sick vs demented

sick is a noun but demented is not a noun.

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

As adjectives, sick and demented are synonyms defined as:

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

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

sick (adjective)	demented (adjective)
affected by an impairment of normal physical or mental function	affected with madness or insanity
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

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