Which act mandated that new drugs have demonstrated effectiveness before marketing?

The Kefauver-Harris Amendment is the correct answer because it specifically requires that new drugs not only be proven safe but also demonstrate efficacy before they can be marketed. This amendment was enacted in response to the thalidomide tragedy in the late 1950s and early 1960s, which raised significant public health concerns about the marketing of drugs that had not undergone adequate testing for effectiveness.

Prior to this amendment, the Food, Drug, and Cosmetic Act allowed for the marketing of drugs based only on safety, which led to a significant number of ineffective medications being available to the public. The Kefauver-Harris Amendment thus represented a substantial change in regulatory requirements, ensuring that pharmaceutical companies must provide evidence of how well their drugs work, not just how safe they are.

The other acts mentioned in the question have different focuses. The Pure Food and Drug Act was primarily aimed at prohibiting adulterated and misbranded foods and drugs, without a specific requirement for drug effectiveness. The Food, Drug, and Cosmetic Act established the foundation for drug regulation but did not initially include efficacy as a requirement for new drugs. The Durham-Humphrey Amendment distinguished between prescription and over-the-counter drugs, but it also did not address the effectiveness of