Tamoxifen does not prevent recurrence for 1 in 5 people. Are you one of them?

Tamoxifen is used to prevent the return of ER+ breast cancer (recurrence) by blocking the effects of estrogen in your body. However, tamoxifen doesn’t work for all people and can cause serious side effects. Find out how your body uses tamoxifen to make sure you are receiving the best possible cancer treatment.

How Kailos can help you

With the Kailos test, you will discover how your body responds to tamoxifen. If you are just beginning treatment, this can help you and your doctor know if tamoxifen is right for you from the start. If you are currently taking tamoxifen, genetic testing can tell you whether your body is able to use this medicine and how you might respond to certain side effects. This lets you find the best treatment plan based on your genes.

Order the Kailos test today and make your treatment more effective!

Tamoxifen, $299 $99

Why would I respond differently?

Approximately 20 percent of women do not respond to tamoxifen properly. This is because some women have a variation of a specific gene in their body called “CYP2D6” that makes them resistant to this medicine. People with this variation can’t use tamoxifen well or at all – putting them at risk of recurring ER+ breast cancer. That’s why it’s important for you to learn what genes you have and how your body uses tamoxifen.

Order the Kailos test today to take control of your cancer treatment!

Tamoxifen, $299 $99


What is Tamoxifen?

It is a drug most commonly used to treat women with estrogen receptor (ER)-positive early-stage breast cancer in an attempt to reduce the risk of recurrence. Studies show that it reduces recurrence and mortality by greater than 30%. Tamoxifen is metabolized into it’s active form called endoxifen by a gene called CYP2D6. There have been investigations that have identified genetic changes, or variants, of CYP2D6. These variants can affect how well CYP2D6 works.

What Kind of Metabolizer Are You?

The ability of CYP2D6 to change tamoxifen to endoxifen, turning it into the active medicine to prevent cancer recurrence, depends on the specific changes in the CYP2D6 gene you possess. Everyone has 2 copies of CYP2D6 in their DNA. At Kailos, we sequence your copies to determine what changes you have.

If you have 2 non-functioning copies of CYP2D6, you are a poor metabolizer of tamoxifen. Poor metabolizers have an increased risk for relapse of breast cancer. At 7-10%, caucasians are found to be poor metabolizers of tamoxifen more often than other ethnic populations. In addition to considering aromatase inhibitors, physicians treating patients that are poor metabolizers may consider the selection of an alternative drug, therapeutic drug monitoring, increased alertness to adverse drug events and to reduced efficacy.

Intermediate metabolizers (someone who is between poor and normal) also have an increased risk for relapse of breast cancer. In this case, tamoxifen is still changed to endoxifen but much less efficiently. So, an alternative treatment with aromatase inhibitors is often recommended or changes to the tamoxifen dose can be helpful. Importantly, intermediate metabolizers should avoid the use of CYP2D6 inhibitors during tamoxifen treatment. You don’t want an already impaired CYP2D6 gene to be further reduced by an inhibitor. Strong inhibitors of CYP2D6 which should be avoided include: Prozac, Paxil, Cardioquin and Wellbutrin. More moderate inhibitors include: Cymbalta, Benadryl, Mellaril, Cordarone, Tagamet and Zoloft. For a complete list and a specific Tamoxifen guide to CYP2D6 inhibitors visit Indiana University School of Medicine. At 30% Asian populations are found to be intermediate metabolizers, much more than other ethnic groups.


PharmGKB summary: tamoxifen pathway, pharmacokinetics. Pharmacogenetics and Genomics. 2013;23:643-647.

Association Between CYP2D6 Genotypes and the Clinical Outcomes of Adjuvant Tamoxifen for Breast Cancer. Pharmacogenomics. 2014;15(1):49-60.

Prediction of tamoxifen outcome by genetic variation of CYP2D6 in post-menopausal women with early breast cancer. Br J Clin Pharmacol. 2013;77(4):695-703.

Loss of Heterozygosity at the CYP2D6 Locus in Breast Cancer: Implications for Tamoxifen Pharmacogentic Studies. JNCI J Natl Cancer Inst. 2015;107(2):dju437.

Did you know?

You may have had a lot of tests at the start of your cancer treatment. While tamoxifen is typically prescribed after treatment or surgery, you shouldn’t wait to be tested to find out if you respond. If tamoxifen doesn’t work for your body, breast cancer is likely to recur in 5-7 years. Getting your Kailos test can help you feel more confident about treating your ER+ breast cancer with tamoxifen.

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