This is a double-blind, randomized, placebo-controlled trial of melatonin as an add-on therapy to antipsychotics will be performed to examine the effects of melatonin on tardive dyskinesia symptoms and cognitive deficits in 120 patients with established tardive dyskinesia (TD). This study addresses a free radical hypothesis of TD.

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.

Inclusion Criteria:

1. diagnosis of both schizophrenia and TD;
2. duration of TD symptoms longer than 1 year;
3. on stable doses of antipsychotic drug for at least 6 months;
4. between 18 and 70 years of age.

Exclusion Criteria:

1. comorbid neurological illness other than TD;
2. if they have received vitamin C or vitamin E within 1 month before the start of the study;
3. alcohol/drug abuse;
4. acute, unstable medical condition;
5. pregnant or breastfeeding female;
6. use of other antioxidants.