Which of the following is considered a Schedule I controlled substance?

Mescaline is classified as a Schedule I controlled substance due to its high potential for abuse and the lack of accepted medical use in the United States. Schedule I substances are characterized by the fact that they are deemed to have a significant risk of addiction and do not have legitimate medical use recognized by the FDA. In the case of mescaline, which is a naturally occurring psychedelic compound found in certain cacti, it remains illegal for medical use and recreational use in many jurisdictions.

In contrast, the other substances listed in the options have legitimate medical applications. Codeine is often prescribed for pain relief, diazepam is used for anxiety and muscle spasms, and loperamide is commonly utilized to treat diarrhea. These differences in medical use and abuse potential are crucial in determining their classification under the Controlled Substances Act.