Final answer:
A Schedule I drug has a high potential for abuse and no accepted medical use, making it illegal for all purposes including prescription and over-the-counter availability.
Step-by-step explanation:
Regarding the regulation of controlled substances, a Schedule I drug is characterized by having a high potential for abuse and no accepted medical use in the United States.
This makes such drugs illegal to manufacture, possess, or distribute for any purpose, including over-the-counter or prescription availability.
Despite the complexities presented by state and federal laws, where certain states have legalized marijuana either for medical or recreational use, it remains a Schedule I substance under federal law, with no accepted medical use and a high potential for abuse.