A 71-year-old woman starting clozapine for schizophrenia. Which adverse reaction is most likely to be observed when taking or using the medication?

• A. Constipation
• B. Agranulocytosis
• C. Seizures
• D. Tachycardia

Answer: (A)

Clozapine carries a distinctive and serious safety concern: agranulocytosis, a marked drop in neutrophils that greatly increases infection risk. Because of this, ongoing monitoring of white blood cell counts, especially the absolute neutrophil count, is essential from the start of therapy. If the neutrophil count falls too low, clozapine must be stopped and appropriate medical management initiated. This risk drives all the important safety precautions and monitoring requirements around clozapine use.

Other adverse effects listed—constipation, tachycardia, and seizures—can occur with clozapine, but they are not the defining safety issue that dictates monitoring and management in the same way. Constipation, in particular, is a common and potentially serious side effect due to anticholinergic action, especially in older adults, and should be anticipated and treated proactively. However, the immediate, high-stakes concern that governs safe use of clozapine is agranulocytosis.