This is a 16-week, randomized, double-blind, parallel group, placebo-controlled study comparing adjunctive vortioxetine with identically appearing adjunctive placebo pills in 88 stable patients with a research diagnosis of schizophrenia determined with the Structured Clinical Interview for DSM (SCID). Patient randomization will be stratified by illness duration (i.e., </=5 years and >5 years) in order to allow for post-hoc analyses examining whether earlier illness moderates greater negative and /or cognitive symptom reduction in response to vortioxetine.

This is a 16-week, randomized, double-blind, parallel group, placebo-controlled study comparing adjunctive vortioxetine with identically appearing adjunctive placebo pills in 88 stable patients with a research diagnosis of schizophrenia determined with the Structured Clinical Interview for DSM (SCID). Patient randomization will be stratified by illness duration (i.e., </=5 years and >5 years) in order to allow for post-hoc analyses examining whether earlier illness moderates greater negative and /or cognitive symptom reduction in response to vortioxetine.

Patients with clinical stability for at least 3 months and stable antipsychotic and psychotropic medication treatment for at least the last 4 weeks and without relevant depression, positive psychotic symptoms or extrapyramidal symptoms EPS will be eligible. After written informed consent, patients will undergo a screening visit to confirm full study eligibility. If all in- and exclusion criteria are fulfilled, patients will undergo a baseline assessment and then be randomized 1:1 to 16 weeks of adjunctive treatment with vortioxetine or placebo. Subsequently, patients will be seen at week 1, 2, 4 and monthly for the next 3 months.

Inclusion Criteria:

1. Outpatient
2. SCID diagnosis of schizophrenia
3. Age 18-65 years old
4. Treated with an adequate dose of one or a maximum of two antipsychotic medications for at least 3 months, with a stable dose for at least the last 4 weeks, and willing to continue for up to 16 weeks.
5. No antidepressant treatment for at least 8 weeks prior to randomization.
6. Treatment with other concomitant, non-antipsychotic, psychiatric medications is allowed, but must be unchanged for at least 8 weeks, with a stable dose for at least the last 4 weeks, and willing to continue for 16 weeks. Benzodiazepines must not be given within 8 hours of the cognitive assessment. Treatment with non-psychiatric medications is unrestricted.
7. PANSS Negative subscore >14 with at least two of the items at a level >/=4 (moderate)
8. PANSS Positive subscore </=14 with not more than one of the items at a level >/=4 (moderate) for patients who have been stable outpatients for <1 year. A PANSS Positive subscore of </=18 is acceptable for patients who have been stable outpatients for >/=1year and whose psychotic symptoms do not affect behavior in a clinically relevant way as per investigator assessment. - . In order to avoid simple double counting, any rating on P1 ("delusions") will not be considered as an additional symptom for determination of eligibility based on PANSS positive subscore when a specific delusion is present that is rated either on P5 ("grandiosity") or P6 ("suspiciousness/paranoia"), whereas delusions not captured by P5 ("grandiosity") or P6 ("suspiciousness/paranoia"), will be rated under P1 ("delusions") and counted toward the sum score used to determine study eligibility).
9. HAMD-17 total score \