Inclusion criteria:
Patients who are hospitalized in psychiatric centers of the province and are in the range of 18-65 years old.
Patients who were diagnosed with schizophrenia based on DSM-5 criteria, got at least two years of anti-psychotic drug treatment but still show symptoms.
Patients who are treated with anti-psychotics for at least one year and in the last month, the type and the dosage of anti-psychotics has remained permanent.
If the patients receives mood stabilizer drugs and antidepressants with their anti-psychotic treatment, the type and the dosage of the drug will remain constant for one month before the study and during the study.
Consent from patient's guardian.
Exclusion criteria:
Patients with suicidal thoughts and acute suicidal behavior, a history of suicidal thoughts or attempted suicide during the past year, patients with thoughts or attempts to harm themselves or others at the beginning of the study or 6 months before the study, serious illness, major depression during the last 12 months that requires treatment, a history of hospitalization due to the episode of major depression over the past 6 months.
Patients who received ECT during the last 6 months.
Patients who suffer from psychiatric disorders such as schizophrenia or other psychiatric disorders' bipolar disorder, anxiety disorders such as panic disorder or obsessive-compulsive disorder, post traumatic stress disorder, eating disorder.
Current or past (past 6 months) diagnosis, severe personality disorders that compromise the ability of participants to fulfill the study requirements
Dependence or acute drug, substance or alcohol abuse in the past 12 months, except nicotine (DSM-5) or positive urine specimen testing at the beginning of the study
Acute phase of the disease ((an increase of 20% in the total score of PANSS) criteria for assessing the positive and negative symptoms of schizophrenia)
Sensitization to varenicline or other substances in the placebo
Other treatments that can interfere with the results. for example quit smoking (bupropion, varicellin or nicotine), the use of varnicline and other medication less than 30 days before the study or during the study. patients treated with anticholinergic drugs (except bipyridine and trihexafenidil) based on ACB scale (26) and obtaining the history of the drugs patient used.
Physical illness, mental retardation or other cognitive impairment and neurological disorders such as delirium dementia, uncontrolled bleeding, head injury, seizure disorder (other than the febrile type), and neurodegenerative diseases (such as Alzheimer's, Parkinson's disease , Stroke, and multiple sclerosis), and severe cardiovascular or pertussocular ailments in the past 6 months, recent history of cancer (in the past 5 years), BMI less than 15 kg/m2 or more than 38 kg/m2 or weight more than 45.5 kg or any severe and unstable medical or psychiatric conditions and disorders. uncontrolled ilnesses and patients with NMS. this information can be obtained through PMH.