Final answer:
The Kefauver-Harris Amendment's efficacy requirement applies to drugs marketed in the United States since 1962, ensuring they are both safe and effective for use.
Step-by-step explanation:
The efficacy requirement stipulated by the Kefauver-Harris Amendment affected all drugs marketed in the United States since its enactment in 1962. This amendment was in response to the thalidomide disaster, where the drug caused birth defects in thousands of babies. The requirement mandates that pharmaceutical companies provide substantial evidence of a drug's efficacy for its intended use before it can be approved by the Food and Drug Administration (FDA). This legislation marked a significant turning point in FDA regulation, ensuring that medications not only had to be safe but also effective for their advertised purposes.