Which of the following substances is classified as a Schedule I drug?

Heroin is classified as a Schedule I drug due to its high potential for abuse, lack of accepted medical use in treatment in the United States, and a lack of accepted safety for use under medical supervision. Schedule I substances are deemed to have no medical value and are illegal to manufacture, distribute, or possess. This classification reflects a stringent regulatory stance towards opioid drugs that are not recognized for any therapeutic purposes, in contrast to substances like morphine and pentobarbital, which have accepted medical uses and are classified in lower schedules.