2 Pink breast cancer bands

What is this drug?

Tamoxifen is a treatment for breast cancer in both men and women that works by blocking estrogen from stimulating breast tissue.  That stimulation leads in part to roughly 2/3 of breast cancers, and so blocking it is quite important, and Tamoxifen is the most popular breast cancer medication for that reason.

We have genetic tests these days and – if your cancer is positive for estrogen receptors (ER) – Tamoxifen is quite an effective treatment.    Additionally, it has important protective effects in women who are at high risk.  That said, for post-menopausal women, a different class of medication called the aromatase inhibitors may be a more effective treatment for certain types of breast cancer.

One study showed that over five years of treatment with Tamoxifen, your risk of developing ER+ cancer dropped almost 70%.  That said, it has almost no effect on cancers which don’t have the ER, so its final effect is to reduce your risk of breast cancer by 50%.

Quite good.  Unfortunately, Tamoxifen is technically a SERM – a selective estrogen receptor modulator.  While it blocks estrogen from stimulating your breast, it actually acts like estrogen in your bone and uterine areas.

That means that treatment for extended periods of time can highly increase your risk of uterine cancer.  The exact numbers aren’t entirely clear, but they range from doubling the risk to quintupling it.  In terms of absolute numbers, 2.3 out 1,000 women will develop uterine cancer over the course of treatment.

As such, preventative use of tamoxifen should be reserved for people at high risk.  One guide describes that as having a 1% or greater chance of developing cancer over a year if you have a uterus, and .5% or greater if you don’t.  Issues that put you at high risk include BRCA1 or P53 genetic mutations, or simply having had strange growth in your breast.

What are the side effects of Tamoxifen?

Hormonal:  Women taking tamoxifen are at increased risk of birth defects.  This is because the drug interferes with the body’s hypothalamic-pituitary system, which can interfere with the menstrual cycle.  Hot flashes are a common side effect, as well as ovarian cysts.

Skeletal: On the plus side, tamoxifen has positive effects on bones.  Treatment with it is associated with a slight reduction in risk for bone fractures.  The effect is not as strong as typical treatments and should not be sought after for itself.

Heart: Treatment causes a slight reduction in LDL cholesterol, but does not effect HDL levels.  This effect has not been shown to have any cardiovascular benefits, however.

Blood: Tamoxifen has similar blood clotting risks as typical estrogen therapy.  That means that you are at a higher risk of developing a clot which lodges in your veins or lungs, causing deep venous thrombosis or a pulmonary embolism, respectively.

Eyes:  Treatment is associated with a 60% increase of risk for cataracts.