Opioids are the most important drugs for postoperative pain control. In the post-anesthesia care area , especially when faced with a patient with severe or unbearable pain (14,15) , the analgesic opioid titration is the most effective strategy for controlling postoperative pain ( 16). The most studied and used today opioid is morphine, which by its pharmacokinetic characteristics provides an important balance between speed of onset and maintenance of analgesia ( 17). However, there are other alternatives for analgesic titration , such as fentanyl , which has favorable pharmacokinetics to have a faster response and thus may shorten the time needed to decrease pain from severe to mild ( 14,15). There are comparative studies between fixed-dose fentanyl and morphine in the emergency services and some preliminary tests in the recovery area (18,19). The most recent of analgesic titration trials comparing appropriate doses of opioids to patient weight which would improve its efficacy and safety ( 20). The tests that are known to have used fentanyl and morphine compared fixed-dose and 10 minute intervals .