lapse vs drop off

lapse is a noun but drop off is not a noun.

Word	Noun	Adjective	Verb	Adverb
lapse	Yes	No	Yes	No
drop off	No	No	Yes	No

As verbs, drop off is a hypernym of lapse; that is, drop off is a word with a broader meaning than lapse:

Other hypernyms of lapse include drop away, fall away, slip.

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

lapse (verb)	drop off (verb)
go back to bad behavior	change from a waking to a sleeping state
drop to a lower level, as in one's morals or standards	fall to a lower standard
end, at least for a long time	fall or diminish
for time to move forward	retreat
pass into a specified state or condition; sink into	remove (cargo, people, etc.) from and leave
let slip