Which law established the distinction between prescription and over-the-counter medications?

The correct choice is the Durham-Humphrey Amendment, which was enacted in 1951. This amendment established the legal distinction between prescription medications, which require a licensed healthcare provider's authorization, and over-the-counter (OTC) medications, which can be purchased without a prescription.

The importance of this amendment lies in its role in protecting the public by ensuring that certain medications, which could pose risks if misused or taken without appropriate medical guidance, remain available only through a pharmacist or physician's oversight. This amendment is pivotal in shaping medication accessibility and safety practices in the pharmacy field.

In contrast, the Controlled Substances Act focuses on the regulation of substances with potential for abuse and does not address the prescription versus OTC distinction. The Drug Enforcement Administration Act primarily pertains to the enforcement of the Controlled Substances Act rather than defining medication categories. Meanwhile, the Food, Drug, and Cosmetic Act provides foundational regulations concerning the safety and efficacy of foods and drugs but was the framework into which the Durham-Humphrey Amendment fit, creating specific classifications for medications.