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UGT1A1 and Medicines

What is UGT1A1?

When you take a medicine, your body needs a way to handle it. One way is for enzymes to metabolize (break down) the medicine. Uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1) is an enzyme that breaks down some medicines. UGT1A1 also removes a substance called bilirubin from your body. Bilirubin is made in the body during the normal breakdown of red blood cells.

UGT1A1 function

People who take certain medicines or have certain health conditions may have decreased UGT1A1 function.

  • Atazanavir - Atazanavir is a drug used to treat human immunodeficiency virus (HIV) infection. Atazanavir decreases the function of UGT1A1. As a result, certain people who take atazanavir may have yellowing of the skin and the whites of the eyes, or jaundice. This effect usually is not harmful, and the jaundice goes away if the person stops taking atazanavir.

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 UGT1A1 gene is a section of DNA that instructs how well UGT1A1 enzymes will work.

The study of how genes like UGT1A1 affect the way your body interacts with medicines is called pharmacogenomics. Differences in your DNA that make up the UGT1A1 gene can affect how well you are able to break down some 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 UGT1A1 enzyme will work. The test results can help your doctor and pharmacist choose the correct type or dose of medicine to give you.

UGT1A1 gene groups

The results of your UGT1A1 test will place you into one of three groups:

Normal UGT1A1 function – People in this group have normal working UGT1A1 enzymes. Atazanavir may be prescribed based on this UGT1A1 test result. About 43 in 100 people are in this group.

Intermediate UGT1A1 function – People in this group have slightly lower than normal UGT1A1 enzyme function. Atazanavir may still be prescribed based on this UGT1A1 test result. About 44 in 100 people are in this group.

Poor UGT1A1 function – People in this group have much lower UGT1A1 function. This result is associated with Gilbert syndrome. If people in this group take atazanavir, they are at high risk for jaundice. It is usually best to avoid atazanavir in people with poor UGT1A1 function. About 13 in 100 people are in this group.

Find more information on Gilbert syndrome. 

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 about UGT1A1

  • UGT1A1 is an enzyme that breaks down some medicines and also removes bilirubin from the body.
  • A pharmacogenomic test can help your medical team know how well your UGT1A1 enzyme will work.
  • Some people have poor UGT1A1 enzyme function and should avoid taking atazanavir.
  • Pharmacogenomic testing can help doctors and pharmacists know if you need a different medicine or dose.

Reviewed: October 2022