In the United States, the Food and Drug Administration requires regular blood count monitoring of all patients taking clozapine. An uncommon but serious side effect of clozapine is severe neutropenia, defined as an absolute neutrophil count (ANC) less than 500/μL. Neutrophils are a type of white blood cell that combats infection. The most dangerous form of neutropenia is agranulocytosis, which occurs in 1% or less of patients taking clozapine, most commonly between 6 and 18 weeks after starting the medication. Agranulocytosis is usually a sudden drop of ANC to zero, is thought to be autoimmune, is not permanent, is not related to dose, and is a medical emergency. Any severe neutropenia can be fatal if not detected and managed, since it makes people vulnerable to infection. With required monitoring, clozapine is safe and fatalities from neutropenia are very rare. To prescribe clozapine outside a hospital, clinicians must register with the national clozapine REMS program at https://www.clozapinerems.com , complete a training, and pass a quiz. This generally takes less than an hour. To receive clozapine, patients must be part of a system that includes regular ANC monitoring and REMS registry reporting of neutrophil counts. When starting clozapine, ANC is measured every week for 6 months, then every other week for 6 months, then monthly, if all ANC results are adequate. Monitoring or treatment are changed when an ANC is below 1500/μL. The monitoring protocol, including how and when to respond to low ANC results, is at https://www.clozapinerems.com/CpmgClozapineUI/rems/pdf/resources/ANC_Table.pdf . When patients’ ANC counts are not dangerous but near a monitoring threshold, labs are sometimes drawn later in the day, when white blood counts can be higher. There is a different monitoring protocol for patients who have been diagnosed with Benign Ethnic Neutropenia (BEN). BEN is more common in certain ethnic groups, especially people of African descent. People with BEN have safe, normal ANCs that are lower than the standard reference ranges.