Background Polymorbidity induces polypharmacy in older patients may lead to potential drug–drug interactions (DDI) which can modify the tolerance and safety of oncological treatments and alter the intended therapeutic effect. The objective of our study was to describe the decision-making process for oncological treatment and related outcomes, in a population of older adults undergoing a comprehensive geriatric assessment (CGA) associated to a comprehensive medication reconciliation (CMR) prior to initiating oncological treatment. Methods ChimioAge is a prospective observational study conducted between 01/2017 and 07/2018 at Marseille University Hospital and approved by the French National Ethics Committee. It comprised all consecutive patients aged 70 years and over who were referred for a CGA as part of CMR, before initiating systemic treatment. Results One hundred and seventy-one cancer patients were included. Mean age was 79.2 years, over half had metastatic cancers, 75% had an ECOG performance status zero or one, and two-thirds were independent in daily activities. Two-thirds of the patients had polypharmacy and the CMR identified potential DDI with systemic treatment in 43.3% of patients. Following the CGA, the CMR and the hospital oncologists decision, 30% of the patients received adapted systemic treatment with reduced doses at initiation. They presented fewer toxicities – irrespective of grade and type – than patients who received standard treatment (p<0.001) and had comparable overall survival (Log rank p=0.21). Conclusion This is one of the first studies to highlight the value in conducting CMR and a CGA simultaneously before initiating systemic treatment in older patients with cancer. These two evaluations could give oncologists decisive information to personalize cancer treatment of older patients and optimize treatment dose to offer the best efficacy and minimize toxicity.