Which amendment defined the categories of legend and over-the-counter drugs?

The correct answer is found in the Durham-Humphrey Amendment of 1951, which established the legal distinctions between prescription (legend) drugs and over-the-counter (OTC) medications. This amendment was pivotal in determining how drugs are classified based on their safety, efficacy, and the need for medical supervision during their use.

Prior to this amendment, there was no formal legal framework distinguishing these categories, which led to confusion regarding which medications required a prescription and which were safe for consumer use without professional guidance. The Durham-Humphrey Amendment required that drugs intended for use by the public either be dispensed by prescription or be available OTC, laying the groundwork for safe medication practices today.

The other amendments and acts listed do address important aspects of drug regulation and marketing, but none specifically define the categories of legend and OTC drugs. The Kefauver-Harris Amendment of 1962 focused on drug efficacy and safety, the Prescription Drug Marketing Act of 1987 dealt with the distribution and marketing of prescription drugs, and the FDA Modernization Act of 1997 updated various provisions regarding drug approval and regulation but did not address the fundamental classification established by the Durham-Humphrey Amendment.