Clozapine

The label says clozapine's mechanism is unknown but has been proposed to involve blocking dopamine D2 and serotonin 5-HT2A receptors. It also blocks adrenergic, cholinergic, histamine and other dopamine and serotonin receptors [1].

Because of the risks of severe neutropenia (a drop in infection-fighting white blood cells) and seizures, the label limits clozapine to people whose schizophrenia has not responded adequately to standard antipsychotics. Treatment needs a baseline absolute neutrophil count (ANC), and the ANC is monitored regularly throughout treatment. The starting dose is 12.5 mg once or twice a day, increased slowly to 150 to 225 mg twice a day by the end of 2 weeks, with a maximum of 450 mg twice a day. The slow start reduces the risk of low blood pressure, slow heart rate and fainting [1].

Clozapine is almost completely metabolized before it leaves the body. CYP1A2, CYP2D6 and CYP3A4 are the main enzymes involved. The main metabolite, norclozapine, has only limited activity. The half-life at steady state averages 12 hours but ranged from 4 to 66 hours [1].

CYP2D6 is a liver enzyme with many inherited variants. People with two non-working copies are poor metabolizers. CPIC estimates that about 6.5% of people of European ancestry are poor metabolizers, compared with about 2% to 3% in most other groups and under 1% of people of East Asian or Oceanian ancestry [3].

The label reasons that clozapine levels "may be increased" in poor metabolizers because the drug is almost completely metabolized [1]. The human data are thin. A 2021 meta-analysis of drug levels by genotype found one study with 4 poor metabolizers, whose clozapine exposure was the same as in normal metabolizers (ratio 1.00), and two studies of intermediate metabolizers with a ratio of 1.22 that was not statistically significant. The authors described the clozapine differences as marginal, ambiguous or based on fewer than 3 studies [4].

The DPWG reached a similar view. Its recommendation text says variation in CYP2D6 can affect clozapine levels but has no clinical consequences, so no action is needed for ultrarapid, intermediate or poor metabolizers [2]. The DPWG also concluded that testing before treatment provides no benefit for this pair [5].

What about CYP1A2?

CYP1A2 is another of clozapine's main enzymes, and the label describes tobacco smoke as a moderate CYP1A2 inducer [1]. The DPWG once advised adjusting the dose by blood level for carriers of the CYP1A2*1C allele, but in 2022 it concluded that there is no gene-drug interaction between any CYP1A2 allele and clozapine [6]. The Gene2Rx clozapine result uses CYP2D6 only.

The FDA label says it may be necessary to reduce the clozapine dose in CYP2D6 poor metabolizers, as it may be with significant kidney or liver impairment. It gives no specific reduced dose [1]. The FDA's Table of Pharmacogenetic Associations lists clozapine and CYP2D6 in its first section with the wording "dosage reductions may be necessary" [7]. A Gene2Rx report shows that FDA statement and the DPWG recommendation for your CYP2D6 result. For poor metabolizers the two sources differ: the FDA label says a lower dose may be necessary, and the DPWG recommends no action.

The FDA wording is permissive ("may be necessary") and sets no number, and the label already starts every patient at 12.5 mg and raises the dose gradually. The DPWG position comes from its 2024 antipsychotic guideline [5]. CPIC has no clozapine guideline; its gene-drug list gives CYP2D6 and clozapine a provisional level of B/C [8].

For clozapine, other medicines and smoking usually change levels more than CYP2D6 genes do. The label's largest adjustment is for strong CYP1A2 inhibitors such as fluvoxamine, ciprofloxacin or enoxacin: the dose drops to one-third. It names CYP2D6 and CYP3A4 inhibitors, including paroxetine, fluoxetine, bupropion, duloxetine, sertraline, escitalopram, quinidine, terbinafine, cimetidine and erythromycin, as drugs that can raise clozapine levels, and advises caution and a possible dose reduction. Strong CYP3A4 inducers such as carbamazepine are not recommended with clozapine [1].

The label also warns that stopping a CYP1A2 inducer such as tobacco smoke, or a CYP3A4 inducer such as carbamazepine, may call for a lower clozapine dose [1]. It also says inducers, including tobacco smoke, can lower clozapine levels and its effect. Tell your prescriber before you stop or cut down smoking.

Do not start, stop or change clozapine or any other medicine because of a genetic result without talking to your prescriber.

Consumer arrays (23andMe, AncestryDNA, MyHeritage) are not clinical-grade tests, and their raw data is labeled not for medical use. They read fixed positions, and Gene2Rx uses imputation to infer some variants that were not measured directly. A normal result means none of the assessed variants was found among the positions your data covers; it cannot rule out a variant at a position the data does not cover. If a clinical test (GeneSight, Genomind or another lab) has already reported your CYP2D6 genotype, those results can be entered for a Panel Report.

The clozapine label sets a lower starting neutrophil threshold for people with benign ethnic neutropenia, also called Duffy-null associated neutrophil count [1]. Gene2Rx does not report Duffy status. A CYP2D6 result also does not predict whether clozapine will work or whether you will have side effects such as neutropenia or seizures.

I am a CYP2D6 poor metabolizer. Do I need a lower clozapine dose?

Possibly, but not necessarily. The two sources in your report disagree: the FDA label says a reduction may be necessary, and the DPWG recommends no action. Clozapine is started low and raised gradually in everyone, so your prescriber adjusts the dose by response and tolerability either way.

Why does smoking matter for clozapine?

Tobacco smoke induces CYP1A2, one of the main enzymes that clears clozapine, and the label warns that stopping an inducer may require a lower dose [1].

1. Clozaril (clozapine) tablets: prescribing information (HLS Therapeutics). DailyMed. Indications, dosing, monitoring, interactions and CYP2D6 poor metabolizers.
2. ClinPGx. Annotation of DPWG Clinical Guideline for clozapine and CYP2D6. The Dutch recommendation text: no action for any CYP2D6 result.
3. CPIC. CYP2D6 frequency table (spreadsheet). Estimated phenotype frequencies by biogeographic group.
4. Milosavljevic F et al. Association of CYP2C19 and CYP2D6 poor and intermediate metabolizer status with antidepressant and antipsychotic exposure: a systematic review and meta-analysis. JAMA Psychiatry (2021). Clozapine exposure by CYP2D6 group.
5. Beunk L et al. Dutch Pharmacogenetics Working Group (DPWG) guideline for the gene-drug interaction between CYP2D6, CYP3A4 and CYP1A2 and antipsychotics. Eur J Hum Genet (2024). No action for CYP2D6 and clozapine; no benefit from testing before treatment.
6. ClinPGx. Annotation of DPWG Clinical Guideline for clozapine and CYP1A2. The 2022 conclusion that CYP1A2 alleles do not interact with clozapine.
7. US Food and Drug Administration. Table of Pharmacogenetic Associations. The FDA statement for clozapine and CYP2D6 poor metabolizers.
8. CPIC. Gene-drug pairs and CPIC levels (spreadsheet). Provisional level B/C for CYP2D6 and clozapine.