Older adults with cancer are typically prescribed multiple medications (i.e., polypharmacy) and are therefore at risk of adverse drug events (ADE). In this study, the medical records of older adults (those at least 70 years old) receiving cancer care who had an unplanned hospital admission were reviewed to determine if it was potentially related to an ADE. Of the hospitalized patients, more than half were potentially due to ADE at three and six months after the initial oncology visit. Including a clinical pharmacist may help reduce ADEs and hospitalizations.