CYP2D6 (Cytochrome P450 2D6) is a crucial liver enzyme responsible for metabolising approximately 25% of all clinically used drugs, including antidepressants, opioids, beta-blockers, and antipsychotics. Variations in CYP2D6 activity—due to genetic differences or CYP2D6 inhibitors—can significantly impact how medications are processed in the body.

Real-World Example:

Consider a patient prescribed codeine, a common pain reliever. Codeine is a prodrug that requires conversion to morphine by CYP2D6 to exert its analgesic (pain killer) effects. If this patient is concurrently taking fluoxetine (Prozac), a known CYP2D6 inhibitor, the conversion is hindered, leading to inadequate pain relief. Conversely, in individuals with multiple copies of the CYP2D6 gene (ultrarapid metabolizers), codeine is rapidly converted to morphine, increasing the risk of toxicity, including life-threatening respiratory depression.

Understanding CYP2D6 Inhibitors

CYP2D6 inhibitors are substances that decrease the metabolic activity of the CYP2D6 enzyme, leading to altered plasma concentrations of drugs metabolized by this pathway. As shared by PubMed, according to the FDA, these inhibitors are categorized based on their potency:

• Strong Inhibitors: Strong CYP2D6 inhibitors cause a greater than or equal to 5-fold increase in plasma AUC (area under the curve) or more than 80% decrease in clearance.
• Moderate Inhibitors: Causes a greater than or equal to 2-fold but lower than 5-fold increase in AUC or 50-80% decrease in clearance.
• Weak Inhibitors: Cause a greater than or equal to 1.25-fold but less than 2-fold increase in AUC or 20-50% decrease in clearance.

It’s important to note that these classifications are based on pharmacokinetic parameters rather than specific dosages (mg). The inhibitory effect is influenced by the drug’s affinity for CYP2D6 and its concentration at the enzyme site, which may not directly correlate with the administered dose.

Common CYP2D6 Inhibitors in the USA

CYP2D6 inhibitors are prescribed for various therapeutic indications. Below is a list of commonly used inhibitors along with their typical dosages and primary uses:

Generic Name	Brand Name	Typical Dosage	Primary Use
Fluoxetine (SSRI)	Prozac	20–80 mg/day	Depression, OCD, Bulimia
Paroxetine (SSRI)	Paxil	10–60 mg/day	Depression, Anxiety Disorders
Bupropion (NDRI)	Wellbutrin	150–300 mg/day	Depression, Smoking Cessation
Quinidine (Ia antiarrhythmics)	Quinidex	200–400 mg every 6 hours	Arrhythmias
Ritonavir (protease inhibitor)	Norvir	100–600 mg/day	HIV Infection
Cinacalcet (type II calcimimetics)	Sensipar	30–180 mg/day	Hyperparathyroidism

Note: Dosages may vary based on individual patient factors and clinical judgment.

Brand Names, Potencies, and Contraindications

Following is a famous CYP2D6 inhibitors list, covering potencies and their contraindications:

Brand Name	Generic Name	Potency	Typical Dosage	Contraindications
Prozac	Fluoxetine	Strong	20–80 mg/day	Concurrent use with MAOIs, Thioridazine
Paxil	Paroxetine	Strong	10–60 mg/day	Concurrent use with MAOIs, Pimozide
Wellbutrin	Bupropion	Moderate	150–300 mg/day	Seizure disorders, Eating disorders
Quinidex	Quinidine	Strong	200–400 mg every 6 hours	AV block without pacemaker, Myasthenia gravis
Norvir	Ritonavir	Strong	100–600 mg/day	Concurrent use with certain sedatives, Ergot derivatives
Sensipar	Cinacalcet	Moderate	30–180 mg/day	Do not consume if less than normal (Hypocalcemia)

Potency classifications are based on their inhibitory effect on CYP2D6 activity, not on dosage amounts.

Therapeutic Indications for CYP2D6 Inhibitors

CYP2D6 inhibitors are utilised in the management of various medical conditions:

• Depression and Anxiety Disorders: SSRIs like fluoxetine and paroxetine are first-line treatments for major depressive disorder, generalised anxiety disorder, and obsessive-compulsive disorder.
• Smoking Cessation: Bupropion is approved for aiding smoking cessation efforts. Here is how to use Bupropion SR.
• Cardiac Arrhythmias: Quinidine is used to treat certain types of irregular heartbeats.
• HIV Infection: Ritonavir is a protease inhibitor used in combination antiretroviral therapy.
• Hyperparathyroidism: Cinacalcet is prescribed for secondary hyperparathyroidism in patients with chronic kidney disease.

It’s crucial to consider the CYP2D6 inhibitory effects of these drugs, especially when patients are on multiple medications metabolized by this enzyme, to avoid potential drug interactions and adverse effects.

Long-Term Side Effects of CYP2D6 Inhibitors

While CYP2D6 inhibitors are effective for managing specific health conditions, their long-term use—especially in patients with polypharmacy—can lead to various complications. These effects are often due to altered metabolism of co-administered drugs or from cumulative toxicity.

Common Long-Term Issues:

• Increased Plasma Levels of Other Drugs: For example, paroxetine can significantly raise blood levels of drugs like metoprolol, increasing the risk of bradycardia (slow heart rate).
• Reduced Efficacy of Prodrugs: Codeine and tamoxifen rely on CYP2D6 for activation. Chronic inhibition can render these drugs less effective, compromising pain relief or cancer treatment outcomes.
• Neuropsychiatric Effects: Long-term use of SSRIs such as fluoxetine has been associated with emotional blunting, sexual dysfunction, and in some cases, persistent depressive symptoms.
• Cardiac Risks: Quinidine’s CYP2D6 inhibition combined with its own cardiotoxic potential increases the risk of arrhythmias, particularly in older adults.

Studies Suggest:

Several studies highlight that chronic CYP2D6 inhibition may increase the risk of adverse drug reactions in elderly patients taking multiple medications.

Monitoring liver function, cardiac status, and mental health in patients on long-term CYP2D6 inhibitors is essential. Dose adjustments, drug alternatives, and routine evaluations can minimize risks.

How Do CYP2D6 Inhibitors Work?

To understand how CYP2D6 inhibitors function, it’s helpful to grasp how the enzyme itself works. CYP2D6 metabolizes drugs by adding oxygen atoms to make them more water-soluble for excretion. Inhibitors interfere with this metabolic process, leading to higher concentrations of the substrate drug.

Mechanism of Inhibition:

• Competitive Inhibition: The inhibitor binds to the active site of the CYP2D6 enzyme, blocking the substrate from accessing it, e.g., fluoxetine CYP2D6 interaction.
• Mechanism-Based (Irreversible) Inhibition: The inhibitors, such as paroxetine CYP2D6 inhibitor, form a reactive intermediate that binds covalently to the enzyme, deactivating it permanently.
• Non-Competitive Inhibition: The inhibitor binds elsewhere on the enzyme, changing its shape and reducing its function.

Pharmacokinetics Impact:

CYP2D6 inhibition can lead to delayed drug clearance, accumulation, and prolonged half-life of the affected medications. This can result in enhanced effects (sometimes dangerously so) or decreased therapeutic efficacy in the case of prodrugs.

Clinical Example:

• Codeine: Normally converted to morphine by CYP2D6. If fluoxetine is co-administered, morphine production is reduced, leading to inadequate pain relief.
• Tamoxifen: Requires CYP2D6 for conversion to its active form (endoxifen). Patients taking paroxetine may have diminished cancer-protective benefits.

Understanding these pathways is crucial in avoiding negative drug interactions and optimizing therapeutic results.

CYP2D6 Polymorphisms and Individual Response

One of the most fascinating aspects of CYP2D6 is its genetic variability. The CYP2D6 gene has over 100 known alleles, and individuals are categorized into different metabolizer types based on their genotype:

Metabolizer Type	Description	Clinical Impact
Ultrarapid Metabolizer (UM)	>2 copies of active gene	May metabolize drugs too quickly; reduced efficacy
Normal (Extensive) Metabolizer (EM)	2 functional alleles	Expected drug response
Intermediate Metabolizer (IM)	1 functional, 1 defective allele	Reduced enzyme activity
Poor Metabolizer (PM)	2 non-functional alleles	High drug levels; increased risk of side effects

Why This Matters:

A CYP2D6 poor metabolizer on a standard dose of a CYP2D6-metabolized drug could experience toxicity, while an ultrarapid metabolizer might not respond at all due to rapid clearance.

Drug Implications:

• Codeine: Ineffective in PMs (no morphine conversion), potentially dangerous in UMs.
• SSRIs: Increased risk of side effects in PMs due to slower metabolism.
• Antipsychotics (e.g., risperidone): Elevated plasma levels in PMs can lead to extrapyramidal symptoms.

Some FDA-labelled gene-drug interaction medicines (including the ones for the CYP2D6 gene).

Pharmacogenomics (PGx) Testing

Pharmacogenomics (PGx) testing uses DNA analysis to determine how a person will respond to specific drugs based on their genetic makeup. For CYP2D6, PGx testing can identify the individual’s metabolizer status—UM, EM, IM, or PM. An at-home PGx test from RPh LABS (CLIA-accredited lab) requires just your saliva and shows how you may respond to 250+ medications.

PGx Test Clinical Relevance:

• Tailored Dosing: Adjust dosages based on metabolizer status.
• Drug Selection: Avoid certain drugs in poor or ultrarapid metabolizers.
• Minimize ADRs: Helps prevent adverse reactions, especially in polypharmacy.

PGx Test Insurance and Cost:

In certain cases, PGx tests are covered by insurance. However, to create ease, RPh LABS offers other payment options as well, such as paying in 3 equal installments with 0 interest, or FSA/HSA payment eligibility. The testing costs generally vary between $200 – $500.

Benefits of PGx Testing for Tailored Therapy

Using PGx testing to guide therapy decisions is rapidly becoming a cornerstone of precision medicine. Benefits include:

• Improved Efficacy: Matching the right drug at the right dose to the patient’s genetic profile ensures better therapeutic outcomes.
• Reduced Adverse Effects: Identifying poor or ultrarapid metabolizers helps avoid drugs that may cause harm.
• Cost Savings: By reducing trial-and-error prescribing and minimizing hospitalizations due to adverse reactions.

Case Example:

A patient with breast cancer is prescribed tamoxifen. PGx testing reveals a poor metabolizer status for CYP2D6. Instead of tamoxifen, the clinician opts for an alternative that doesn’t rely on CYP2D6 for activation, enhancing the treatment’s efficacy.

Clinical Scenarios Involving CYP2D6 Inhibitors

Real-life clinical situations help illustrate how CYP2D6 inhibitors influence patient care. Let’s look at a few key scenarios:

a. Antidepressants

• Case: A 45-year-old woman is prescribed paroxetine for depression. She also takes tramadol for chronic back pain.
• Problem: Paroxetine, a strong CYP2D6 inhibitor, prevents tramadol from being converted into its active metabolite. Result? Inadequate pain control.
• Solution: Switch tramadol (opioid) to a non-CYP2D6-dependent analgesic like acetaminophen (non opioid) or adjust antidepressant therapy.

b. Antipsychotics

• Case: A patient with schizophrenia on risperidone (a CYP2D6 substrate) develops severe tremors and rigidity.
• Problem: The patient is also on bupropion, which inhibits CYP2D6. The risperidone levels become elevated, increasing extrapyramidal side effects.
• Solution: Reduce risperidone dose or switch to a different antidepressant with minimal CYP2D6 inhibition.

c. Beta-Blockers

• Case: A hypertensive patient on metoprolol also takes fluoxetine for depression.
• Problem: Fluoxetine slows down metoprolol metabolism, leading to bradycardia and fatigue.
• Solution: Consider switching to a beta-blocker not metabolized by CYP2D6, like atenolol.

These cases show how critical it is to recognise drug interactions involving CYP2D6 inhibitors and adjust therapy accordingly to avoid treatment failures or toxicity.

Alternatives to CYP2D6 Inhibitors

In cases where CYP2D6 inhibition could pose a risk, alternatives can be considered to mitigate interactions.

a. Antidepressants

• Avoid: Fluoxetine, Paroxetine
• Use: Sertraline, Citalopram (minimal CYP2D6 inhibition)

b. Pain Management

• Avoid: Codeine, Tramadol
• Use: Morphine, Hydromorphone (don’t require CYP2D6 metabolism)

c. Antipsychotics

• Avoid: Risperidone
• Use: Olanzapine, Quetiapine (less CYP2D6 interaction)

d. Beta-Blockers

• Avoid: Metoprolol, Carvedilol
• Use: Atenolol, Bisoprolol

Drug-Drug Interactions

Polypharmacy is common, especially in the elderly or chronically ill, increasing the risk of CYP2D6-mediated drug-drug interactions.

Common Drug-Drug Interaction Risks with CYP2D6 Inhibitors:

CYP2D6 Inhibitor	Interacting Drug	Result
Fluoxetine	Codeine	Reduced pain relief
Paroxetine	Tamoxifen	Reduced anti-cancer activity
Bupropion	Risperidone	Elevated antipsychotic side effects
Ritonavir	Beta-blockers	Bradycardia and hypotension
Quinidine	Opioids	Increased sedation, respiratory depression

Recommendation: Monitor closely or use alternative medications that bypass CYP2D6.

Patient Counseling and Monitoring

Patient education is key to preventing adverse effects from CYP2D6 inhibitors. Here’s how healthcare providers should guide patients:

What to Tell Patients:

• Inform them if their medication affects drug metabolism.
• Encourage them to disclose all medications, including over-the-counter drugs and supplements.
• Emphasize not to stop or start any medication without professional advice.

Monitoring Tips:

• Liver Function Tests (LFTs): To assess liver enzyme performance.
• ECG Monitoring: For drugs affecting heart rhythm.
• Therapeutic Drug Monitoring (TDM): For narrow therapeutic index drugs like antidepressants, antipsychotics.

Red Flags for Patients:

• Sudden fatigue, dizziness, changes in mood, or abnormal heartbeats.
• Unusual symptoms when starting a new drug or changing doses.

CYP2D6 Inhibitors Food List

*This list includes CYP2D6 inhibitor substances as well.

Food/Substance	Details	Source
Grapefruit Juice	Contains furanocoumarins that inhibit CYP enzymes, including CYP2D6.	FDA
Curcumin (Turmeric)	Found in turmeric; shown to inhibit CYP2D6 activity.	PMC
Goldenseal	Herbal supplement containing berberine; inhibits CYP2D6.	FDA
St. John’s Wort	Herbal supplement; known to affect CYP enzymes, including CYP2D6.	Cancer Research UK
Green Tea	Contains compounds that may inhibit CYP2D6 activity.	ScienceDirect
Black Tea	Similar to green tea; may affect CYP2D6 activity.	ScienceDirect
Garlic	Contains allicin; potential to inhibit CYP2D6 recombinants.	ScienceDirect
Cruciferous Vegetables	Includes broccoli, cabbage; may modulate CYP enzyme activity.	ScienceDirect
Chamomile	Herbal tea; potential to interact with CYP enzymes.	Wikipedia
Peppermint	Herbal tea; may influence CYP enzyme activity.	Wikipedia
Ginkgo Biloba	Herbal supplement; known to have weak effects on CYP enzyme inhibition.	Cancer Research UK
Ginseng	Herbal supplement; potential to interact with CYP enzymes.	Cancer Research UK

As shared by the PubMed, ‘’Coleus forskohlii extract- and collagen-based products also inhibit CYP2D6.’’

Recommendations for Patients

• Consult Healthcare Providers: Before consuming these foods or supplements, especially if you are on medications metabolized by CYP2D6, consult with your healthcare provider.
• Monitor for Side Effects: Be vigilant for any unusual side effects when consuming these foods alongside medications.
• Maintain Consistency: If you regularly consume any of these foods, maintain consistency in your diet to help your healthcare provider manage potential interactions.

Conclusion

CYP2D6 inhibitors play a vital role in modern pharmacotherapy, especially in psychiatry, cardiology, and pain management. However, these are metabolized by the CYP2D6 enzymes, the activity of which can vary in individuals. Thus, if administered without knowing a patient’s genetic insights, their current medications, and a few more factors, these inhibitors can lead to significant therapeutic challenges, including treatment failure or toxicity. Getting tested from a CLIA certified lab like RPh LABS can help doctors and patients avoid trial & error with the help of tailored treatments. Still unsure? Here is how these at-home PGx tests work.

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