Clozaril and CBCs: Monitoring Matters

Clozaril (clozapine) requires regular blood monitoring, specifically complete blood counts (CBC) and absolute neutrophil counts (ANC), to check for two potential side effects:

  1. Agranulocytosis: A sharp drop in white blood cells (neutropenia) which increases infection risk. Clozaril can cause agranulocytosis by suppressing production of granulocytes (including neutrophils) in the bone marrow. Incidence is 0.5-1% but weekly blood monitoring for 18 weeks, then biweekly up to 1 year helps detect this early.


  • Baseline CBC/ANC before starting Clozaril
  • Weekly CBC/ANC for 18 weeks
  • Biweekly CBC/ANC weeks 19-52
  • Monthly CBC/ANC thereafter
  • Immediately stop Clozaril if ANC <1000 cells/μL or lower over successive tests
  1. Leukocytosis: An elevated white blood cell count, often over 15,000 cells/μL. Usually benign but may cause symptoms or indicate toxicity at very high levels. Temporary dose reduction or cessation of Clozaril is required until levels return to normal range.

The Clozaril Risk Evaluation and Mitigation Strategy (REMS) program mandates regular blood monitoring to access this medication. Requirements include:

  • Registering patients in a national database to track blood results
  • Educating patients and caregivers on the importance of regular blood tests
  • Reporting blood results and any abnormal values promptly to the registry
  • Alerting the registry to any signs of infection or related hospitalizations
  • Adhering to protocols for when to withhold, reduce or cease Clozaril based on blood counts

Benefits of monitoring:

  • Detects early warning signs of potentially serious side effects to allow prompt intervention
  • Guides dosage adjustments to minimize risks based on frequent measurement of hematological response
  • Provides an ongoing assessment of suitability and safety for each patient on an individual basis
  • Supports a collaborative approach to risk management between patients, clinicians, pharmacists and the drug registry

While a barrier for some, blood monitoring plays an essential role in mitigating the risks associated with Clozaril relative to benefits for treatment resistant schizophrenia. Compliance with testing protocols aims to ensure maximum safety through frequent reviews and dose adjustments, with cessation mandated immediately if markers fall outside specified ranges based on the severity of changes.

Ongoing research into equally effective yet safer alternatives remains key to overcoming current restrictions on access from both safety requirements and monitoring costs. New options may expand treatment to less severe groups where risks currently outweigh benefits without the intense follow up and interventions Clozaril demands to achieve optimal outcomes for eligible patients. Even minor improvements in safety and tolerability could greatly increase the feasibility of administration, especially long term.

In summary, mandatory blood monitoring with stringent protocols in place provides the only way to balance significant risks, like agranulocytosis, with the proven and unsurpassed efficacy of Clozaril for treatment resistant schizophrenia. While a barrier, the safety assurances offered through close supervision and the ability to individually manage risks for each patient also make this important treatment option possible where needs are greatest and few alternative choices exist.

There are various forms of drugs available, such as tablets or liquids, and each may have a separate patient information leaflet (PIL) for different doses. It is important to refer to the PIL for the specific form and dose of the drug that you have been prescribed.

You can search for further information and PILs on websites such as:

  • The British National Formulary (BNF)
  • Electronic medicines compendium (emc)
  • The National Library of Medicine's DailyMed
  • The Food and Drug Administration (FDA) 
  • Different drug forms like tablets or liquids have specific patient information leaflets (PIL) for various doses. Refer to the PIL for your prescribed drug form and dose.

    Search for PILs on websites like:


    • ANSM (Agence nationale de sécurité du médicament et des produits de santé):


    • AEMPS (Agencia Española de Medicamentos y Productos Sanitarios):


    • BfArM (Bundesinstitut für Arzneimittel und Medizinprodukte):


    For questions about your medication:

    • Consult your doctor, a healthcare professional, or a pharmacist


    • Contact SOS Médecins: 3624
    • For mental health support, contact SOS Suicide: 01 45 39 40 00

    Belgium (French)

    Switzerland (French)

    • La Main Tendue: 143
    • Website:

    Canada (French)

    • Centre de prévention du suicide du Québec: 1 866 APPELLE (1 866 277-3553)
    • Website:


    • Contact emergency number: 112
    • For mental health support, contact Teléfono de la Esperanza: 717 003 717


    • Contact emergency number: 112
    • For mental health support, contact Telefonseelsorge: 0800 111 0 111 or 0800 111 0 222


    • Contact emergency number: 112
    • For mental health support, contact Telefono Amico: 199 284 284


    • Contact emergency number: 112
    • For mental health support, contact 113 Zelfmoordpreventie: 0800 0113


    • Contact emergency number: 112
    • For mental health support, contact Sos Voz Amiga: 21 354 45 45, 91 280 26 69, or 96 352 46 60

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