<p>Aprepitant is a selective high affinity antagonist of human substance P neurokinin 1 (NK1) receptors. When substance P attaches to these receptors, it causes nausea and vomiting. Aprepitant stops substance P from binding to the NK1 receptors. By blocking the receptors, Aprepitant can prevent nausea and vomiting, which often happens after chemotherapy or as a complication of surgery.</p>