Which of the following is a controlled substance?

Lomotil is classified as a controlled substance because it contains diphenoxylate, which is an opioid used to treat diarrhea. It is considered a Schedule V controlled substance due to its potential for abuse, although it has a lower risk compared to more tightly controlled substances. The presence of the opioid component necessitates regulation to prevent misuse.

On the other hand, Bystolic, Cleocin, and Pamelor do not have the same level of abuse potential associated with controlled substances. Bystolic is a beta-blocker typically used for hypertension, Cleocin is an antibiotic that treats various infections, and Pamelor is a tricyclic antidepressant. These medications, while they may have side effects or interactions, do not fall under the categories of controlled substances, and therefore, they are not subject to the same regulatory restrictions as Lomotil.