That's a pretty willfully naive answer though. The fact that the CSA was passable at all is indicative that something had changed. Because if the legislative branch can grant the executive branch the right to make drugs illegal, the legislative branch must have had that ability to do so without a constitutional amendment on its own already. The question is what changed that made the CSA possible?
The correct answer is that in the 30s the courts started taking the position that the federal government could do things that weren't explicitly granted as within their powers, and most types of trade were now within "interstate commerce"
It's also generally just a wrong answer that misses big things like the Federal Food Drug and Cosmetic Act in the 30s.