Background : Why do patients practice fasting? The effects of fasting before treatment with chemotherapy for cancer in humans are currently unknown. However, there is an apparent enthusiasm for fasting among cancer patients. This qualitative study provides data on the motivations to fast and the experience of fasting among a population of women with breast cancer.Method : Sixteen semi‐structured interviews were conducted, and two researchers independently performed a thematic analysis. To ensure the internal validity of the study, patients had the possibility to rate their agreement with the study results through a satisfaction questionnaire.Results : Six main themes were identified in this study: main reasons to fast, alternative authorities to the oncologist, adapting the fast to social and lifestyle constraints, fasting effects felt during chemotherapy, barriers and facilitators of fasting during chemotherapy, and seeking for a more integrative medicine. Patients’ primary motivation to fast was to lower the negative side effects of chemotherapy. Fasting was also reported as a coping strategy to give them a greater sense of control over their treatment and to reduce their anxiety.Clinical implications : Results from the study suggest that, if discouraged from fasting, patients may turn to complementary health care practitioners for support. Medical professionals may thus not know of patients’ fasting practice. Health psychologists could play a key role fostering the dialogue between different health professionals and the patient. They could also help to meet patients’ needs during cancer treatment to reduce treatment anxiety.