Adult dosage (ages 18–64 years): Typical dosage for chemotherapy that’s highly likely to cause nausea and vomiting: A single 24-mg dose given 30 minutes before chemotherapy. Typical dosage for chemotherapy that’s somewhat likely to cause nausea and vomiting: 8 mg, 30 minutes before chemotherapy. Eight hours later, you can take another 8 mg. For 1–2 days after chemotherapy, you can take 8 mg twice per day.Child dosage (ages 12–17 years): Typical dosage for chemotherapy that’s somewhat likely to cause nausea and vomiting: 8 mg given 30 minutes before chemotherapy. Four and eight hours after the first dose, your child can take another 8 mg. For 1–2 days after chemotherapy, they can take 8 mg three times per day.Child dosage (ages 4–11 years): Typical dosage for chemotherapy that’s somewhat likely to cause nausea and vomiting: 4 mg given 30 minutes before chemotherapy. Four and eight hours after the first dose, your child can take another 4 mg. For 1–2 days after chemotherapy, your child can take 4 mg, three times per day.Child dosage (ages 0–3 years): It has not been confirmed that ondansetron is safe and effective for use in children younger than 4 years. It should not be used in children of this age range.Senior dosage (ages 65 years and older): The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, a higher amount of a drug stays in your body for a longer time. This raises your risk of side effects.Your doctor may start you on a lowered dosage or a different dosing schedule. This can help keep levels of this drug from building up too much in your body.

For people with risk factors for heart arrhythmias: If you have conditions such as heart failure or congenital long QT syndrome, this drug may increase your risk of arrhythmias. Ask your doctor if you have risk factors for arrhythmias.For people with phenylketonuria: The ondansetron orally disintegrating tablet contains phenylalanine. This amino acid can cause dangerous effects in people with a condition called phenylketonuria. Don’t take the orally disintegrating tablet if you have phenylketonuria.For pregnant women: There haven’t been enough studies done in humans to be certain how ondansetron might affect a fetus when the mother takes it. Research in animals has not shown a risk to the fetus. However, animal studies do not always predict the way humans would respond. Talk to your doctor if you’re pregnant or planning to become pregnant. This drug should only be used in pregnancy if clearly needed.For women who are breastfeeding: Ondansetron may pass into breast milk and may cause side effects in a child who is breastfed. Talk to your doctor if you breastfeed your child. You may need to decide whether to stop breastfeeding or stop taking this medication.For seniors: The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, a higher amount of a drug stays in your body for a longer time. This raises your risk of side effects.For children: This medication has not been studied in children younger than 4 years. It should not be used in children of this age range.

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