Fosaprepitant is primarily used in the prevention of nausea and vomiting caused by chemotherapy and post-operative procedures. It is often administered intravenously before chemotherapy sessions or surgery to block the action of substance P/neurokinin 1 (NK1) receptors in the brain, which are involved in triggering nausea and vomiting. This medication is typically part of a combination therapy that includes other antiemetic drugs, such as corticosteroids and 5-HT3 receptor antagonists, to enhance its effectiveness. It is particularly useful in managing acute and delayed phases of chemotherapy-induced nausea and vomiting (CINV), as well as post-operative nausea and vomiting (PONV), improving patient comfort and adherence to cancer treatment regimens.