1. Since it has been proposed that neuroleptic-induced increases in free-radical production may relate to the development of TD, the investigators hypothesize that melatonin, an effective antioxidant, may attenuate the severity of tardive dyskinesia symptoms.
2. Due to increased cognitive deficits in patients with TD and implication of oxidative stress in cognitive impairment, the investigators hypothesize that both cognitive impairment and tardive dyskinesia symptoms may be induced by the same pathophysiological stimulus--oxidative stress. Hence, the investigators further hypothesize that both tardive dyskinesia symptoms and cognitive deficits in patients with TD may be improved by melatonin simultaneously.
