Stop drug being injected into patient (5)
I believe the answer is:
cease
'stop' is the definition.
(synonyms)
'drug being injected into patient' is the wordplay.
'drug' becomes 'e' (E is another name for ecstasy).
'being injected into' is an insertion indicator.
'patient' becomes 'case' (synonyms).
'e' going into 'case' is 'CEASE'.
(Other definitions for cease that I've seen before include "Come to an end" , "Discontinue" , "Desist" , "Stop, terminate" , "Finish, end" .)