Fentanyl is a potent opioid, it is theoretically 100 times more potent that morphine and in severe acute postoperative pain acts faster than its congener (morphine ) for pain relief . In the literature there is no study that corroborates this theoretical assumption and proposes to compare which has fewer adverse effects. This drug produces effects similar to those reported for morphine but less magnitude and has the advantage that during the postoperative period respiratory depression, antitussive effect , gastrointestinal discomfort and physical dependence are manifested in a significantly less degree.