Deteriorates, as a user may (4,2,3)
I believe the answer is:
goes to pot
This clue is a double definition.
'deteriorates' is the first definition.
(I've seen this before)
'as a user may' is the second definition.
(a drug user might smoke 'pot')
(Other definitions for goes to pot that I've seen before include "Becomes bad" , "Becomes ruined" , "Become ruined" , "Prepares to follow snooker player" , "Approaches the snooker table" .)