Due to the risk of QT interval prolongation, GSK recently announced new dosage information for IV ondansetron in elderly patients:

Patients 75 years or older:
A single IV dose for the prevention of chemotherapy-induced nausea and vomiting (CINV) must not exceed 8 mg (infused over at least 15 mins)

Adult patients less than 75 years old:
A single IV dose for the prevention of CINV must not exceed 16 mg (infused over at least 15 mins)

Patients 65 years or older:

All IV doses must be diluted in 50-100mL saline or other compatible fluid and infused over at least 15 minutes.

Repeat IV doses on ondansetron should be given no less than 4 hours apart.