The most common and most likely answer to this crossword clue is the 11 letter word AMPHETAMINE
I've seen this crossword clue in The Times
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More clues leading to the result AMPHETAMINE
Buddy explains!

The straight part
We know that "different drug" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case different drug = amphetamine.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 AMPHETAMINE mean?
AMPHETAMINE noun- (proper) The racemic freebase of 1-phenylpropan-2-amine; an equal parts mixture of levoamphetamine and dextroamphetamine in their pure amine forms.
- Any mixture of the two amphetamine enantiomers, dextroamphetamine and levoamphetamine.
- Referring to a substituted amphetamine; a member of the amphetamine class of chemicals.
- A potent central nervous system stimulant of the phenethylamine chemical class that is used to treat ADHD, narcolepsy, and obesity.