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 in refractory cases but carrying a significant risk of severe adverse effects, most notably agranulocytosis, necessitating strict hematological monitoring.
| Onset | Duration | Bioavailability |
|---|---|---|
| Psychotic symptom improvement typically begins within 1-4 weeks, with full therapeutic effect often taking 6-12 weeks or longer. | The pharmacodynamic effects last beyond its plasma half-life, allowing for once or twice-daily dosing. Steady-state plasma levels are achieved in about 1 week. | Approximately 50-60% following oral administration, but subject to significant first-pass metabolism. |
2. Mechanism of Action
Clozapine's exact mechanism in schizophrenia is unknown but is believed to be mediated through a combination of antagonistic activity at dopamine type 2 (D2) and serotonin type 2A (5-HT2A) receptors, with a higher affinity for 5-HT2A receptors. This atypical profile is associated with a lower incidence of extrapyramidal symptoms (EPS). It also has affinity for adrenergic, cholinergic, and histaminergic receptors, contributing to both its therapeutic and side effect profile.
3. Indications & Uses
- Treatment-resistant Schizophrenia (failure to respond adequately to at least two different antipsychotics).
- Reduction in the risk of recurrent suicidal behavior in patients with schizophrenia or schizoaffective disorder.
4. Dosage & Administration
Adult Dosage: **Initial:** 12.5 mg once or twice daily, increased gradually by 25-50 mg/day to a target dose of 300-450 mg/day by end of week 2. **Therapeutic range:** 200-600 mg/day in divided doses. Some patients may require up to 900 mg/day. Dose titration must be slow to minimize hypotension, sedation, and seizures.
Administration: Administer with or without food. Food may slow absorption and reduce peak concentration, potentially minimizing side effects. The total daily dose is usually divided into 2-3 doses. The evening dose can be larger to promote sleep and manage daytime sedation. Tablets should be swallowed whole with a full glass of water.
5. Side Effects
Common side effects may include:
- Sedation/Somnolence
- Hypersalivation (Sialorrhea) - especially nocturnal
- Dizziness, Vertigo
- Tachycardia (Increased heart rate)
- Constipation
- Weight Gain
- Orthostatic Hypotension
- Fever (especially during initial titration)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Carbamazepine | Induces CYP1A2/3A4, significantly reducing clozapine plasma levels by 50% or more. Also increases risk of agranulocytosis. CONTRAINDICATED. | High |
| Fluvoxamine, Ciprofloxacin | Potent CYP1A2 inhibitors, can increase clozapine levels 5-10 fold, leading to toxicity. | High |
| Smoking/Tobacco | Polycyclic hydrocarbons in smoke induce CYP1A2, reducing clozapine levels by ~30-50%. Smoking cessation can lead to a rapid rise in levels. | High |
| Benzodiazepines (e.g., Lorazepam, Clonazepam) | Increased risk of profound sedation, respiratory depression, and collapse, especially during initial titration. | High |
| Warfarin | Clozapine may potentiate anticoagulant effect; monitor INR closely. | Moderate |
| Lithium | May increase risk of seizures, NMS, and encephalopathy. Use with caution. | Moderate |
| Selective Serotonin Reuptake Inhibitors (SSRIs) like Fluoxetine, Paroxetine | CYP2D6 inhibition can increase clozapine levels. | Moderate |
| Anticholinergic drugs (e.g., Trihexyphenidyl) | Additive anticholinergic effects, increasing risk of constipation, urinary retention, and delirium. | Moderate |
7. Patient Counselling
- **DO** get your blood tests done strictly as scheduled by your doctor. No medicine will be dispensed without a valid blood test report.
- **DO** take the medicine exactly as prescribed, even if you feel better.
- **DO** inform all your doctors and dentists you are on clozapine.
- **DO** rise slowly from sitting/lying position to avoid dizziness.
- **DON'T** stop taking clozapine suddenly without consulting your doctor.
- **DON'T** smoke or change your smoking habits without informing your doctor, as it affects dose.
- **DON'T** consume alcohol.
- **DON'T** take any other prescription, OTC, or herbal medicine without checking with your doctor.
8. Toxicology & Storage
Overdose: Profound sedation, delirium, coma, tachycardia, hypotension or hypertension, respiratory depression, hypersalivation, seizures, arrhythmias. Fatal outcomes have occurred.
Storage: Store below 30°C, in a cool, dry place, protected from light and moisture. Keep the container tightly closed. Keep out of reach of children and pets. Do not use after the expiry date printed on the pack.