Presently no generally effective treatments for tardive dyskinesia (TD) are available. D-serine is a naturally occurring amino acid that acts in-vivo as positive allosteric modulator at the glycine site associated with the glutamatergic NMDA receptor. Previous studies have suggested that D-serine may improve motor symptoms, including dyskinesias, which are caused by treatment with presently used antipsychotics drugs.

The hypothesis under investigation in the present study is that D-serine adjuvant treatment may improve TD in schizophrenia patients diagnosed with this disorder.

Inclusion Criteria:

1. age 18-70;
2. diagnosis of schizophrenia/schizoaffective disorder according to DSM-IV criteria; diagnosis will be made on the basis of SCID interview and information from medical records, previous treating psychiatrists, and family informants;
3. history of ≥3 months antipsychotic drugs treatment and present stable dose antipsychotic treatment for at last 4 weeks;
4. fulfillment of Schooler-Kane TD research criteria on a first evaluation performed 2-12 weeks prior to study entrance and on a subsequent evaluation performed prior to allocation to experimental treatment.

Exclusion Criteria:

1. meeting criteria for other DSM-IV Axis I diagnoses;
2. presence of a neurological disorder or history of significant head injury;
3. substance abuse or alcoholism during entire lifetime;
4. are judged clinically to be at suicidal or homicidal risk;
5. female patients who are pregnant or lactating; female patients who are not pregnant or lactating, if sexually active, must be using medically accepted means of contraception.