Tamoxifen (Efficacy)

Tamoxifen is a selective estrogen receptor modulator used in the treatment of breast cancer to reduce the risk of tumor recurrence. Despite being a very effective drug, it is estimated that 50% of patients do not obtain the expected benefit and this could be conditioned by the presence of polymorphisms in the genes involved in its metabolism.

Tamoxifen is a drug used to treat breast cancer. It is a non-steroidal anti-estrogen agent, which means it prevents estrogen from binding to estrogen receptors on cells. This helps prevent the growth of cancer cells in breast tissue.

It is also used to prevent the possible development of the disease in individuals who are at high risk for the disease. As an adjuvant it decreases 5-year mortality by 20% with few adverse reactions.


Although tamoxifen is an effective drug for the treatment of breast cancer, it may also have some contraindications. It can have serious side effects, such as increased risk of endometrial cancer, deep vein thrombosis, liver cancer, bone changes, increased risk of cataracts and eye problems, and problems with liver function.

In addition, tamoxifen is not safe for everyone. Pregnant women should not take it, and women who are breastfeeding should not use it. People with kidney or liver problems should also avoid tamoxifen.

Before prescribing tamoxifen, family history and risk factors for thrombosis should be assessed.

Precautions should be taken in patients at risk of endometrial cancer and uterine sarcoma.

In patients with breast cancer, before prescribing tamoxifen, it should be taken into account whether there is a risk of second primary tumors in the endometrium and breast.

The use of potent CYP2D6 inhibitors (paroxetine, fluoxetine, quinidine, cinacalcet or bupropion) should be avoided during treatment with tamoxifen.

Not recommended in children.


Hot flushes, ischemic stroke, venous thromboembolism, deep vein thrombosis, pulmonary embolism; vaginal bleeding, vaginal discharge, vulvar pruritus, endometrial changes (including hyperplasia and polyps), menstrual irregularities.lipos), menstrual irregularities; gastrointestinal intolerance, alopecia, rash, headache, lightheadedness, tumor-related symptoms, fluid retention, muscle cramps in lower extremities.


Tamoxifen plasma levels are decreased by rifampicin.

The anticoagulant effect of tamoxifen is increased by concomitant treatment with coumarin-type anticoagulants.

There is a risk of thromboembolic effects if tamoxifen is taken together with other cytotoxic agents.

In combination with an aromatase inhibitor (as adjuvant therapy) no greater efficacy has been shown than in comparison with tamoxifen alone.


  • Nolvadex ®

Genes analyzed



Goetz MP, Sangkuhl K, Guchelaar HJ, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2D6 and Tamoxifen Therapy. Clin Pharmacol Ther. 2018 May;103(5):770-777.

Saladores P, Mürdter T, Eccles D, et al. Tamoxifen metabolism predicts drug concentrations and outcome in premenopausal patients with early breast cancer. Pharmacogenomics J. 2015 Feb;15(1):84-94.

Schroth W, Goetz MP, Hamann U, et al. Association between CYP2D6 polymorphisms and outcomes among women with early stage breast cancer treated with tamoxifen. JAMA. 2009 Oct 7;302(13):1429-36.

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