Objective To explore the experience of time toxicity among young and middle-aged breast cancer patients, to provide reference for developing effective intervention strategies to alleviate time toxicity in this population. Methods A descriptive qualitative study was designed. Young and middle-aged breast cancer patients were selected through purposive sampling during March-April 2025, then face-to-face semi-structured in-depth interviews were conducted, and the interview data were analyzed using content analysis methodology. Results A total of 21 young and middle-aged breast cancer patients were interviewed, yielding three themes and ten sub-themes: abnormal experiences of time perception (distorted time perception, loss of control over time perception), drivers of time toxicity (complex follow-up procedures, inadequate doctor-patient communication, moving between multiple departments increases time costs, time costs of travel), and impacts of time toxicity (negative psychological emotions, disruption of social roles, disruption of daily rhythms, exacerbated economic toxicity). Conclusion Young and middle-aged breast cancer patients experience altered perceptions of time under the influence of time toxicity. Healthcare professionals should pay attention to the adverse effects of time toxicity, establish a patient-centred care model, and integrate outpatient appointments, remote follow-ups, health education and psychological support, so as to realize systematic restructuring and dynamic optimization of service delivery processes, and improve patients′ experience of time toxicity ultimately.