Which of the following best defines a "controlled substance"?

The definition of a "controlled substance" primarily pertains to drugs and chemicals that are regulated by law due to their potential for abuse and addiction. Controlled substances are categorized into different schedules based on their medical use, safety, and potential for abuse or dependence.

Choosing the option that states a drug regulated for risk of abuse accurately reflects the criteria that classify substances as controlled. These substances are monitored closely by regulatory agencies, and their distribution is controlled to mitigate the risks associated with misuse and illegal trafficking. This option captures the essence of why certain substances are classified under this category, distinguishing them from other medications that may not carry the same risk.

In contrast, the other options either misinterpret the criteria or do not fully encompass the regulatory framework surrounding controlled substances. A medication requiring a prescription might include controlled substances, but not all prescribed medications fall under this category. A substance with no medical use can be prohibited, but this does not inherently classify it as a controlled substance; it depends on the laws governing its status. Non-prescription medications are typically not regulated to the same degree as controlled substances, reflecting the differences in risk associated with their use.