Background Breakthrough chemotherapy–induced vomiting (CIV) is defined as CIV occurring after adequate antiemetic prophylaxis. Olanzapine and metoclopramide are two drugs recommended for the treatment of breakthrough CIV in children, without adequate evidence. We conducted an open‐label, single‐center, phase 3 randomized controlled trial comparing the safety and efficacy of olanzapine and metoclopramide for treating breakthrough CIV. Procedure Children aged 5‐18 years who developed breakthrough CIV after receiving highly emetogenic chemotherapy or moderately emetogenic chemotherapy were randomly assigned to the metoclopramide or olanzapine arm. The primary objective of the study was to compare the complete response (CR) rates between patients receiving olanzapine or metoclopramide for treating breakthrough CIV during 72 hours after the administration of the study drug. Secondary objectives were to compare CR rates for nausea and toxicities between the two arms. Results Eighty patients were analyzed (39 in the olanzapine arm and 41 in the metoclopramide arm). CR rates were significantly higher in the olanzapine arm compared with the metoclopramide arm for vomiting (72% vs 39%, P = 0.003) and nausea (59% vs 34%, P = 0.026). Seven patients in the metoclopramide arm crossed over to the olanzapine arm and none crossed over in the olanzapine arm (P < 0.001). The mean nausea score in the olanzapine arm was significantly lower than the metoclopramide arm after the initiation of the rescue antiemetic (P = 0.01). Hyperglycemia and drowsiness were more commonly seen in the olanzapine arm. Conclusion Olanzapine is superior to metoclopramide for the treatment of breakthrough CIV in children. Drowsiness and hyperglycemia need to be monitored closely in children receiving olanzapine for breakthrough CIV.

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