Perphenazine

Perphenazine belongs to the piperazine group of phenothiazines. The label notes that in this group extrapyramidal symptoms (movement side effects such as stiffness, tremor and restlessness) are more common, and sedation is less common, than with other phenothiazines. Because extrapyramidal symptoms rise with dose, the label asks for the lowest effective dose [1].

Usual doses in the label are 4 to 8 mg three times a day for outpatients with schizophrenia, 8 to 16 mg two to four times a day in hospital (no more than 64 mg a day), and 8 to 16 mg a day in divided doses for severe nausea and vomiting. Long-term doses above 24 mg a day are reserved for people in hospital or under continued observation [1].

The liver breaks perphenazine down extensively by sulfoxidation, hydroxylation, dealkylation and glucuronidation. Peak levels come 1 to 3 hours after a dose, and the half-life is 9 to 12 hours. One metabolite, 7-hydroxyperphenazine, reaches peak levels about half those of the parent drug. Blood levels per milligram rise with age, and the label recommends lower starting doses in older people [1].

CYP2D6 is a liver enzyme with many inherited variants. People with two non-working copies are poor metabolizers; people with reduced function are intermediate 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 [2].

The label says perphenazine's pharmacokinetics follow CYP2D6 activity and that poor metabolizers "will metabolize perphenazine more slowly and will experience higher concentrations" [1]. Studies put numbers on this. In a 1996 Swedish study of 36 patients, oral clearance was three times higher in people with two working CYP2D6 genes than in poor metabolizers [3]. In a 2021 Norwegian drug-monitoring study of 564 samples, dose-adjusted perphenazine levels were 3.9 times higher in poor metabolizers and 1.6 times higher in intermediate metabolizers than in normal metabolizers. The same study found that the common decreased-function allele *41 raised levels about as much as a non-working allele when paired with a normal one [4].

The link to side effects rests on less data. The label describes a study of 45 older people with dementia: the 5 poor metabolizers reported significantly more side effects during the first 10 days of treatment than the other 40, after which the groups tended to converge [1]. That is a small study. Perphenazine is not approved for dementia-related psychosis, and the label warns of higher mortality with antipsychotics in that group.

The FDA's Table of Pharmacogenetic Associations lists perphenazine and CYP2D6 in its second section, for associations where the data indicate a potential impact on safety or response, with the statement that poor metabolizers have higher systemic concentrations and a higher risk of adverse reactions [5]. Unlike drugs in the first section, there is no dose instruction. The label itself sets no genotype-specific dose; it notes that prospective phenotyping of older patients may identify those at risk of adverse events [1].

CPIC has not published a perphenazine guideline; its gene-drug list gives the pair a provisional level of B/C [6]. The DPWG's 2024 antipsychotic guideline does not cover perphenazine [7].

Bring the report to your prescriber or pharmacist. Because the label already asks for the lowest effective dose, a poor metabolizer result is most useful as a reason to start low and watch closely for movement side effects and sedation, especially in older adults.

Other medicines can make CYP2D6 behave as if your genes were different, an effect called phenoconversion. The perphenazine label names tricyclic antidepressants and SSRIs such as fluoxetine, sertraline and paroxetine as CYP2D6 inhibitors that may raise antipsychotic levels, and advises close monitoring and possible dose reduction when they are added [1]. The FDA classes fluoxetine, paroxetine and bupropion as strong CYP2D6 inhibitors and sertraline as weak [8].

Do not start, stop or change perphenazine 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.

A genotype does not predict whether perphenazine will help, and the label's warnings about movement side effects, including tardive dyskinesia, apply whatever the CYP2D6 result.

I am a poor metabolizer. Is perphenazine unsafe for me?

Neither the label nor the FDA table says to avoid it. Poor metabolizers have higher levels and may have more early side effects, so a lower starting dose and closer monitoring are reasonable topics for your prescriber.

Does this apply to perphenazine for nausea?

The drug and its metabolism are the same, so the higher levels apply. Doses for nausea are lower than for schizophrenia (see How perphenazine works).

1. Perphenazine tablets, USP: prescribing information (Sandoz). DailyMed. Indications, dosing, pharmacokinetics, CYP2D6 and drug interactions.
2. CPIC. CYP2D6 frequency table (spreadsheet). Estimated phenotype frequencies by biogeographic group.
3. Jerling M et al. The CYP2D6 genotype predicts the oral clearance of the neuroleptic agents perphenazine and zuclopenthixol. Clin Pharmacol Ther (1996). Threefold difference in perphenazine clearance.
4. Waade RB et al. Impact of CYP2D6 on serum concentrations of flupentixol, haloperidol, perphenazine and zuclopenthixol. Br J Clin Pharmacol (2021). Perphenazine levels by CYP2D6 group and the effect of *41.
5. US Food and Drug Administration. Table of Pharmacogenetic Associations. The FDA statement for perphenazine and CYP2D6 poor metabolizers (section 2).
6. CPIC. Gene-drug pairs and CPIC levels (spreadsheet). Provisional level B/C for CYP2D6 and perphenazine.
7. 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). The drugs the DPWG antipsychotic guideline covers.
8. US Food and Drug Administration. For healthcare professionals: FDA's examples of drugs that interact with CYP enzymes and transporter systems. Strength of CYP2D6 inhibitors.