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.