If the DEA does have this power … isn't there overlap between the FDA and the DEA?
their domains overlap because the prohibition culture of the U.S. government does not treat "drugs" rationally (i.e. as if they were simply medicines) but rather has tied the entire concept up in a gordian knot of fear, racism, social engineering, and in-group selection.
similarly, alcohol and tobacco, despite obviously being "drugs" even according to the logic of the federal government bureaucracy, are given their own police agency as well, because the DEA was tasked with persecuting hippies and brown-skinned people, whereas the ATF was tasked with shooting up disobedient white people.
The FDA is part of the Department of Health and Human Services, also serving the executive branch.
Sorry, I don't know, but it is a good question, about what policy determines, which department is responsible for what.