A 25-year-old man is brought to the emergency department after being found unresponsive by friends. He has a history of intravenous heroin use. Vital signs reveal a blood pressure of 90/60 mm Hg, heart rate of 50 beats/min, respiratory rate of 6 breaths/min, and oxygen saturation of 82% on room air. Physical examination shows pinpoint pupils and shallow respirations. Which of the following is the most appropriate immediate intervention?