Search drugs

ACRONYMS AND DEFINITIONS

CYP - Cytochrome P450

Sensitive substrate - a drug whose exposure has been shown to be significantly altered by CYP2D6 inducers and/or inhibitors in studies. For other substrates, the clinical significance of inducer/inhibitor interactions is not as well-defined.

NOT A COMPLETE LIST

The information presented here is NOT A COMPLETE LIST of CYP2D6 inducers, inhibitors, and substrates
Not all drug interactions are clinically significant. Potential drug interactions should be researched, and medication changes should only be made after consulting a health professional.

CYTOCHROME P450

Drug metabolism

Cytochrome P450 (often abbreviated "CYP") is a class of liver enzymes involved in the metabolism of many medications. CYP enzymes are divided into subtypes (e.g. 2D6, 3A4, 2C8) based on their structure.
Drugs may be metabolized by one or several different CYP enzymes. In some cases, one CYP enzyme is responsible for a majority of the drug's metabolism (major pathway), while other CYP enzymes contribute a small amount (minor pathway). Some drugs undergo no CYP metabolism, and some require CYP metabolism to become active. For example, clopidogrel (Plavix) has no antiplatelet effect until it is converted to its active metabolite by CYP2C19.

Illustration of drug metabolism

CYP drug interactions

Inducers and Inhibitors

Inducers - CYP inducers increase enzyme activity and may decrease exposure to substrates
Inhibitors - CYP inhibitors reduce enzyme activity and may increase exposure to substrates
Certain chemicals and foods (e.g. tobacco smoke, grapefruit juice) may also act as CYP inducers and inhibitors
One drug may inhibit or induce multiple enzymes and transporters (e.g. p-glycoprotein, OAT)
Drugs may inhibit or induce the same enzyme they are metabolized by
Inducers and inhibitors can be subdivided into strong, moderate, or weak depending on the degree of their effect

Competitive inhibition

If two drugs are metabolized by the same CYP enzyme, they may "compete" for the enzyme, and this can alter the metabolism of one or both of the drugs

Genetic factors

Genetic differences in the genes that code for CYP enzymes can lead to variations in their activity. Patients with alleles that increase enzyme activity are referred to as "rapid metabolizers," and those with suppressive alleles are called "poor metabolizers." People in the middle are called "intermediate" or "normal" metabolizers.

RAPID METABOLIZERS AND OPIOIDS

Overview

CYP2D6 metabolizes codeine to morphine, its active form. Likewise, dihydrocodeine is metabolized by CYP2D6 to dihydromorphine. Tramadol is metabolized by CYP2D6 to an active metabolite (M1) that is more potent than the parent compound.
Patients who are CYP2D6 rapid metabolizers may experience opioid toxicity (e.g. respiratory depression) when given codeine, dihydrocodeine, or tramadol because they achieve quick and high levels of the active compound. Conversely, poor metabolizers may not receive adequate analgesia.
Respiratory depression and death have occurred in children who received codeine after surgery. These children were often found to be ultra-rapid CYP2D6 metabolizers. Because of this unforeseeable risk, codeine, dihydrocodeine, and tramadol are contraindicated in children under twelve.

Prevalence of ultra-rapid CYP2D6 metabolizers by ethnicity

North Africans, Ethiopians, and Arabs 16 - 28%
African-Americans 3%
Caucasians 1 - 10%
Chinese and Japanese 0.5 - 1%
Hispanics 0.5 - 1% [13]

IMPORTANT POINTS ABOUT DRUG INTERACTIONS

Drug interactions are challenging

Information on drug interactions can be difficult to assimilate
Certain drug interactions and metabolic pathways are well-defined while others are not

Factors that can make drug interactions challenging

New drugs

During drug development, the FDA requires interaction testing with medications that are known to have significant effects on CYP enzymes and transporters. Obviously, there is no way to test a new drug with every medication available, meaning most drugs come to market with incomplete interaction profiles. After the medication is prescribed to a large number of people, other drug interactions are inevitably discovered.

Research

Drug research often occurs in a laboratory setting (in vitro) with animals and cell cultures. Findings from these experiments do not always reflect what happens in the human body (in vivo).

Evolving information

Drug metabolism is an evolving field, and researchers are just beginning to understand all the different ways the body eliminates medications. Cytochrome P450 enzymes have been studied for a while, but cell transport systems (e.g. p-glycoprotein, OAT) are a relatively new area of pharmacology, and their role in drug elimination has not been completely elucidated.

Important points

Not all drug interactions are known or can be predicted
Good information on possible drug interactions may not be available
Not all drug interactions are clinically significant, and patients should consult their healthcare provider if they are concerned about a possible interaction
Drug interaction checkers provide the most efficient and practical way to check for interactions among multiple medications. A free interaction checker is available from Drugs.com (see Drugs.com interactions checker).

CYP2D6 INDUCERS

CYP2D6 inducers (class not well-defined)

Tocilizumab (Actemra®) - indirect induction through inflammation suppression [6]
Tumor Necrosis Factor inhibitors - indirect induction through inflammation suppression - (adalimumab, Humira®, certolizumab, Cimzia®, etanercept, Enbrel®, golimumab, Simponi®, infliximab, Remicade®) [6]

CYP2D6 INHIBITORS

CYP2D6 strong inhibitors

Bupropion (Wellbutrin®) [1]
Fluoxetine (Prozac®) [1]
Metoclopramide (Reglan®) [8]
Paroxetine (Paxil®) [1]
Quinidine [1]

CYP2D6 moderate inhibitors

Cinacalcet (Sensipar®) [1]
Dronedarone (Multaq®) [6]
Duloxetine (Cymbalta®) [1]
Mirabegron (Myrbetriq®) [6]
Terbinafine (Lamisil®) [1]

CYP2D6 weak inhibitors

Amiodarone (Cordarone®) [1]
Amphetamines (in vitro evidence) [6]
Asenapine (Saphris®) [6]
Celecoxib (Celebrex®) [1,6]
Cimetidine (Tagamet®) [1]
Desipramine (Norpramin®) [11]
Desvenlafaxine (Pristiq®) [1]
Diltiazem (Cardizem®) [1]
Diphenhydramine (Benadryl®) [1]
Echinacea [1]
Escitalopram (Lexapro®) [1]
Febuxostat (Uloric®) [1,6]
Gefitinib (Iressa®) [1]
Hydralazine (Apresoline®) [1]
Hydroxychloroquine (Plaquenil®) [1]
Imatinib (Gleevec®) [1]
Imipramine (Tofranil®) [11]
Methadone [1]
Nortriptyline (Pamelor®) [11]
Oral contraceptive pills [1]
Propafenone (Rythmol®) [1]
Ranitidine (Zantac®) [1]
Ritonavir (Norvir®) [1]
Sertraline (Zoloft®) [1]
Telithromycin (Ketek®) [1]
Venlafaxine (Effexor®) [6]
Viloxazine (Qelbree®) [6]
Verapamil (Calan®, Isoptin®, etc.) [1]

CYP2D6 inhibitors (class not well-defined)

Amitriptyline (Elavil®) [3]
Chlorpheniramine (Chlor-Trimeton®) [7]
Chlorpromazine (Thorazine®) [8]
Citalopram (Celexa®) [8]
Clobazam (Onfi®) [6]
Clomipramine (Anafranil®) [2]
Cobicistat (part of Stribild®) [6]
Doxepin [2,9]
Fluphenazine [8]
Fluvoxamine (Luvox®) [8]
Haloperidol (Haldol®) [8]
Hydroxychloroquine (Plaquenil®) (possible, not well-defined) [12]
Hydroxyzine (Vistaril®) [2,10]
Lorcaserin (Belviq®) [6]
Niacin (Niaspan®, Slo-Niacin®) [5]
Nicardipine (Cardene®) [6]
Peginterferon alfa-2b (PegIntron®) [6]
Perphenazine [8]
Sertraline (Zoloft®) [8]
Thioridazine (Mellaril®) [8]
Ticlopidine (Ticlid®) [2, 8]
Vilazodone (Viibryd®) - in vitro studies only [6]

CYP2D6 SUBSTRATES

CYP2D6 sensitive substrates

NOTE: A sensitive substrate is a drug whose exposure has been shown to be significantly altered by CYP2D6 inducers and/or inhibitors in studies

Amphetamines (Adderall®, Vyvanse®, etc.) [6]
Aripiprazole (Abilify®) [6]
Atomoxetine (Strattera®) [1,6]
Brexpiprazole (Rexulti®) [6]
Codeine (see CYP2D6 rapid metabolizers) [8]
Desipramine (Norpramin®) [1]
Dextromethorphan (Robitussin DM®, etc.) [1]
Dihydrocodeine (Trezix®) - see CYP2D6 rapid metabolizers [6]
Iloperidone (Fanapt®) [6]
Methadone [6]
Metoprolol (Toprol®) [1]
Nebivolol (Bystolic®) [1]
Perphenazine [1]
Pitolisant (Wakix®) [6]
Propafenone (Rythmol®) [6, 8]
Risperidone (Risperdal®) [6, 8]
Timolol (Timoptic®, Blocadren®) [6]
Tolterodine (Detrol®) [1]
Tramadol (Ultram®) - see CYP2D6 rapid metabolizers [6,8]
Venlafaxine (Effexor®) [1]
Viloxazine (Qelbree®) (major substrate) [6]
Vortioxetine (Trintellix®) [6]
Thioridazine (Mellaril®) - has a narrow therapeutic window [8]

CYP2D6 substrates

NOTE: Compared to sensitive substrates, the clinical significance of inducer/inhibitor interactions with these drugs is not as well-defined

Amitriptyline (Elavil®) [8]
Arformoterol (Brovana®) [6]
Benzhydrocodone (Apadaz®) [6]
Cariprazine (Vraylar®) (minor substrate) [6]
Carvedilol (Coreg®) [6]
Chlorpheniramine (Chlor-Trimeton®) [8]
Chlorpromazine (Thorazine®) [8]
Ciclesonide (Alvesco®, Omnaris®) [6]
Clomipramine (Anafranil®) [8]
Clonidine (Catapres®) [4]
Clozapine (Clozaril®) [8]
Donepezil (Aricept®) [8]
Doxazosin (Cardura®, Cardura XL®) [6, Cardura XL® PI]
Doxepin [8]
Duloxetine (Cymbalta®) [8]
Flecainide (Tambocor®) [6]
Fluoxetine (Prozac®) [8]
Fluphenazine [8]
Fluvoxamine (Luvox®) [8]
Formoterol (Foradil®) [6]
Haloperidol (Haldol®) [8]
Ibrutinib (Imbruvica®) - minor substrate [6]
Imipramine (Tofranil®) [8]
Ivermectin (Stromectol®) - in vitro only - minor substrate [6]
Letermovir (Prevymis®) [6]
Metaxalone (Skelaxin®) [6]
Metoclopramide (Reglan®) [8]
Mirtazapine (Remeron®) [6]
Mexiletine [6]
Nicardipine (Cardene®) [6]
Nortriptyline (Pamelor®) [8]
Olanzapine (Zyprexa®) (minor substrate) [6]
Ondansetron (Zofran®) [8]
Oxycodone (Oxycontin®, Percocet®) (minor substrate) [2,6]
Paliperidone (Invega®) (minor substrate)[6]
Paroxetine (Paxil®) [8]
Promethazine (Phenergan®) [8]
Propranolol (Inderal®) [6]
Tamoxifen [2,8]
Tamsulosin (Flomax®) [6]
Tiotropium (Spiriva®) (minor substrate) [6]
Trimipramine (Surmontil®) [8]
Umeclidinium (Anoro Ellipta®) [6]

BIBLIOGRAPHY

1 - FDA drug development and drug interactions - CLICK HERE
2 - Flockhart DA. Drug Interactions: Cytochrome P450 Drug Interaction Table. Indiana University School of Medicine (2007). http://medicine.iupui.edu/clinpharm/ddis/table.aspx. Accessed [2011].
3 - SuperCYP website
4 - PMID 20570945
5 - PMID 15081432
6 - Manufacturer's package insert (for drug listed)
7 - PMID 11936702
8 - PMID 19601803
9 - PMID 15520506
10 - PMID 9616188
11 - PMID 17471183
12 - PMID 10848718
13 - Codeine sulfate PI

CYTOCHROME P450 2D6