The safety of ondansetron has been reviewed based on experience in its use as an anti-emetic treatment in about 1,400 patients receiving cancer treatment and further experience in about 650 volunteers and patients with other medical conditions. Evidence from animal pharmacology and toxicology had indicated that ondansetron had a wide therapeutic index, no interaction with commonly co-prescribed drugs, and no dependence liability. No end-organ toxicity had been seen. Clinical experience showed that ondansetron is well tolerated; the principal side effects being constipation and headache, which in the context of cancer treatment were not troublesome. Increases in liver function tests were observed, undoubtedly due in many instances to the underlying cancer or metastases and to chemotherapy, and the incidence was similar on ondansetron and metoclopramide. No extrapyramidal side effects were reported.