The most common and most likely answer to this crossword clue is the 9 letter word DIGITOXIN
D
I
G
I
T
O
X
I
N
Buddy explains!

The straight part
We know that "drug" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drug = digitoxin.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DIGITOXIN mean?
DIGITOXIN noun- A toxic cardiac glycoside, obtained from digitalis, related to cardenolide.