1. Clinical Overview
Clozapine is a second-generation (atypical) antipsychotic medication, considered the gold standard for treatment-resistant schizophrenia. It is a dibenzodiazepine derivative with a unique receptor binding profile, offering superior efficacy for positive and negative symptoms but carrying a significant risk of severe adverse effects, most notably agranulocytosis, necessitating strict hematological monitoring.
| Onset | Duration | Bioavailability |
|---|---|---|
| Psychotic symptom improvement may be seen within 1-2 weeks, but full therapeutic effect can take 6-12 weeks or longer. | Approximately 12-24 hours, supporting once or twice-daily dosing. | Approximately 50-60% due to first-pass metabolism. |
2. Mechanism of Action
Clozapine's exact antipsychotic mechanism is complex and not fully understood. It demonstrates a broad receptor binding profile with higher affinity for serotonin (5-HT2A) receptors than dopamine (D2) receptors. This atypical profile is thought to contribute to its efficacy in treatment-resistant cases and lower incidence of extrapyramidal symptoms (EPS).
3. Indications & Uses
- Treatment-resistant schizophrenia (failure to respond adequately to at least two different antipsychotics).
- Reduction in risk of recurrent suicidal behavior in patients with schizophrenia or schizoaffective disorder.
4. Dosage & Administration
Adult Dosage: **Initial:** 12.5 mg (half of 25mg tablet) once or twice daily on Day 1, increased cautiously by 25-50 mg/day to a target range of 300-450 mg/day by end of week 2. **Therapeutic:** 200-600 mg/day in divided doses. Maximum daily dose should generally not exceed 900 mg. Dose must be titrated slowly to minimize hypotension and sedation.
Administration: Administer with or without food. To minimize orthostatic hypotension, advise taking at bedtime, especially during initial titration. The tablet can be split. Consistent timing of doses is important. Do not stop abruptly due to risk of rebound psychosis and cholinergic rebound.
5. Side Effects
Common side effects may include:
- Sedation/drowsiness
- Hypersalivation (sialorrhea) - especially nocturnal
- Dizziness/orthostatic hypotension
- Tachycardia
- Constipation
- Weight gain
- Fever (especially in first few weeks)
- Dry mouth
- Blurred vision
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Carbamazepine, Phenytoin, Phenobarbital | Induce CYP1A2/3A4, significantly decreasing clozapine plasma levels, increasing risk of treatment failure AND concurrent bone marrow suppression risk with carbamazepine. | Major - Contraindicated/Requires alternative |
| Fluoroquinolones (Ciprofloxacin), Fluvoxamine | Potent CYP1A2 inhibitors, significantly increasing clozapine plasma levels, leading to toxicity (seizures, sedation, myocarditis risk). | Major |
| Erythromycin, Clarithromycin, Ketoconazole, Itraconazole | CYP3A4 inhibitors, increase clozapine levels. | Moderate |
| Rifampicin | Potent CYP inducer, decreases clozapine levels substantially. | Major |
| Selective Serotonin Reuptake Inhibitors (SSRIs) like Fluoxetine, Paroxetine | CYP2D6 inhibitors, moderate increase in clozapine levels. | Moderate |
| Benzodiazepines, Opioids, Alcohol, other CNS Depressants | Potentiate CNS and respiratory depression, profound sedation. | Major |
| Anticholinergic drugs (e.g., Trihexyphenidyl) | Potentiate anticholinergic side effects (ileus, delirium, hyperthermia). | Moderate |
| Antihypertensives | Enhanced hypotensive effect. | Moderate |
| Lithium | Increased risk of seizures, NMS, encephalopathy. | Moderate |
7. Patient Counselling
- **DO** take your medication exactly as prescribed, at the same time(s) each day.
- **DO** attend ALL scheduled blood test appointments without fail. No blood test, no medication.
- **DO** inform all your doctors and dentists that you are on clozapine.
- **DO** report immediately any signs of infection (fever, sore throat, mouth ulcers).
- **DO NOT** stop taking clozapine suddenly without consulting your doctor.
- **DO NOT** consume alcohol.
- **DO NOT** drive or operate heavy machinery if you feel drowsy or dizzy.
- **DO NOT** take any new prescription, over-the-counter, or herbal medicine without checking with your doctor or pharmacist.
8. Toxicology & Storage
Overdose: Profound CNS depression (drowsiness, delirium, coma), seizures, tachycardia, arrhythmias (torsades de pointes), hypotension, hypersalivation, anticholinergic toxicity (mydriasis, ileus), respiratory depression, and aspiration pneumonia. Fatalities have occurred.
Storage: Store below 30°C, in a cool, dry place, protected from light and moisture. Keep the medicine in its original blister pack or container. Keep out of reach and sight of children. Do not use after the expiry date printed on the pack.