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

A substance classified as a Schedule I drug is defined as having a high potential for abuse, no accepted medical use in treatment in the United States, and a lack of accepted safety for use under medical supervision. Heroin fits this classification as it is an opiate that has no approved medical use in the U.S. and is linked to various health risks and high potential for addiction and abuse.

In contrast, cocaine, while it is a powerful stimulant and has significant potential for abuse, is classified as a Schedule II drug because it has accepted medical uses, specifically for certain medical procedures and as a local anesthetic in some cases. Hydrocodone and diazepam are also classified as Schedule II and Schedule IV drugs, respectively, meaning both have accepted medical uses and varying degrees of abuse potential, but neither is classified as Schedule I. This differentiation is crucial in understanding how substances are regulated and categorized based on their medical utility and risk of dependency.