What defines controlled substances in the pharmaceutical context?

Controlled substances in the pharmaceutical context are specifically defined as drugs that possess a potential for abuse and are subject to legal restrictions due to this potential. These substances are regulated by law in order to manage their distribution, use, and prescription. The classification of controlled substances is based on their potential for dependency, risks associated with misuse, and the therapeutic benefits they can provide, which necessitate careful control.

The classification into schedules (e.g., Schedule I, II, III, IV, V) reflects the varying degrees of risk and legal restrictions, with Schedule I substances seen as having the highest potential for abuse and no accepted medical use, while those in lower schedules may be accepted for medical use but still require close monitoring.

In contrast, other options do not capture the essence of what defines controlled substances. Limited medical use does not inherently dictate control measures without consideration of abuse potential. Over-the-counter drugs are not subject to the same restrictions due to their perceived safety for general use. Lastly, the availability of drugs during specific clinical trials pertains to investigational drugs rather than those classified as controlled substances based on their abuse potential and legal status. Thus, the correct answer highlights the crucial factor of abuse potential and legal regulation that defines controlled substances.