This is a multi-centre randomized, double-blind, placebo-controlled, cross-over study aimed at testing the efficacy of Ondansetron in the low anterior resection syndrome (LARS) occurring as a consequence of surgery for rectal cancer.~Patients will be randomized (1:1 ratio) to receive either 4 weeks of Ondansetron followed by 4 weeks of placebo (O-P sequence) or 4 weeks of placebo followed by 4 weeks of Ondansetron (P-O sequence). Neither the investigator nor the patient will be aware of the sequence (O-P or P-O) assigned to the patient. One week will elapse between the end of the first treatment and start of the second treatment.~The hypothesis to be tested is that 4 weeks of oral Ondansetron at the dose of 4mg BID induce a variation of the LARS Score at least 7 points (20%) greater than the variation occurring after 4 weeks of placebo.~The sample size calculated for the primary analysis is 38 evaluable patients. Considering an expected attrition rate of about 20%, it is expected to enrol approximately 46 patients in total.
Patients will be randomized (1:1 ratio) to receive either 4 weeks of Ondansetron followed by 4 weeks of placebo (O-P sequence) or 4 weeks of placebo followed by 4 weeks of Ondansetron (P-O sequence). It will be one week of washout between the two treatments.~During the treatment questionnaires will be completed by the patients to evaluate the efficacy of the study treatment and the quality of live.