Synonyms for Demented:

adj.

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

n.

childish, laughable, ludicrous, over-the-top, ridiculous, silly, not in your right mind/not right in the head. around the bend, bonkers, buggy, cracked, crackers, cuckoo, daft, disordered, distraught, fruity, loco, loony, maniac, mentally ill, moonstruck, non compos mentis, nuts, of unsound mind, off, sane, screwy, touched, unsound, wacky, wrong, Daffy, bananas, not all there, crazy as a loon, mad as a hatter, nutty as a fruitcake, sick in the head, stark raving mad. crazy (noun)
bemused.
demented (noun)
brainsick, crazy, distracted, disturbed, insane, mad, sick, unbalanced, unhinged.
mentally ill (noun)
bipolar, confused, depressed, diminished responsibility.

Other synonyms:

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

Usage examples for demented

  1. When I saw you in the carriage downtown, with that poor, demented child, I thought, for just a moment- oh, can you forgive me? – In the Bishop's Carriage by Miriam Michelson