Interactions between Clozapine and other drugs

Potential to increase Clozapine levels

  • Antiviral treatments for COVID-19 disease
    • Paxlovid® is contraindicated in people taking clozapine, with the risk of serious clozapine toxicity.
  • SSRIs for example:
    • fluvoxamine (very large effect)
    • fluoxetine
    • paroxetine
    • sertraline (large doses)
  • Ciprofloxacin
  • Cimetidine
  • some macrolide antibiotics including:
    • erythromycin
    • clarithromycin
    • azithromycin
  • caffeine (large doses)

Potential to decrease Clozapine levels

  • Carbamazepine
  • Rifampicin
  • St Johns Wort
  • Omeprazole
  • Phenytoin
  • Phenobarbitone

Potential to depress bone marrow

  • Carbamazepine
  • Trimethoprim/sulfamethoxazole
  • Nitrofurantoin
  • Cytotoxic medication
  • Immunosuppressant medication

Potential to depress respiration

  • Benzodiazepines, especially large parenteral doses or at start of therapy

Potential for anticholinergic side effects

For example constipation, urinary retention, delirium

  • Anticholinergic TCAs for example
    • amitriptyline
    • dothiepin
  • Anticholinergic antipsychotics for example
    • chlorpromazine
    • pericyazine
    • quetiapine
  • EPSE medication for example
    • benzhexol
    • benztropine
    • biperiden
  • sedating antihistamines for example
    • diphenhydramine
    • cyproheptadine
    • promethazine
    • trimeprazine
  • gastrointestinal antispasmodics for example
    • atropine
    • hyoscine

Potential for hypotension

  • Antihypertensives
  • TCAs
  • Some antipsychotics for example
    • chlorpromazine
    • pericyazine
    • trifluoperazine
    • risperidone (initially)
    • quetiapine (initially)

Potential for changes due to smoking

Starting or stopping smoking can cause dramatic changes in clozapine blood levels

Prescription medications used for smoking cessation have been associated with destabilisation of mental state in some patients. Consider a strategic approach in consultation with a psychiatrist. Clozapine levels must be monitored in people ceasing smoking to avoid clozapine toxicity. Refer to the Clozapine Management Clinical Guideline (CMCG) (PDF 3.95MB) for required monitoring.