Background The underlying mechanisms of chemotherapy-induced gastrointestinal (GI) symptoms are poorly researched. This study characterised the nature, frequency, severity and treatable causes for GI symptoms prospectively in patients undergoing chemotherapy for GI malignancy. Methods Patients receiving chemotherapy for a GI malignancy were assessed pre-chemotherapy, then monthly for 1 year using the Gastrointestinal Symptom Rating Scale, a validated patient-reported outcome measure. Patients with new, troublesome GI symptoms were offered investigations to diagnose the cause(s). Their oncologist was alerted when investigations were abnormal. Results A total of 241 patients, 60% male, median age 63 years (range 30-88), were enrolled; 122 patients were withdrawn, 93%, because of progressive disease or death. During the study, > 20% patients reported chronic faecal incontinence and > 10% reported moderate or severe problems with taste, dysphagia, belching, heartburn, early satiety, appetite, nausea, abdominal cramps, peri-rectal pain, rectal flatulence, borborygmi, urgency of defecation or tenesmus. Thirty percent reported continuing passage of hard stools and 30% ongoing diarrhoea. Moderate or severe fatigue affected 40% participants at its peak and persisted in 15% at 1 year. Toxicity dictated change in chemotherapy for 13-29% patients/month. Common Terminology Criteria for Adverse Events underestimated gastrointestinal morbidity. Pre-chemotherapy screening identified previously undiagnosed pathology: exocrine pancreatic insufficiency (9%), vitamin B 12 deficiency (12%) and thyroid dysfunction (20%). Patients often refused investigations to diagnose their chemotherapy-induced symptoms; however, for every three investigations performed, one treatable cause was diagnosed: particularly small intestinal bacterial overgrowth (54%), bile acid malabsorption (43%), previously not described after chemotherapy, and unsuspected urinary tract infection (17%). Conclusions Patients undergoing chemotherapy for GI malignancy commonly have difficult GI symptoms requiring active management which does not occur routinely. The underlying causes for these symptoms are often treatable or curable. Randomised trials are urgently needed to show whether timely investigation and treatment of symptoms improve quality of life and survival. Trial registration ClinicalTrials.gov Identifier: NCT02121626

The results of the MAAS score indicate that mindfulness was enhanced by the sessions. The feedback from the participants highlighted the impact on their professional development. This is encouraging to support developing similar facilitated reflective practice programmes in different settings.