relapse vs lapse

relapse and lapse both are nouns.

Word	Noun	Adjective	Verb	Adverb
relapse	Yes	No	Yes	No
lapse	Yes	No	Yes	No

As nouns, relapse and lapse are synonyms defined as:

Other synonyms of relapse include backsliding, lapsing, relapsing, reversion, reverting.

As verbs, relapse and lapse are synonyms defined as:

Other synonyms of relapse include fall back, recidivate, regress, retrogress.

relapse (noun)	lapse (noun)
a failure to maintain a higher state	a mistake resulting from inattention
	a failure to maintain a higher state
	a break or intermission in the occurrence of something

relapse (verb)	lapse (verb)
go back to bad behavior	go back to bad behavior
deteriorate in health	drop to a lower level, as in one's morals or standards
	end, at least for a long time
	for time to move forward
	pass into a specified state or condition; sink into
	let slip