The most common and most likely answer to this crossword clue is the 4 letter word METH
I've seen this crossword clue in The Guardian
M
E
T
H
More clues leading to the result METH
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 = meth.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 METH mean?
METH noun- Methamphetamine, especially in the form of the crystalline hydrochloride.
- Methadone.
- (Mancunian) A tramp.
- A spiced mead, originally from Wales.