Consolidated Standards of Reporting Trials (CONSORT) diagram. *Exclusion criterion removed as a part of protocol version 1.04, dated January 11, 2012. †Exclusion criterion removed as a part of protocol version 1.03, dated July 5, 2011. ‡Exclusions with fewer than 10 occurrences included: intolerance to IV morphine and hydromorphone, known kidney/liver failure, pulmonary hypertension, pregnancy, diagnosis of hemodynamic instability or sepsis, taking oral magnesium, or current/planned use of neuromuscular blockers. §267 unique subjects; more than 1 exclusion possible on a visit; subjects may have had multiple visits. ǁReasons not approached with fewer than 5 occurrences included: physician request, inability to consent because of medical condition, language barrier, admitted to unit with untrained staff, unable to complete follow-up, research pharmacy closed, or patient refused to allow research team to speak with parents. ¶Reasons for not receiving all allocated doses included study withdrawal, hypotension, no 25-hour safety laboratory (blinded magnesium level), physician request, found to be ineligible, and site miscommunication.