Clozapine is the single most examined drug in MRCPsych Paper B. Its unique efficacy in treatment-resistant schizophrenia, combined with its complex monitoring requirements and serious side effect profile, makes it a rich source of exam questions. You must know the indications, the initiation process, the monitoring schedule, the major side effects, and the rechallenge protocol.
Indications
Clozapine is indicated for treatment-resistant schizophrenia — defined as failure to respond adequately to at least two different antipsychotics (at least one atypical) given at adequate dose for adequate duration (at least 6 to 8 weeks each). It is the only antipsychotic with proven efficacy in treatment-resistant schizophrenia.
Clozapine is also indicated for psychosis in Parkinson's disease (at low doses) where other interventions have failed, and it is sometimes used off-label for treatment-resistant psychosis in other conditions. The exam primarily tests its use in schizophrenia.
CPMS registration
In the UK, clozapine can only be prescribed through the Clozaril Patient Monitoring Service (CPMS) or equivalent brand-specific monitoring systems (Zaponex Treatment Access System for zaponex). Registration requires baseline blood tests and a commitment to ongoing haematological monitoring. The pharmacy cannot dispense clozapine without a current green blood result in the monitoring system.
Blood monitoring schedule
The monitoring schedule is the most commonly tested aspect. Full blood count monitoring is mandatory throughout treatment.
Weeks 1 to 18: weekly FBC. Weeks 19 to 52: fortnightly (every 2 weeks) FBC. After 52 weeks: monthly FBC, continuing for the duration of treatment and for 4 weeks after discontinuation.
The traffic light system classifies results as green (safe to continue), amber (requires more frequent monitoring), or red (stop clozapine immediately). The thresholds are based on neutrophil count and white cell count.
Green: WCC above 3.5 and neutrophils above 2.0. Amber: WCC 3.0 to 3.5 or neutrophils 1.5 to 2.0 — increase monitoring to twice weekly, do not interrupt treatment. Red: WCC below 3.0 or neutrophils below 1.5 — stop clozapine immediately, do not rechallenge without haematological advice.
Major side effects
Neutropenia and agranulocytosis: the most serious haematological risk. Agranulocytosis (neutrophils below 0.5) occurs in approximately 0.8 per cent of patients, most commonly in the first 18 weeks. This is why weekly monitoring is required for the first 18 weeks.
Metabolic syndrome: clozapine causes significant weight gain, dyslipidaemia, and insulin resistance. Metabolic monitoring (weight, fasting glucose, lipid profile) should be performed at baseline, at 3 months, at 12 months, and annually thereafter.
Myocarditis and cardiomyopathy: myocarditis typically presents in the first 4 to 6 weeks of treatment. Monitor for unexplained tachycardia, fever, chest pain, or raised troponin and CRP. Baseline and periodic echocardiography is recommended. Myocarditis requires immediate discontinuation.
Constipation and ileus: clozapine causes severe constipation through its anticholinergic action. Clozapine-induced intestinal hypomotility can progress to ileus, bowel obstruction, and death. Proactive laxative prescribing is mandatory — the exam tests whether you would prescribe a laxative alongside clozapine initiation.
Sedation and hypersalivation: dose-related. Hypersalivation is particularly troublesome at night and can be managed with hyoscine hydrobromide.
Seizures: dose-related. The seizure risk increases above 600 mg per day. If seizures occur, reduce the dose and consider adding sodium valproate (noting the haematological monitoring interaction).
Rechallenge after neutropenia
Rechallenge after a red result is a high-yield exam topic. In general, rechallenge is not recommended after confirmed agranulocytosis. However, in cases where the benefits are considered to outweigh the risks (severe treatment-resistant schizophrenia with no alternative), rechallenge may be attempted under specialist haematological supervision with enhanced monitoring. The Maudsley Prescribing Guidelines provide a structured rechallenge protocol.
The exam tests your knowledge that rechallenge requires specialist input and enhanced monitoring — not that it is absolutely contraindicated in all circumstances.
What the exam tests
Typical MRCPsych Paper B clozapine questions present a clinical scenario and ask about the monitoring schedule (especially the transition points at 18 weeks and 52 weeks), the appropriate response to an amber or red blood result, the management of specific side effects (particularly constipation, metabolic syndrome, and myocarditis), and the rechallenge decision.
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