Which of the following drugs is NOT classified as a schedule IV drug?

Heroin is classified as a Schedule I substance under the Controlled Substances Act and is not available for medical use in the United States. This classification indicates that heroin has a high potential for abuse and no accepted medical use, distinguishing it from Schedule IV drugs, which are recognized for their legitimate medical uses but still have a potential for abuse and dependence.

Schedule IV drugs include substances that are deemed to have a lower potential for abuse compared to Schedule I to III drugs. Phenobarbital, clonazepam, and zolpidem are all recognized for their medical uses; they are prescribed for conditions like seizures, anxiety, and insomnia, respectively. Since they can be abused but are still deemed safe for medical use when properly prescribed, they fall into the Schedule IV category. Therefore, heroin’s classification as a Schedule I substance makes it the correct answer to the question regarding which drug is NOT classified as a Schedule IV drug.