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CYP2D6 gene and medication reactions

Key Takeaways:
~ The CYP450 family of genes encodes enzymes that break down (metabolize) foreign substances, like medications and toxicants.
~ CYP2D6 metabolizes about 25% of prescription drugs.
~ Genetic variants in CYP2D6 affect how medications work for an individual.

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CYP2D6: Breaking down medications

Say you aren’t feeling well, have had a cold for a week, and can’t sleep… you’re just plain miserable. In your sleep-deprived state, you decide to take some Nyquil (or another cough syrup containing dextromethorphan). Some people may get relief and finally get some sleep. Others may wake up the next morning feeling like they were hit by a truck.

This example is just one of many medications metabolized by the CYP2D6 enzyme. Many genetic variants impact the function of CYP2D6, causing a wide variety of reactions to some commonly used medications.

Medications: The CYP2D6 enzyme metabolizes (breaks down) about 25% of pharmaceutical drugs, including SSRIs, opioids, tamoxifen, dextromethorphan, and beta-blockers.

Which drugs are metabolized by CYP2D6?

Here is a partial list of drugs metabolized by CYP2D6:

  • Dextromethorphan (cough syrup)
  • Hydrocodone
  • Methadone
  • Tamoxifen (breast cancer, estrogen blocker)
  • Pimozide (Tourette’s medication)
  • Metoprolol (beta-blocker)
  • Propranolol (beta-blocker)
  • Risperidone (schizophrenia, bipolar medication)

You can find a full list with details at PharmGKB.

Environmental toxicants metabolized by CYP2D6:

Many chemicals and toxicants that we are regularly exposed to are broken down by CYP450 enzymes. For many substances, more than one CYP enzyme can be involved in the metabolism.

Some of the toxicants that are metabolized (partially or fully) by or impact CYP2D6 include:

  • Organophosphate and organochlorine pesticides [ref]
  • Fipronil (pesticide)[ref]
  • Carbendazim (fungicide)[ref]

Natural substances metabolized by CYP2D6:

The CYP2D6 enzyme also metabolizes substances that your body produces naturally, including[ref][ref]

  • Serotonin (neurotransmitter)
  • Tyrosine into dopamine[ref] (neurotransmitter)
  • Pregnenolone (hormone)
  • Cholesterol
  • Androsterone (steroid hormone)
  • Eicosanoids (inflammatory fatty acids)

Speeding up and slowing down enzyme function:

Several important variants in the CYP2D6 gene can cause the enzyme to function differently — either by speeding up or slowing down the rate by which the medications break down.

  • A fast CYP2D6 enzyme function is usually called an ‘extensive metabolizer
  • Slow (or no) enzyme function is referred to as a ‘poor metabolizer‘.

A fast or ultrarapid metabolizer will clear the drug from their system more rapidly. It can mean that the drug has less of an effect than expected.

Pro-drugs: For a few medications, it is the metabolite (or what the drug is converted into) that gives the effect. For these types of pro-drugs, an extensive metabolizer can have a more intense effect from the drug.

Slower enzyme function, for many drugs, can mean that the medication sticks around in the body longer than normal. It may affect how much you need to take or how often to take the drug.

A variant that slows down the CYP2D6 isn’t always bad. Being a poor metabolizer may reduce the risk of cancers such as bladder or lung cancer.

On the other hand, it also may significantly increase the risk of Parkinson’s disease for those exposed to higher levels of pesticides.[ref]

Knowing whether you’re a fast or slow metabolizer may make it easier to find the proper dosage of certain medications. But you also need to understand how the drug works in the body:

  • Some drugs, such as tamoxifen, need to be metabolized to their active form by CYP2D6 to work.
  • Other drugs are turned into their inactive form by CYP2D6.

Remember that many drugs, toxins, and endogenous substances can be metabolized using multiple CYP enzymes. Thus, many drugs metabolized by CYP2D6 may also be broken down with other enzymes.

What does CYP2D6 do in the body?

We don’t have specific drug-metabolizing genes that have been waiting around for medications to be invented.

Instead, the CYP enzymes are produced in the liver (mainly) to break down molecules produced by the body.

In the body, CYP2D6 is involved in[ref][ref]

  • The metabolism of tyrosine to dopamine.
  • The turnover rate of serotonin.
  • The metabolism of anandamide, which binds to the cannabinoid receptor.

Anxiety traits and CYP2D6 variants:

With the impact on neurotransmitters, researchers subsequently discovered that CYP2D6 genetic variants are linked to personality traits. For example, several studies show that CYP2D6 poor metabolizers are more likely to score lower in socialization and higher in anxiety symptoms.[ref]

While the information on possible links to personality traits is interesting, the research also points out that medications that utilize CYP2D6 may also (somewhat) affect your mood.

CYP2D6 Genotype Report:

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The main takeaway is that if you carry a non-functioning variant, you need to be aware that drugs metabolized through CYP2D6 may not work as well for you.

  • It could mean varying the dosage or timing.
  • Or you may need to discuss alternative medications with your doctor.

Talk with your doctor or pharmacist if you have questions. I don’t want to steer anyone wrong here, but people need to know that their genetic variants impact drug metabolism rates.

There is clinical genetic testing available through your doctor for checking gene-drug interactions. Talk with your doctor about pharmacogenomic (PGx) testing for a more thorough, clinically useful report.

Other genes to check:

The rest of this article is for Genetic Lifehacks members only. Consider joining today to see the rest of this article.

Natural CYP2D6 Inhibitors:

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Originally published 6/2015. Revised on 4/26/19.

About the Author:
Debbie Moon is the founder of Genetic Lifehacks. Fascinated by the connections between genes, diet, and health, her goal is to help you understand how to apply genetics to your diet and lifestyle decisions. Debbie has a BS in engineering from Colorado School of Mines and an MSc in biological sciences from Clemson University. Debbie combines an engineering mindset with a biological systems approach to help you understand how genetic differences impact your optimal health.