From Medical News Today:
Health Canada takes additional safety measures to ensure the safe use of Clozapine
Health Canada is taking additional steps to ensure the safe use of clozapine, a drug used in the treatment of schizophrenia.

Clozapine is used for patients that are unresponsive to, or not tolerant to, conventional drugs. The use of clozapine is known to potentially cause a decrease in white blood cells, known as agranulocytosis, which impairs the body's defense mechanism against infection. As such, patients must have regular blood tests to monitor their white blood cell count. Results are recorded in patient registries, which are accessed by treating physicians and pharmacists to ensure that the drug is not given to a patient at risk for agranulocytosis.