Which of the following best describes a controlled substance?

A controlled substance is best described as a substance that requires a prescription due to its potential for abuse, dependency, or addiction. These substances are regulated by law, and their distribution and use are monitored to ensure safety and control.

Controlled substances are classified into different schedules based on their potential for abuse and accepted medical use. For instance, Schedule I substances have no accepted medical use and a high potential for abuse, while Schedule II-V substances have accepted medical uses but still present risks associated with misuse. Because of these risks, individuals cannot purchase them freely; they must obtain a prescription from a licensed healthcare provider who assesses the necessity and appropriateness for the patient's condition.

Other choices describe substances in ways that do not align with the definition of controlled substances. Substances available over-the-counter do not require a prescription, unrestricted substances imply no legal limitations, and substances labeled as safe may still have restrictions based on potential side effects or misuse. Thus, the requirement for a prescription is a defining characteristic of controlled substances.