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Cytochrome P450 2C9 (CYP2C9) and medicines


When you take a medicine (drug), your body has to have a way to handle the medicine. One way is for enzymes to metabolize (break down) the medicine. A family of enzymes called cytochrome P450s have the ability to break down certain medicines. By metabolizing a medicine, cytochrome P450 enzymes make the medicine either more or less active, depending upon the medicine. Cytochrome P450 2C9 (CYP2C9) is part of the cytochrome P450 family of proteins in the body. It is responsible for breaking down several medicines that are commonly used. 

Pharmacogenetic testing

DNA is like a set of instructions for your body that can help decide how well your enzymes will work. Each person differs from another at the DNA (gene) level. This means that each person has small differences in the genes that code for enzymes. The part of DNA that instructs how well the CYP2C9 enzyme will work is called the CYP2C9 gene. The study of how genes like CYP2C9 affect the way you break down medicines is called pharmacogenetics (FAR mah coh je NEH tiks).

Differences in your DNA that make up the CYP2C9 gene can change how well you are able to break down certain medicines. If you break down a medicine too fast or too slowly, this may cause a bad reaction to the medicine. By testing your DNA (with a pharmacogenetic test), we may find DNA differences that can allow us to predict how well your CYP2C9 enzyme will work. The result of this test will guide your doctor to choose the correct dose of medicine to give you.

The results of your CYP2C9 test will place you into one of three groups:

  • Poor metabolizers – People in this group have little or no active CYP2C9 enzyme. People who are poor metabolizers break down some medicines slowly and are likely to need altered doses or even a different medicine in some cases. About 2 percent of people are poor metabolizers of CYP2C9.
  • Intermediate metabolizers – People in this group break down some medicines at a rate in between the poor and normal metabolizers. About 25 percent of people are intermediate metabolizers of CYP2C9.
  • Normal metabolizer – People in this group have normal working CYP2C9 enzymes. About 73 percent of people are normal metabolizers of CYP2C9.

Medicines that may be affected

CYP2C9 enzymes break down several commonly used medicines including celecoxib, phenytoin (used for seizures), and warfarin (used to prevent blood clots). The result of your CYP2C9 genotype test will place you into one of the three groups above. Knowing what group you are in may help your doctor to pick the right medicine and right dose for you. 

For example, for celecoxib:

  • Poor metabolizers are more likely to be at risk of gastrointestinal bleeding from celecoxib.
  • Normal metabolizers and intermediate metabolizers don’t normally need dosage changes of celecoxib based on CYP2C9 genotype test result.

Scientists continue to find new information about which medicines are affected by gene test results. For details about which medicines are broken down by CYP2C9, please go to or


If you have questions or concerns about pharmacogenetic testing at St. Jude, call one of the Pharmaceutical Sciences research nurses at 901-595-2482 or email If you are calling from outside the Memphis area, dial toll-free 1-866-2ST-JUDE (1-866-278-5833), then dial extension 2482.


This document is not intended to take the place of the care and attention of your personal physician or other professional medical services. Our aim is to promote active participation in your care and treatment by providing information and education. Questions about individual health concerns or specific treatment options should be discussed with your physician.

St. Jude complies with health care-related federal civil rights laws and does not discriminate on the basis of race, color, national origin, age, disability, or sex.

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