Uses: Fosaprepitant is used with other medications to help prevent nausea and vomiting caused by cancer drug treatment (chemotherapy). Fosaprepitant 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 if you already have it. Contact your doctor for further instructions if you are experiencing nausea or vomiting.