Ondansetron hydrochloride is a prescription medication that is used to stop nausea and vomiting following surgery, chemotherapy, or radiation treatment for cancer. To prevent nausea or vomiting before surgery, adults should take two 8 mg tablets or four teaspoons of this medication one hour before anesthesia. To prevent nausea or vomiting before radiation or chemotherapy treatment, an adult should take an 8 mg tablet or two teaspoons of the oral solution twice a day for chemotherapy, or three times a day for radiation therapy. The first dose should be taken one to two hours before receiving radiation treatment or 30 minutes before chemotherapy. If nausea and vomiting are anticipated following chemotherapy treatment, a 24 milligram tablet should be taken prior to treatment. A child's dosage prior to chemotherapy treatment should be a four milligram tablet or one teaspoon of oral solution every eight hours. Take the first dose 30 minutes before receiving chemotherapy. This medication is also available under the brand name Zofran.