Side effects of nalbuphine while reversing opioid-induced respiratory depression: report of four cases. Nalbuphine hydrochloride, an agonist-antagonist opioid, is reported to reverse the respiratory depression of moderate doses of fentanyl (20 micrograms.kg-1) and still provide good analgesia. We report four patients having abdominal aortic aneurysm repair in which we attempted to reverse the respiratory depression of large doses of fentanyl (50-75 micrograms.kg-1) with nalbuphine (0.3 mg.kg-1, 0.1 mg.kg-1 or 0.05 mg.kg-1). Nalbuphine reversed respiratory depression in all four patients and the respiratory rate increased from 10 to 23 breaths per minute, end-tidal CO2 decreased from 7.0 +/- 0.3 per cent to 5.6 +/- 0.7 per cent, and peak inspiratory pressure after 0.1 seconds increased from 4 +/- 1.4 to 13 +/- 2.6 mmHg. However, hypertension, increased heart rate, and significant increase in analogue pain scores accompanied reversal of respiratory depression. Agitation, nausea, vomiting, and cardiac dysrhythmias also were observed frequently. We do not recommend the use of nalbuphine to facilitate early extubation of the trachea after large doses of fentanyl for abdominal aortic surgery. 