What is tamoxifen?
Tamoxifen is a hormone therapy for breast cancer, and works as a preventative for people who are at high risk for developing breast cancer.
SERMs – Agonist-Antagonist Properties
Tamoxifen acts as an agonist-antagonist where estrogen is concerned on different parts of the body.
Tamoxifen citrate is a ‘selective estrogen receptor modulator’, or SERM for short. SERMs act as estrogen antagonists binding to estrogen receptors, and preventing estrogen from binding in the breast. But SERMs conversely act at the same time (‘selectively’) as estrogen agonists on the uterus. Meaning while it significantly decreases the occurrence of breast cancer, it increases the chance of getting endometrial and uterine cancers.
Tamoxifen and Triple Negative Breast Cancers
Triple negative breast cancer is a type of cancer that tests negative for all three hormone receptors (estrogen receptor-negative, progesterone receptor-negative, and HER2-negative) therefore tamoxifen will not help with this cancer since triple negative cancer isn’t estrogen based.
My breast cancer is triple negative, but my oncologist prescribed me tamoxifen because I’m BRCA2 positive. She figured it wouldn’t hurt to have this level of protection to prevent an estrogen based breast cancer from occurring during the time between the end of chemo and the date of my double mastectomy.
Certain drugs may interfere with the potency and effectiveness of tamoxifen. Drugs that inhibit the enzyme CYP2D6 all do this. The enzyme CYP2D6 is needed to break down tamoxifen into molecules, or metabolites, which activates it. These drugs include SSRI antidepressants (selective serotonin reuptake inhibibors) such as Paxil, Prozac and Zoloft. Other common drugs that interfere with this enzyme are diphenydramine (Benedryl, an antihistamine), quinidine (Cardioquin, for abnormal heart rhythm), and cimetidine (Tagamet, reduces stomach upset). Also, people who have abnormal CYP2D6 enzyme function may not get the full benefit of tamoxifen, either.
Benefits of Tamoxifen
Tamoxifen has been proven to lower the chance of high-risk women getting breast cancer, and lowers the chance of the cancer metastasizing, or spreading, for women who currently have ductal carcinoma in situ – meaning breast cancer found only in the milk ducts. Tamoxifen reduces the risk of a new cancer developing in the other breast when cancer was only found in one breast. It can also help shrink large hormone-receptor cancers before surgery.
Risks of Tamoxifen
There are risks with using tamoxifen. The main ones are hot flashes, mood swings, loss of libido, and night sweats. Since this tamoxifen is an estrogen agonist in the uterus, there is an increased risk of developing endometrial and uterine cancers. This side effect increases with use, so the typical prescription for this drug is 5 years tops. Other side effects are blood clots and stroke.
Women who are pregnant or nursing should not take tamoxifen as the drug may damage developing embryos. Women of child bearing age should use non-hormone contraception (condoms, diaphrams with spermicide, and IUDs) for two months following tamoxifen use. Also, women who are breastfeeding should not take tamoxifen as it may pass through the milk to the baby.