demented vs sick

demented is not a noun while sick is a noun.

Word	Noun	Adjective	Verb	Adverb
demented	No	Yes	No	No
sick	Yes	Yes	Yes	No

As adjectives, demented and sick are synonyms defined as:

Other synonyms of demented include brainsick, crazy, disturbed, mad, unbalanced, unhinged.

demented (noun)	sick (noun)
	people who are sick

demented (adjective)	sick (adjective)
affected with madness or insanity	affected by an impairment of normal physical or mental function
	shockingly repellent; inspiring horror
	having a strong distaste from surfeit
	affected with madness or insanity
	deeply affected by a strong feeling
	(of light) lacking in intensity or brightness; dim or feeble
	feeling nausea; feeling about to vomit

demented (verb)	sick (verb)
	eject the contents of the stomach through the mouth