drug user vs substance abuser

drug user and substance abuser both are nouns.

Word	Noun	Adjective	Verb	Adverb
drug user	Yes	No	No	No
substance abuser	Yes	No	No	No

As nouns, drug user and substance abuser are synonyms defined as:

Other synonyms of drug user include user.

drug user (noun)	substance abuser (noun)
a person who takes drugs	a person who takes drugs