Clozapine is an atypical (second-generation) antipsychotic with a unique efficacy profile, primarily reserved for patients with treatment-resistant schizophrenia. It was the first antipsychotic shown to be superior in patients who do not respond to standard therapy, particularly for both positive and negative symptoms. Despite its clinical benefits, clozapine use is limited by potentially serious adverse effects, most notably agranulocytosis, as well as significant metabolic, cardiovascular, and gastrointestinal risks. As a result, clozapine therapy requires careful patient selection, strict monitoring protocols, and continuous risk-benefit assessment in clinical practice. This article will provide a comprehensive overview of clozapine, including its clinical indications, mechanism of action, pharmacokinetics, adverse effects, contraindications, monitoring requirements, and key drug interactions. Indications for Use Clozapine is primarily indicated for: Treatment-resistant schizophrenia (defined as failure of at least two antipsychotics, including one second-generation agent) Reduction of recurrent suicidal behaviour in schizophrenia or schizoaffective disorder Patients intolerant to other antipsychotics, particularly due to extrapyramidal side effects (EPSE) Due to its safety profile, clozapine is reserved for patients who have not responded adequately to standard therapy. Clozapine is effective for both positive and negative symptoms in schizophrenia. Mechanism of Action Clozapine has a unique receptor-binding profile, compared to other antipsychotics. Unlike typical antipsychotics, clozapine produces antipsychotic effects via dopamine receptor antagonism, with lower D2 receptor occupancy, reducing the risk of extrapyramidal side effects (EPSE). Weak affinity at dopamine receptors (D1, D2, D3, D5) High affinity at D4 receptors It also acts on: Serotonin receptors (5-HT2A, 5-HT1A) Histamine (H1) receptors Muscarinic receptors Alpha-adrenergic receptors Sedative effects occur early (due to blockade of histamine and muscarinic receptors), whereas antipsychotic effects are delayed. Created in BioRender. Boucher, M. (2026) https://BioRender.com/4bpdjjr Fig 1: Clozapine receptor activity Pharmacokinetics Clozapine is available as an oral formulation only, and brands for clozapine should be kept the same when possible. If switching is required, additional monitoring is likely required. Clozapine displays linear pharmacokinetics. One metabolite (norclozapine) is active but less potent and possesses a shorter half life (t½). Absorption Distribution Metabolism Excretion Clozapine F = 50-60% (moderate first pass metabolism) Tmax = 2/1 hours Vd = 1.6L/kg Protein plasma binding: 95% Extensive metabolism by CYP1A2 and CYP3A4, and to some extent by CYP2C19 and CYP2D6 T ½ = 14.2 hours once at steady state 50% excreted via urine and 30% faeces Cautions and Adverse Effects Endocrine Hyperglycaemia, dyslipidaemia and weight gain can occur with clozapine therapy, leading to an increased risk of diabetes and cardiovascular disease. The risk is higher with second generation antipsychotics, and this is likely due to activity at histamine, 5HT and muscarinic receptors. Blood disorders Agranulocytosis and leucopenia risk is 1-2% with clozapine therapy, therefore regular blood monitoring is required. Cardiac Potentially fatal myocarditis, pericarditis and cardiomyopathy can occur early into therapy with clozapine. Persistent tachycardia, palpitations, chest pain or signs of heart failure/myocardial infarction should be investigated. If myocarditis/cardiomyopathy are suspected, clozapine therapy should be stopped and not restarted. Neuroleptic malignant syndrome (NMS) NMS is a rare idiosyncratic response characterised by the following signs and symptoms: Hyperthermia Muscle rigidity Autonomic instability Reduced consciousness Increased creatinine phosphokinase There is a small increased risk of NMS with clozapine, which is further increased if clozapine is combined with lithium. Antimuscarinic effects Muscarinic antagonism can lead to sedation, constipation, dry mouth and blurred vision. Patients with prostatic enlargement and narrow angle glaucoma could be monitored closely. Constipation can be severe and lead to an increased risk of intestinal obstruction and ileus (MHRA warning 2017 – Clozapine: reminder of potentially fatal risk of intestinal obstruction, faecal impaction, and paralytic ileus). The sedating effects of olanzapine are less than with clozapine. Other Adverse Effects Clozapine possesses a lower risk of EPSE; patients who have experienced EPSE with other antipsychotics have found to improve with clozapine therapy. Other adverse effects are listed below: With atypical antipsychotics, there is a 3x increased risk of cerebrovascular adverse events in patients with dementia. Furthermore, cases of thromboembolism have been reported with antipsychotic drugs. Orthostatic hypotension due to alpha receptor blockade Dose related seizures Hypersalivation Jaundice associated with raised ALP Erectile dysfunction Abrupt withdrawal can cause onset of a psychotic episode Transient and often benign fever in the first 3 weeks Contraindications The following are important contraindications to note with clozapine therapy, alongside some drug combinations which will be described later in the article: History of agranulocytosis/impaired bone marrow function Uncontrolled epilepsy CNS depression Paralytic ileus Severe cardiac, hepatic, or renal disease Monitoring Monitoring is paramount with clozapine therapy due to the wide range of clinically significant adverse effects possible Weekly weight checks Pulse Blood pressure Baseline fasting glucose Baseline lipids Liver function tests Blood counts At a minimum, white cell count (WBC) and neutrophil count (ANC) should be monitored following the schedule below: Weeks 0-18: weekly Weeks 18-52: 2 weekly 1 year onwards and for 4 weeks after discontinuation: 4 weekly Initiation requires: WBC ≥ 3500/mm³ ANC ≥ 2000/mm³ Stop clozapine if: WBC < 3000/mm³ ANC < 1500/mm³ More frequent monitoring is required (twice weekly) if WBC is between ≥ 3000mm3 and < 3500mm3 and/or ANC is between ≥ 1500mm3 and < 2000mm3, until counts stabilise or increase. Clozapine use is contraindicated if patients are not able to undergo regular blood tests or with history of clozapine induced agranulocytosis. Furthermore, patients should monitor for infections especially flu like symptoms and a sore throat which can indicate neutropenia Clozapine can also cause eosinophilia and thrombocytopenia, generally a full blood count will be taken at the intervals described above to check all parameters. Interactions CYP mediated interactions Smoking tobacco induces CYP1A2, which is responsible for the metabolism of clozapine. Therefore, cessation of smoking can increase clozapine levels. Omeprazole is also a CYP1A2 inducer so concomitant administration can reduce clozapine levels. Caffeine, hormonal contraceptives and fluvoxamine inhibit CYP1A2, concomitant administration with clozapine can increase clozapine exposure. By TeachMeSeries Ltd (2026) Fig 2: CYP1A2 inducers and inhibitors Protein binding Clozapine displaces warfarin and digoxin from proteins leading to increased exposure of both drugs Additive Adverse Effects There is a risk of myelosuppression if clozapine is administered alongside other drugs that can cause myelosuppression, such as long acting depot antipsychotics or carbamazepine. Concomitant administration of alcohol and clozapine is contraindicated due to the risk of sedation. Furthermore, caution should be applied when narcotics, antihistamines or benzodiazepines are administered alongside clozapine, due to the risk of CNS depression. References Clozaril 25 mg Tablets – Summary of Product Characteristics (SmPC) Rang & Dale’s Pharmacology (9th Edition) The Top 100 Drugs (2022) MHRA (2017): Clozapine and risk of intestinal obstruction Do you think you’re ready? Take the quiz below Pro Feature - Quiz Clozapine Question 1 of 3 Submitting... 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