It’s Day 16 of National Breast Cancer Awareness Month! Today’s topic will cover one of the oldest targeted breast cancer drugs developed that is still used in the clinic today—tamoxifen. I’ve been taking it for 3 1/2 years, and millions of other breast cancer survivors with ER+ breast cancer have taken this drug as part of their treatments to prevent recurrence. But how was it discovered? How does it work?

Like many scientific discoveries, the discovery of tamoxifen was an accident. ICI46,474, later named tamoxifen, was first synthesized in 1966 by scientists working for a company that would become AstraZeneca. The goal of the project was to find a new chemical compound that could be made into a birth control drug. Laboratory studies were promising, but they found it didn’t work as a form of contraception in humans. This could have been the end of the story for tamoxifen, but one of the members of the team thought it might work as a breast cancer drug. In 1971, tamoxifen was tested in a clinical trial conducted in the UK on “late or recurrent carcinoma of the breast.” Thankfully, it worked!

How does it work? Since it started out as a candidate contraceptive, it makes sense that it was designed to block estrogen, a female hormone that helps prepare the uterus and uterine lining for pregnancy. It is in a class of drugs known as Selective Estrogen Receptor Modulators (SERMs), which are compounds that compete with the hormone estrogen for binding to its receptor. Normally, when estrogen binds to its receptor in the body, it triggers processes in the cell that make it divide, or produce more cells. This is called proliferation. In cancers with too many estrogen receptors (ER), estrogen in the body makes these cells grow uncontrollably. By binding to estrogen receptors in breast cancer cells, tamoxifen blocks this action and stops breast cancer cells from growing.

Around 70-80% of breast cancers are ER+, meaning that abnormal estrogen receptor activation is a key driver for growth of the breast cancer cells. Tamoxifen was a game changer for women with ER+ disease, reducing the annual breast cancer death rate by 31%. There are other drugs on the market that also block the activity of estrogen or downstream molecules in the estrogen receptor pathway, but tamoxifen remains standard of care for many cases of ER+ breast cancer.

As with any medication, tamoxifen comes with side effects that include: hot flashes, vaginal discharge, nausea, mood swings, fatigue, depression, hair thinning, constipation, loss of libido, dry skin. I experienced hot flashes, vaginal dryness and libido issues, and hair thinning, but they were not as severe as those I experienced with other estrogen blockers (aromatase inhibitors). For me, tamoxifen is a better balance between protection from recurrence and quality of life, but everyone’s physiology and experiences are different.

Be sure to talk to your healthcare providers about any side effects you experience. You don’t have to suffer in silence, and there are options to reduce side effects and improve your quality of life.