MK0557 for the Treatment of Cognitive Impairment in Patients With Schizophrenia (0557-027)

Inclusion Criteria: * Patient is clinically stable, on current antipsychotic medication for at least 3 months and current dose for 2 months * Patient has a 6th grade reading level or better and has completed at least 6 years of formal education * Females are not pregnant, and those who can have children agree to remain abstinent or use acceptable birth control throughout study * Patient has had a stable living arrangement for at least 3 months prior to study start * Patient is in general good health based on screening assessments Exclusion Criteria: * Patient has a major disease/disorder that may interfere with cognitive testing (such as mental retardation) and/or pose a risk upon study participation * Patient has a history of head trauma with loss of consciousness greater than 15 minutes * Patient has had warfarin treatment, MAO inhibitors, clonazepam, clozapine or St. John's wort within 1 month of screening * Patient has had ECT treatment within 6 months of screening * Patient requires treatment with antihistamines or certain other medications listed in the protocol * Patient has a history of liver disease that has been active within the last 2 years, or a history of cancer within past 5 years * Patient has a history of alcohol or drug dependence within past year or alcohol or drug abuse within 3 months of screening