UNLABELLED BACKGROUND AND AIM OF INVESTIGATION: Intramuscular (IM) administration has been considered to be safer than intravenous (IV) for opioids on wards, but a comparative knowledge of patient safety and analgesic potency following a single dose of IV and IM administration is lacking. This study was carried out to compare patient safety and analgesic efficacy of a single and high dose of morphine given IM or IV for post-operative pain management. MATERIALS AND METHODS Thirty-eight patients with post-operative pain following hip replacement surgery were given IM or IV morphine 10 mg at a specified pain level. The study was randomized and double blinded. Time to onset of analgesic effect (11-point numeric rating scale), respiratory function (p(a)CO2, p(a)O2, and respiratory rate), level of sedation (5-point verbal rating scale), and hemodynamic function were recorded. RESULTS In the IV group there was a slight but significant increase in p(a)CO2 after 5, 10, and 15 min compared with the IM group (5.2 vs. 4.8, 5.4, vs. 5.0 and 5.5 vs. 5.1 kPa, respectively). The IV group had a significantly faster onset of analgesic effect than the IM group (5 vs. 20 min). Between 5 and 25 min after morphine administration, pain status in the IV group was significantly improved compared with the IM group. Patients in the IV group were slightly more sedated than the IM group 5 and 10 min after morphine. CONCLUSION A 10 mg bolus dose of IV morphine given to patients with moderate pain after surgery does not cause severe respiratory depression, but provides more rapid and better initial analgesia than 10 mg given IM. IV morphine even at a dose as high as 10 mg IV is well tolerated if there is a certain level of pain at its administration. The safety of IV morphine on the general ward needs to be further explored in adequately controlled studies.