The most common and most likely answer to this crossword clue is the 8 plus 5 letter word SLEEPING PILLS
I've seen this crossword clue in The Guardian
S
L
E
E
P
I
N
G
P
I
L
L
S
More clues leading to the result SLEEPING PILLS
Buddy explains!
The straight part
We know that "If you’re on them, you should get off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case if you’re on them, you should get off = sleeping pills.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.