Q: Which commonly use drug in ICU may provide synergistic effect to ondansetron (Zofran) in preventing postoperative nausea and vomiting?

Answer:  Dexamethasone (Decadron)

Dexamethasone and Ondansetron, together are more effective than Ondansetron alone in preventing postoperative nausea and vomiting.

Reference:

Song (2011). "The effect of combining dexamethasone with ondansetron for nausea and vomiting associated with fentanyl-based intravenous patient-controlled analgesia.". Anaesthesia 66 (4): 263–7