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Cytochrome P450 2C9 (CYP2C9) and Medicines

What is CYP2C9?

When you take a medicine, your body needs a way to handle it. One way your body does this is by using enzymes to break down (metabolize) the medicine. A family of enzymes called cytochrome P450 breaks down certain medicines. The enzymes make the medicine more or less active, depending on the specific medicine.

Cytochrome P450 2C9, known as CYP2C9, enzymes break down several commonly used medicines including celecoxib, meloxicam (used for pain and inflammation), phenytoin (used for seizures), and warfarin (used to prevent blood clots).

Pharmacogenomic testing

Each person differs from another at the DNA level. Genes are segments of DNA that act as a set of instructions and tell the body how to work. The CYP2C9 gene is a section of DNA that instructs how well CYP2C9 enzymes will work.

The study of how genes like CYP2C9 affect the way your body interacts with medicines is called pharmacogenomics. Differences in your DNA that make up the CYP2C9 gene can affect how well you are able to break down certain medicines.

If you break down a medicine too fast or too slow, the medicine may not work as well, or you may have more side effects.

A pharmacogenomic test looks for differences that can help your medical team know how well your CYP2C9 enzyme will work. The test results can help your doctor and pharmacist choose the correct type or dose of medicine to give you.

CYP2C9 gene groups

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

  • Normal metabolizer – People in this group have normal working CYP2C9 enzymes. About 72 in 100 of people are in this group.
  • Intermediate metabolizer – People in this group have lower than normal enzyme function. They break down some medicines more slowly. About 27 in 100 people are in this group.
  • Poor metabolizer – 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 a different dose or type of medicine. About 1 in 100 people are this group.

Scientists continue to find new information about which medicines are affected by gene test results. Talk to your doctor or pharmacist if you have questions about your medicines, side effects, or pharmacogenomic testing.

Find more information about genes that are being used to make medication therapy decisions for patients at St. Jude.

If you have questions about pharmacogenomic testing done at St. Jude, you can email the Clinical Pharmacogenomics Program at

Key Points

  • CYP2C9 is part of the cytochrome P450 family of enzymes that helps the body break down certain medicines.
  • A pharmacogenomic test can help your medical team know well your CYP2C9 enzyme will work.
  • Some people have little or no active CYP2C9 enzyme and break down some medicines slowly.
  • Pharmacogenomic testing can help doctors and pharmacists know if you need a different dose or type of medicine.

Reviewed: August 2022