Which act required that drugs be safe for humans and also effective?

The Kefauver-Harris Amendment is the correct choice because it was enacted in 1962 and specifically mandated that all new drugs must not only be proven safe for human use but also effective for their intended purposes. This legislation was significant in response to the thalidomide tragedy, where a drug caused severe birth defects due to its inadequately tested safety and efficacy.

This amendment strengthened the regulatory framework of the FDA, ensuring that rigorous testing and clinical trials were required to demonstrate both safety and effectiveness before a drug could be marketed. It marked an important shift in pharmaceutical regulation, emphasizing the responsibility of manufacturers to provide proof of efficacy alongside safety.

The other options, such as the Pure Food and Drug Act, addressed the purity and quality of food and drugs but did not establish the requirement for efficacy. The Durham-Humphrey Amendment primarily focused on the distinction between prescription and over-the-counter medications, while the Orphan Drug Act encouraged the development of drugs for rare diseases but did not specifically address safety and effectiveness in the same comprehensive manner as the Kefauver-Harris Amendment.