Q: In severe nausea which drug may provide good synergism with ondansetron (zofran)?

Answer: Dexamethasone (Decadron)

Dexamethasone and Ondansetron (Zofran) may be 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