This qualitative study aimed to explore cancer patients’ perceived tolerance of side effects in phase I drug trials. Patients with solid tumours receiving molecularly targeted agents with/without chemotherapy were eligible for inclusion. In-depth semi-structured interviews were carried out with 17 patients with a median [range] age of 63 [41-72] years. Treatment was discontinued in seven patients. Verbatim transcripts of the audio-taped interviews were analysed using a constructivist grounded theory approach. Four conceptual categories emerged from data analysis, labelled “suffering from side effects” comprising a range of symptoms, psychosocial or role disturbances; “striving to cope with side effects” reflecting psychological strategies for managing side effects; “hoping” reflecting expectations about treatment efficacy and relief from side effects; and “appraisal of care.” Among patients remaining in the trial, treatment was currently perceived as fairly tolerable. For most respondents, whether still in a trial or not, treatment discontinuation could not be justified by the non-tolerance of treatment side effects. These results question the adequacy of patient-perceived tolerance reports to determine an optimal drug dose for phase II trials. Confronted with patients’ hopes and inappropriate beliefs, communication is challenging in phase I trials and could benefit from facilitating psychosocial interventions.