Efficacy of Ondansetron in LARS Treatment

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.