Emergency medical personnel bring an unconscious patient to the emergency department. The patient's pupils are pinpoint and respirations are depressed. Intoxication of which of the following substances could contribute to these clinical signs?

Actions of opioids include constriction of pupils (secondary to parasympathetic stimulation) and depression of respirations (secondary to decreased respiratory center responsiveness to carbon dioxide). The other drugs are stimulants, which cause pupil dilation, excitability, increased heart rate and blood pressure.