher medication-related OOP costs, and chemotherapy was significantly associated with polypharmacy at end of life. Evaluation of polypharmacy at end of life may represent an important opportunity to improve quality of life and reduce costs for patients and families.