lethargy vs inanition

lethargy and inanition both are nouns.

Word	Noun	Adjective	Verb	Adverb
lethargy	Yes	No	No	No
inanition	Yes	No	No	No

As nouns, lethargy and inanition are synonyms defined as:

lethargy and inanition: weakness characterized by a lack of vitality or energy

Other synonyms of lethargy include lassitude, slackness.

lethargy (noun)	inanition (noun)
weakness characterized by a lack of vitality or energy	exhaustion resulting from lack of food
inactivity; showing an unusual lack of energy	weakness characterized by a lack of vitality or energy
a state of comatose torpor (as found in sleeping sickness)