A 20-year-old man is brought to the hospital emergency department by a friend who found him unconscious in his apartment after trying to contact him for 3 days. On arrival, the patient is in a state of respiratory depression. He experiences convulsions for 2 minutes, followed by cardiac arrest. Advanced cardiac life support measures are instituted, and he is stabilized and intubated. On physical examination, there are needle tracks in the left antecubital fossa, miosis, and a loud diastolic heart murmur. His temperature is 39.2deg C. Use of which of the following substances by this man most likely produced these findings?