Under the CSA, which of the following is considered a Schedule II controlled substance?

Morphine is classified as a Schedule II controlled substance under the Controlled Substances Act (CSA) due to its high potential for abuse and the risk of severe psychological or physical dependence it poses. Schedule II substances are often used medically but are strictly regulated. Morphine is an opioid analgesic frequently prescribed for severe pain management, which reflects its medical usefulness as well as the reason for its careful classification.

On the other hand, medications like diazepam and alprazolam fall under Schedule IV, indicating a lower potential for abuse, while diphenhydramine is an over-the-counter antihistamine and is not classified as a controlled substance at all. The classification system of the CSA aims to manage drugs based on their accepted medical uses and the potential risks associated with their misuse, which is why morphine stands out as a Schedule II substance.