This study involves taking a drug Aprepitant by mouth 4 hours before the ERCP and once a day for two days after the ERCP, in an attempt to reduce the risk of pancreatitis caused by ERCP. Pancreatitis, or injury and inflammation of the pancreas, is the most common complication of ERCP and occurs in approximately 6 out of every 100 patients undergoing ERCP. It may result in pain and lead to hospitalization and, in some cases, a need for further procedures such as surgery.