The ondansetron dose used is determined by a number of factors including clinical features of the patient, what it is being used for and the route of administration. Ondansetron is used to control nausea and vomiting, especially in the post-operative period and in patients undergoing chemotherapy treatment for cancer. It may be given by injection, either intravenously (IV) or intramuscularly (IM), or orally. Often it is given by injection initially, followed up by oral dosing.
While the mechanism of action of ondansetron in the treatment and prophylaxis is not fully known, it is thought to act on the 5HT3 receptors both in the brain and gastrointestinal tract. 5HT3 is one of the chemicals involved in causing nausea and triggering vomiting. Ondansetron, by blocking the 5HT3 receptors, may prevent this. Other 5HT3-receptor antagonists in the class include granisetron, tropisetron and dolasetron.
In patients undergoing chemotherapy, the ondansetron dose will be determined by the prescribing doctor, who will first take into consideration which chemotherapy will be used. Some chemotherapy regimens have a higher potential for causing nausea and vomiting and, in these patients, a higher ondansetron dose may be given. This may be oral or injectable and will usually be given before treatment is started and then followed up with oral dosing for up to five days. In patients on regimens that are less likely to cause nausea and vomiting the initial ondansetron dose, before chemotherapy, may be smaller and given orally.
For prevention of post-operative nausea and vomiting, a small ondansetron dose may be given either just before the anesthetic or just after the operation is completed. Alternatively, a higher oral ondansetron dose may be given an hour before the anesthetic is given. Patient choice, the procedure being performed and the doctor's clinical input will influence which of the regimens is chosen. In children, the ondansetron dose is determined, again, by what it is being used for, and then calculated by body weight or body surface area.
As with any medication, ondansetron may be contraindicated in patients with some clinical conditions and may interact with other medication. Any other medication, including homeopathic, complementary or over-the-counter medicines should be discussed with the prescribing doctor before using ondansetron. Pregnancy, desired pregnancy and breastfeeding should also be disclosed.
As with any medication, ondansetron may have adverse side effects, which should be discussed with the doctor, should they occur. The prescribed dose of ondansetron should never be exceeded. If nausea and vomiting is not controlled on the prescribed dose, medical advice should be sought.