What Testing for CYP2D6 and CYP2C19 Tells You
Looking for variations in the CYP2D6 and CYP2C19 genes can find people who metabolize – or breakdown – certain drugs faster or slower than usual. Sometimes this information can be used by your doctor to predict if you are more likely to have side effects or less likely to get relief of your symptoms from certain drugs. The goal of this kind of testing is to help your doctor choose the best drug and dose for you with less trial and error.
If You Are a Poor Metabolizer
If You Are an Ultrarapid Metabolizer
How Might Your Doctor Use This Information?
There are two different enzymes that metabolize many antidepressants, antipsychotics, and at least one drug used to treat ADHD. These enzymes are called CYP2D6 and CYP2C19. Variations in the genes that make these enzymes can cause people to metabolize certain drugs less quickly or more quickly than usual. If you metabolize less quickly, you are called a “Poor Metabolizer”. If you metabolize more quickly, you are called an “Ultrarapid Metabolizer”.
If You Are a Poor Metabolizer
People can be a poor metabolizer for the CYP2D6 or the CYP2C19 enzyme. It is rare that people are poor metabolizers for both enzymes.
People who are poor metabolizers:
- May consider avoiding drugs metabolized by the enzyme that isn’t working normally – if there are other good options available.
- May be able to take a drug metabolized by the deficient enzyme, but may do better with a dose that is lower than average.
- May be at increased risk for specific drug interactions.
- May actually need a higher dose of a drug, in the rare case where a drug has to be broken down before it works.
- May never have any side effects or other problems from antidepressants, because several other factors affect how drugs work in your system.
If You Are an Ultrarapid Metabolizer
In some groups of people, it is common to be an ultrarapid metabolizer for CYP2D6. It is rare to be a CYP2C19 ultrarapid metabolizers, so most laboratories do not test for gene variants that would cause this.
People who are ultrarapid metabolizers for CYP2D6:
- May consider avoiding drugs metabolized by CYP2D6 – if there are other good options available.
- Are often able to take a drug metabolized by CYP2D6, but may do better with a dose that is higher than average.
- May need a lower dose of a drug, in the rare case where the metabolized (broken down) form of the drug is what actually does the work.
- May never have any side effects or other problems from antidepressants because several other factors affect how drugs work in your system.
How Might Your Doctor Use This Information?
We know that these enzymes play an important part in breaking down certain drugs in the body. But, for most drugs, we don’t know that simply finding out a person’s metabolizer status will improve how doctors prescribe drugs.
How you respond to a drug is complex – it involves more than just metabolism. Other factors affect how a drug works – like additional drugs, age, gender, medical conditions, and variations in other genes that interact with a drug. We also usually have more than one way to process a drug, so different enzymes may be able to step in when there is a problem with another enzyme.
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Many drugs are prescribed in a way that maximizes safety and minimizes the risk of side effects already. For instance, your doctor will often start you at a lower dose than you probably need to control your symptoms. Then, the dose may be increased as you show that you can tolerate it and you need more to control symptoms. Knowing a person’s metabolizer status is probably most helpful for prescribing drugs that only have a small range of a safe dose for a specific person – and the risks of not treating fully and fast enough are high. Doctors may also find that simply monitoring drug levels or using other ways to measure a patient’s response helps more than knowing a metabolizer status.
Ideally, studies compare how well people do with a treatment when the metabolizer status is known or unknown. Unfortunately, most drugs haven’t been studied this way. As a result, doctors have to use their clinical judgment to decide who may get better drug therapy by knowing their CYP2D6 or CYP2C19 metabolizer status.
