Two milligrams i.v. hydromorphone provides efficacious and rapid pain relief in nonelderly adults presenting to the ED with acute severe pain. However, oxygen desaturation below 95 percent occurred in about one third of patients. Although no noticeable clinical signs of hypoxemia occurred, a conservative interpretation of this finding suggests that 2 mg i.v. hydromorphone is too much opioid to be given routinely to patients in pain as a single initial dose.