- Treated with atypical antipsychotic drug (in any approved dosage form) other than Clozapine at a stable dose for at least 8 weeks prior to screening and be clinically stable; the subject must remain clinically stable (in the opinion of the principal investigator) through randomization. The use of up to 2 atypical antipsychotic drugs is permitted, as long as in the opinion of the investigator, the second medication is not required to control treatment-resistant or intractable psychotic symptoms. No subject will be washed off antipsychotic therapy to become eligible for this study.