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

The classification of substances into schedules is based on their potential for abuse, medical use, and safety or dependence risks. Schedule I drugs are those that have a high potential for abuse, no accepted medical use in the United States, and a lack of accepted safety for use under medical supervision.

Heroin is classified as a Schedule I drug because it is an opioid that is illegal to manufacture, distribute, or possess in the U.S. It has no accepted medical use and is associated with high levels of addiction and potential for overdose. The characteristics of heroin align with the criteria for Schedule I substances, making it the correct answer in this context.

In contrast, codeine and hydromorphone are classified as Schedule II drugs, meaning they have a high potential for abuse but also have accepted medical uses. Xanax, which contains alprazolam, is classified as a Schedule IV drug due to its lower potential for abuse and accepted medical use for anxiety and panic disorders. This distinction highlights the non-medical legality and higher risk associated with heroin compared to the other substances listed.