List of products in the National Drug Code with proprietary name . Aprepitant is used with other medications to help prevent nausea and vomiting caused by cancer drug treatment (chemotherapy). Aprepitant works by blocking one of the body's natural substances (substance P/neurokinin 1) that causes vomiting. This medication will not treat nausea or vomiting that has already started. Ask your doctor what you should do if you already have nausea or vomiting.