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CYP2C9 gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

CYP2C9 gene

The CYP2C9 gene provides instructions for making an enzyme essential for metabolizing various compounds, including medications like warfarin and ibuprofen.

Normal Function

The CYP2C9 gene provides instructions for making an enzyme found in the endoplasmic reticulum, a cell structure involved in protein processing and transport. This enzyme is responsible for breaking down (metabolizing) compounds such as steroid hormones and fatty acids. It plays a major role in metabolizing the blood-thinning drug warfarin and anti-inflammatory medications like ibuprofen.

Health Conditions Related to Genetic Changes

Warfarin sensitivity

Certain common CYP2C9 gene variations (polymorphisms) are associated with warfarin sensitivity, a condition requiring lower-than-standard doses of the drug. Because the altered enzyme slows the metabolism of warfarin, the drug remains active in the body for a longer period.

  • European ancestry: The most common polymorphisms are CYP2C9*2 and CYP2C9*3, which typically necessitate a reduction in prescription doses by one-third and one-fifth, respectively.
  • African ancestry: The most common polymorphisms are CYP2C9*5, CYP2C9*6, CYP2C9*8, and CYP2C9*11, which may require a dose reduction of one-third to one-sixth.

In rare cases where individuals possess more than one polymorphism, enzyme activity is severely decreased. Taking an average dose of warfarin in these cases can lead to an overdose, causing abnormal bleeding in the brain, gastrointestinal tract, or other tissues, which may result in serious health complications or death.

Other Names for This Gene

  • CYPIIC9
  • cytochrome P-450MP
  • cytochrome P450 2C9
  • cytochrome P450 MP-4
  • cytochrome P450 MP-8
  • cytochrome P450 PB-1
  • cytochrome P450, family 2, subfamily C, polypeptide 9
  • cytochrome P450, family 2, subfamily C, polypeptide 9 gene
  • P450 MP-4
  • S-mephenytoin 4-hydroxylase
  • S-mephenytoin 4-hydroxylase, human
  • warfarin-7-hydroxylase, human

Frequently Asked Questions

If you have a CYP2C9 variant and are on warfarin, watch for signs of excessive bleeding, which indicate the drug is not being metabolized properly. These include unusual bruising, nosebleeds that do not stop, blood in urine or stool, vomiting blood, or severe headaches. If you experience sudden, severe headache, confusion, weakness on one side of the body, or difficulty speaking, these could be signs of a brain bleed. In these cases, call emergency services immediately. Do not wait for your next scheduled INR appointment if you notice these acute symptoms.
Having a CYP2C9 variant does not strictly prohibit the use of ibuprofen or other NSAIDs, but it may alter how quickly your body processes them. Since CYP2C9 is involved in metabolizing ibuprofen, a variant could lead to higher levels of the drug in your blood, potentially increasing the risk of side effects like stomach irritation or kidney issues. It is not an absolute contraindication, but you should inform your doctor about your genetic status. They may recommend a lower dose, a shorter duration of use, or an alternative pain reliever that is not primarily metabolized by CYP2C9, such as acetaminophen, depending on your overall health profile.
Genetic variants like CYP2C9*2 and *3 reduce the enzyme's ability to break down warfarin, causing the drug to stay in your system longer. This means you require a significantly lower dose to achieve the same therapeutic effect. However, you should never adjust your warfarin dose based on assumptions or without medical guidance. A healthcare provider must order a pharmacogenetic test to identify your specific variant, then use that data alongside your INR results to calculate a safe, personalized starting dose. Self-adjusting can lead to dangerous under-dosing (clot risk) or over-dosing (bleeding risk).
CYP2C9 variants are relatively common, but their prevalence varies significantly by ancestry. In individuals of European descent, the *2 and *3 variants are the most frequent, affecting a substantial portion of the population. In individuals of African descent, different variants such as *5, *6, *8, and *11 are more common. Because of these differences, pharmacogenetic testing is particularly important for accurate dosing across all ethnic groups. Knowing your ancestry can help your doctor anticipate which variants are more likely, but direct genetic testing is the only way to confirm your specific profile.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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