30% of the clozapine patients responded compared to 4% of the controls, with significantly greater improvement on the Brief Psychiatric Rating Scale, Clinical Global Impression Scale, and Nurses Observation Scale for Inpatient Evaluation; this improvement included "negative" as well as positive symptom areas. Following this study, the US Food and Drug Administration (FDA) approved its use in 1990. Cautious of this risk, however, the FDA required a black box warning for specific side effects including agranulocytosis, and took the unique step of requiring patients to be registered in a formal system of tracking so that blood count levels could be evaluated on a systematic basis.In December 2002, clozapine was approved in the US for reducing the risk of suicide in people with schizophrenia or schizoaffective  judged to be at chronic risk for suicidal behavior. In 2005, the FDA approved criteria to allow reduced blood monitoring frequency. In 2015, the individual manufacturer Patient Registries were consolidated by request of the FDA into a single shared Patient Registry Called The Clozapine REMS Registry. Despite the demonstrated safety of the new FDA monitoring requirements, which have lower neutrophil levels and do not include total white cell counts, international monitoring has not been standardised. Chemistry
Clozapine is a dibenzodiazepine that is structurally very similar to loxapine (originally deemed a typical antipsychotic). It is slightly soluble in water, soluble in acetone, and highly soluble in chloroform. Its solubility in water is 0.1889 mg/L (25 °C). Its manufacturer, Novartis, claims a solubility of <0.01% in water (<100 mg/L).
Investigations used to monitor the possibility of clozapine related side effects such as myocarditis are also performed including baseline troponin, CRP and BNP and for neuroleptic malignant syndrome CK.The dose of clozapine is initially low and gradually increased over a number of weeks. Initial doses may range from 6.5 to 12.5 mg/d increasing stepwise typically to doses in the range of 250–350 mg per day at which point an assessment of response will be performed. In the UK the average clozapine dose is 450 mg/d. But response is highly variable and some patients respond at much lower doses and vice versa. Monitoring
During the initial dose titration phase the following are typically monitored; usually daily at first; pulse, blood pressure and since orthostatic hypotension can be problematic this should be monitored both sitting and standing. If there is a significant drop then the rate of the dose increase may be slowed, temperature.Weekly tests include; Mandatory full blood counts are performed weekly for the first 18 weeks. In some services there will also be monitoring of markers that might indicate myocarditis; troponin, CRP and BNP although the exact tests and frequency vary between services. Weight is usually measured weekly. Thereon other investigations and monitoring will always include full blood counts (fortnightly for 1 year then monthly). Weight, waist circumference, lipids and glucose or HbA1c may also be monitored. Clozapine Response and Treatment Optimisation
As with other antipsychotics, and in contrast to received wisdom, responses to clozapine are typically seen soon after initiation and often within the first week.