Final answer:
The correct answer is the Food, Drug, and Cosmetic Act; it defines a prescription and the accessibility of drugs. Schedule I drugs have no medicinal use and cannot be prescribed, while Schedule III drugs can be prescribed despite some potential for addiction.
Step-by-step explanation:
The first piece of legislation that defined what a prescription is and who could authorize the use of prescription drugs is the Food, Drug, and Cosmetic Act. Some drugs, like Schedule I drugs, are classified as having no medicinal use and are therefore not available even with a prescription. Although Schedule III drugs are considered to be moderately addictive, the risk of abuse is deemed acceptable for patients in need of their therapeutic benefits. Therefore, the correct completion of the sentence is: "The Food, Drug, and Cosmetic Act was the first piece of legislation that defined what a prescription is and who could authorize the use of prescription drugs. Some drugs, like Schedule I drugs, have no medicinal use and are not available even with a prescription. Although Schedule III drugs are moderately addictive, the risk of abuse is worth it for patients in need of this treatment."