Yes — the active ingredient is metabolized by a gene known to vary between individuals.
Relevant genes: CYP2D6
Used for: Treatment-resistant schizophrenia, reducing suicide risk in schizophrenia or schizoaffective disorderGene2Rx covers this medication using the same CPIC and FDA guidelines GeneSight uses, costs $5-$49 instead of several hundred, and works with your existing 23andMe data.
Clozaril (clozapine) is the most effective antipsychotic for treatment-resistant schizophrenia, and also the most complex to prescribe. It requires ongoing blood monitoring for agranulocytosis, careful titration, and attention to several drug-drug and drug-gene interactions. Clozapine is metabolized primarily by CYP1A2, with contributions from CYP3A4 and CYP2D6. CYP1A2 activity is the most clinically important variable. Smokers induce CYP1A2 significantly, meaning they often need higher clozapine doses to achieve therapeutic plasma levels. When a long-term smoker quits, clozapine levels can rise substantially within days, and patients and clinicians who don't anticipate that shift sometimes see toxicity or sedation that looks inexplicable. Genetic CYP1A2 variants add another layer to this already-dynamic picture.
Clozapine's plasma levels are tracked through therapeutic drug monitoring, which is standard of care. Target trough levels of roughly 350 to 600 ng/mL are associated with efficacy; levels above 1000 ng/mL raise seizure and toxicity risk. CYP1A2 poor and intermediate metabolizers reach target levels at lower doses than rapid metabolizers. CYP2D6 has a smaller role, but CYP2D6 poor metabolizers may also need slightly lower doses, particularly when strong CYP1A2 inhibitors (like the SSRI fluvoxamine) are co-prescribed.
Read the full clozapine genetics guide →Published guidance from FDA on how clozapine should be dosed or substituted based on your CYP2D6 phenotype.
| Phenotype | What it means | Recommendation | Evidence |
|---|---|---|---|
|
Ultrarapid Metabolizer
CYP2D6
|
Your body processes clozapine faster than average, but no specific dosing changes are recommended by the FDA at this time. |
FDA
Initiate therapy with recommended starting dose.
|
— |
|
Normal Metabolizer
CYP2D6
|
Your body processes clozapine at a normal rate. The standard dose should work as expected. |
FDA
Initiate therapy with recommended starting dose.
|
— |
|
Intermediate Metabolizer
CYP2D6
|
Your body processes clozapine slightly slower than average, but no specific dosing changes are recommended at this time. |
FDA
Initiate therapy with recommended starting dose.
|
— |
|
Poor Metabolizer
CYP2D6
|
Your body breaks down clozapine much more slowly than normal, causing the drug to build up to higher levels in your system. A lower dose may be needed to reduce the risk of side effects. |
FDA
Consider dosage reductions. Monitor for adverse reactions. Refer to FDA labeling for specific dosing recommendations.
|
Moderate |
|
Indeterminate
CYP2D6
|
The impact of your genotype on response to this drug is unknown |
FDA
Initiate therapy with recommended starting dose.
|
— |
|
Not available
CYP2D6
|
The impact of your genotype on response to this drug is unknown |
FDA
Initiate therapy with recommended starting dose.
|
— |
Source: FDA
CYP2D6 is the most clinically important pharmacogene. It metabolizes around a quarter of all prescription drugs, including many antidepressants, opioids, and stimulants. The gene is unusually variable: roughly 7 percent of people are poor metabolizers (they barely activate CYP2D6), and another 1 to 3 percent are ultrarapid metabolizers (their enzyme is overactive).
For most CYP2D6 drugs, poor metabolizers feel stronger effects and more side effects at standard doses, while ultrarapid metabolizers may feel almost nothing. For prodrugs like codeine, the relationship flips: poor metabolizers feel less effect because they can't activate the drug.
Browse the full drug-class: Antipsychotics.
If you or a family member is on clozapine, the most actionable pharmacogenetic insight is CYP1A2 status, interpreted alongside smoking history. Psychiatrists familiar with clozapine routinely check plasma levels, but knowing CYP1A2 phenotype gives a faster way to anticipate dose needs and to understand why a patient who was stable for years suddenly becomes toxic after quitting cigarettes. Pharmacogenetic testing doesn't replace clozapine drug-level monitoring; it adds context.
Cigarette smoke contains polycyclic aromatic hydrocarbons that strongly induce CYP1A2, sometimes tripling its activity. Smokers on clozapine often need 50 to 100 percent higher doses than non-smokers to maintain the same plasma level. When a patient quits smoking, the induction fades over 1 to 2 weeks and plasma clozapine levels can rise quickly. This is a well-known clinical scenario that has caused serious toxicity when not anticipated.
None directly established. Clozapine-associated agranulocytosis is primarily an immune-mediated reaction, not a metabolism-driven one, and the mandatory blood monitoring program is what makes clozapine safe enough to use. CYP1A2 genetics affect clozapine plasma levels and dose-related side effects like sedation and seizure risk, not the agranulocytosis risk.
Only with substantial dose reduction and close monitoring. Fluvoxamine is a potent CYP1A2 inhibitor and can increase clozapine plasma levels several-fold. This combination is sometimes used intentionally (to boost clozapine levels in patients who clear the drug too fast), but it's a specialist decision that requires therapeutic drug monitoring and close supervision.
This page describes the general pharmacogenetics. A Gene2Rx report analyzes your own DNA to tell you which metabolizer group you fall into, across every medication.
Get your report Look up another medicationInformational only — not medical advice. Pharmacogenetic guidance describes population-level patterns; your individual response depends on many factors. Never start, stop, or change a medication without talking to your prescribing clinician.