relapse vs turn back

relapse is a noun but turn back is not a noun.

Word	Noun	Adjective	Verb	Adverb
relapse	Yes	No	Yes	No
turn back	No	No	Yes	No

As verbs, turn back is a hypernym of relapse; that is, turn back is a word with a broader meaning than relapse:

Other hypernyms of relapse include regress, retrovert, return, revert.

relapse (noun)	turn back (noun)
a failure to maintain a higher state

relapse (verb)	turn back (verb)
go back to bad behavior	go back to a previous state
deteriorate in health	turn inside out or upside down
	hold back, as of a danger or an enemy; check the expansion or influence of
	retrace one's course
	force to go away; used both with concrete and metaphoric meanings