Synonyms for Demented:

adj.

crazed (adjective)
berserk.
crazy, insane (adjective)
deranged, non compos mentis, frenzied, unsound, distracted, delirious, lunatic, unhinged, unbalanced, maniacal, hysterical, schizoid, daft, maniac, psycho, bananas, bemused, fruity, mad, psychotic, distraught, nutty as a fruitcake.
deranged (adjective)
nutty.
gaga (adjective)
around the bend.
insane (adjective)
daft, babbling, schizoid, berserk, moonstruck, batty, feverish, insane, brainsick, fruity, deranged, loco, unbalanced, nutty, crackers, frenzied, psychotic, delirious, hysterical, irrational, bananas, neurotic, lunatic, crazy, maniacal, psycho, Crack-brained, blithering, mad, brain-sick, kooky.
lunatic (adjective)
schizoid.
of unsound mind (adjective)
non compos mentis.

n.

laughable, not in your right mind/not right in the head, ridiculous, ludicrous, childish, over-the-top, silly. crazy (noun)
bemused.
demented (noun)
crazy, distracted, brainsick, sick, insane, disturbed, unbalanced, unhinged, mad.
mentally ill (noun)
bipolar, confused, diminished responsibility, depressed.

Other synonyms:

crackers, bananas, mentally ill, fruity, bonkers, loco, Daffy. cracked, screwy, nuts, loony, wacky, distraught, buggy, non compos mentis. disordered, moonstruck, unsound, touched. wrong. off. mentally ill
bipolar, confused, diminished responsibility, depressed.
Other relevant words:
cracked, silly, ludicrous, crackers, bonkers, around the bend, buggy, maniac, unsound, disordered, mad as a hatter, wacky, distracted, unhinged, depressed, nutty as a fruitcake, distraught, crazy as a loon, bemused, Daffy, bipolar, sick in the head, bananas, cuckoo, fruity, loony, screwy, ridiculous, loco, off, nuts, sick, not all there, of unsound mind, touched, disturbed, confused, wrong, moonstruck, daft, mentally ill, stark raving mad, brainsick, non compos mentis.