© 2015 Rudolph M. Navari. Despite significant progress in the prevention of chemotherapy-induced nausea and vomiting (CINV) with the introduction of new antiemetic agents, 30-50% of patients receiving moderately or highly emetogenic chemotherapy (MEC or HEC) and guideline directed prophylactic antiemetics develop breakthrough CINV. International guidelines recommend the treatment of breakthrough CINV with an agent from a drug class that was not used in the prophylactic antiemetic regimen and recommend using the breakthrough medication continuously rather than using it on an as needed basis. There have been very few studies on the treatment of breakthrough CINV. A recent double-blind, randomized, phase III study suggested that olanzapine may be an effective agent for the treatment of breakthrough CINV. Refractory CINV occurs when patients develop CINV during subsequent cycles of chemotherapy when antiemetic prophylaxis has not been successful in controlling CINV in earlier cycles. Patients who develop refractory CINV should be considered for a change in their prophylactic antiemetic regimen. If significant anxiety exists, a benzodiazepine may be added to the prophylactic regimen. If a refractory patient is receiving HEC, olanzapine may be added to the prophylactic regimen. If the patient is receiving MEC, olanzapine or an NK-1 receptor antagonist may be added to the prophylactic regimen.