Aprepitant injection and fosaprepitant injection are used along with other medications to prevent nausea and vomiting in adults that may occur within 24 hours or several days after receiving certain cancer chemotherapy treatments. Fosaprepitant injection can also be used in children 6 months of age and older. Aprepitant and fosaprepitant injections are not used to treat nausea and vomiting that you already have. Aprepitant and fosaprepitant injections are in a class of medications called antiemetics. They work by blocking the action of neurokinin, a natural substance in the brain that causes nausea and vomiting.