phlegm vs lethargy

phlegm and lethargy both are nouns.

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

As nouns, phlegm and lethargy are synonyms defined as:

Other synonyms of phlegm include flatness, languor, sluggishness.

phlegm (noun)	lethargy (noun)
inactivity; showing an unusual lack of energy	weakness characterized by a lack of vitality or energy
expectorated matter; saliva mixed with discharges from the respiratory passages; in ancient and medieval physiology it was believed to cause sluggishness	inactivity; showing an unusual lack of energy
apathy demonstrated by an absence of emotional reactions	a state of comatose torpor (as found in sleeping sickness)