downer vs street drug

downer and street drug both are nouns.

Word	Noun	Adjective	Verb	Adverb
downer	Yes	No	No	No
street drug	Yes	No	No	No

As nouns, street drug is a hypernym of downer; that is, street drug is a word with a broader meaning than downer:

downer: a drug that reduces excitability and calms a person
street drug: a drug that is taken for nonmedicinal reasons (usually for mind-altering effects); drug abuse can lead to physical and mental damage and (with some substances) dependence and addiction

Other hypernyms of downer include drug of abuse, medicament, medication, medicinal drug, medicine.

downer (noun)	street drug (noun)
a drug that reduces excitability and calms a person	a drug that is taken for nonmedicinal reasons (usually for mind-altering effects); drug abuse can lead to physical and mental damage and (with some substances) dependence and addiction