Medications are associated with substantial environmental outcomes, yet frequently end up being unused by patients. Waste-minimizing interventions, such as redispensing of quality-approved oral anticancer drugs remaining unused by patients at home, could reduce the environmental footprint of cancer treatment. To assess the environmental outcomes of redispensing quality-assured oral anticancer drugs and to explore how redispensing could be environmentally optimized. In this quality improvement study, a cradle-to-grave life cycle assessment was performed in the outpatient pharmacy of 4 Dutch hospitals, based on a prospective multicenter trial comprising 1071 patients with a clinical diagnosis of cancer and an active prescription for an oral anticancer drug stored at room temperature from February 1, 2021, to February 1, 2023, with a follow-up of 12 months per patient. Participants received prescribed oral anticancer drugs with additional quality-assurance materials (ie, seal bags and time-temperature indicators), so the pharmacy could redispense quality-assured drugs based on authenticity, appearance, remaining shelf life, and/or adequate storage. The estimated environmental outcomes avoided due to waste reduction (ie, production and transport and incineration of redispensed oral anticancer drugs) corrected for outcomes of process burdens (ie, quality assurance materials), quantified in 3 outcome measures: human health damage (disability-adjusted life-years), ecosystems damage (species × year), and climate change (kg of carbon dioxide equivalent [CO2-eq]) per patient per year. A volunteer sample of 1071 patients (median age, 70 years [IQR, 62-75 years]; 622 men [58.1%]) participated in the intervention. Redispensing oral anticancer drugs was initially associated with an environmental burden, mainly because of the high impact of time-temperature indicators. However, when quality-assurance materials were selectively used for temperature-sensitive oral anticancer drugs (ie, maximum storage temperature of 25 °C), redispensing was environmentally beneficial to human health and ecosystems, providing estimated climate benefits of 1.9 kg (95% CI, 1.4-2.6 kg) of CO2-eq per patient per year. In this quality improvement study, redispensing unused oral anticancer drugs was found to be a suitable strategy to reduce waste and improve environmental sustainability of cancer treatment after process optimization. Redispensing unused oral anticancer drugs could contribute to sustainability of cancer treatment through reduced costs and environmental outcomes.