What is Tamoxifen?

Tamoxifen is a well-known drug that was originally discovered by what is now AstraZeneca. This drug was originally screened in a development program centered on the introduction of new contraceptive agents. Although it proved effective in rats, it was not a useful drug for the control of fertility in women; in fact, tamoxifen was found to instead induce ovulation.

Image Credit: LariBat / Shutterstock.com

Mechanism of action

While commonly referred to as an anti-estrogen drug, this is not entirely accurate. Tamoxifen is more appropriately described as a selective estrogen receptor modulator. This is because it expresses both estrogenic and antiestrogenic actions, depending on the target tissue.

On mammary epithelium, its actions are strongly antiestrogenic, which is why it’s used in both the prevention and treatment of breast cancer. In contrast, on the uterine epithelium, tamoxifen exerts proestrogenic actions, which has fuelled the current controversy regarding its safety in cancer prevention. This controversy has been supported by the increased incidence of endometrial carcinoma that has been identified in women treated chronically with tamoxifen.

How is tamoxifen administered?

Tamoxifen comes as a pill to be taken once or twice daily with water at a similar time each day. When taken, the tablets should be whole; they should not be split, chewed, or crushed.

Should a person forget to take a dose of tamoxifen, they should take the missed dose as soon as they remember it, and take the next dose as usual. However, if it is almost time for their next dose, patients should ignore the missed dose and continue following their regular dosing schedule. A double dose should never be taken to compensate for a missed one.


Tamoxifen therapy is used in a number of cases, including:

  • As adjuvant therapy (treatment following successful surgery) in those with lymph node-negative or breast cancers with positive estrogen and progesterone receptors in both females and males.
  • As treatment of both early and advanced estrogen receptor-positive breast cancer in pre- and post-menopausal women.
  • As treatment for women categorized as high risk for the development of breast cancer to reduce the incidence of developing breast cancer.
  • As treatment to reduce contralateral cancer, which s defined as cancer in the opposite breast.
  • Used occasionally to induce ovulation in women who wish to become pregnant but who do not produce eggs naturally.

Tamoxifen can also be given as preventative therapy in women who are at a high risk of developing breast cancer. This may be predisposed due to their age or medical history (both personal and family).

For example, women who have recently undergone treatment for intraductal carcinoma (DCIS), which is a less common type of breast cancer, through surgery, chemotherapy, and/or radiation are at high risk for developing invasive breast cancer in the future. Thus, hormone therapy with tamoxifen prevents the development of this cancer in a significant number of women within the first five years of treatment.

Hormone Therapy for Breast Cancer and the Side Effects of Tamoxifen

Side effects

The safety of tamoxifen therapy has not been established in children or in those breastfeeding due to the potential for carcinogenicity. However, the benefits of tamoxifen therapy generally outweigh the risks in those who are able to have the therapy.

With regards to these risks, the associated adverse reactions tend to be minor, well-tolerated, and often easily controlled by dose reduction. Some of the potential side effects of tamoxifen therapy can include:

  • Depression
  • Hot flashes
  • Weight loss
  • Menstrual irregularities
  • Vaginal discharge
  • Pain/reddening around the site of the tumor
  • Dizziness and severe headache
  • Constipation
  • Muscle weakness
  • Vision problems (e.g., risk of cataract development)
  • Initial adverse reactions, such as increased bone pain, may be a positive tumor response following the onset of tamoxifen therapy.
  • Blood cell changes:
    • Low neutrophil counts (neutropenia)
    • Very low platelet counts (thrombocytopenia)
    • Very low white blood cell counts (leukopenia)
  • Abnormalities in blood clotting due to prolonged tamoxifen therapy at usual doses. This includes deep vein thrombosis and pulmonary embolus. Patients should seek immediate help if they develop sudden shortness of breath, chest pain, and/or only one swollen, red, and painful calf. Fortunately, this is a rare side effect.


Further Reading

Last Updated: Aug 22, 2023

Afsaneh Khetrapal

Written by

Afsaneh Khetrapal

Afsaneh graduated from Warwick University with a First class honours degree in Biomedical science. During her time here her love for neuroscience and scientific journalism only grew and have now steered her into a career with the journal, Scientific Reports under Springer Nature. Of course, she isn’t always immersed in all things science and literary; her free time involves a lot of oil painting and beach-side walks too.


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