Health Conditions A-Z Cancer Breast Cancer 5 Things To Know About Tamoxifen Tamoxifen can help treat and prevent breast cancer, but many people report side effects like hot flashes and weight gain. By Amanda MacMillan Amanda MacMillan Amanda MacMillan is a health and science writer and editor. Her work appears across brands like Health, Prevention, SELF, O Magazine, Travel + Leisure, Time Out New York, and National Geographic's The Green Guide. health's editorial guidelines Updated on December 20, 2022 Medically reviewed by Doru Paul, MD Medically reviewed by Doru Paul, MD Doru Paul, MD, is a board-certified oncologist and hematologist. He is an associate professor of clinical medicine at Weill Cornell Medical College. learn more Share Tweet Pin Email The treatments for breast cancer will be different depending on each individual's cancer profile. Factors in determining treatment include age, overall health, the cancer stage, and other factors. Treatment includes radiation, chemotherapy, surgery, and hormonal therapy such as the use of tamoxifen. Here's what people with breast cancer and their loved ones should know about the pros and cons of tamoxifen use. Goodacre's Story In an interview, former Victoria's Secret model Jill Goodacre spoke about her five-year battle with breast cancer. Specifically, they talked about the medication that they took to keep the disease at bay. Goodacre underwent surgery and radiation in 2012 after an ultrasound detected a tumor. Subsequently, they took the drug tamoxifen. As Goodacre approached her five-year cancer-free mark, she said she had looked forward to stopping tamoxifen. The medication can cause side effects, including weight gain, which Goodacre said she had experienced. "I've always been a pretty fit person, and so to be just rounder and heavier and not to really be able to do much about it—that's been hard," Goodacre said. "It's taken a lot out of my self-confidence." Goodacre's grievances are common problems among breast cancer survivors, Nikita Shah, MD, medical oncology team leader for the Breast Cancer Center at the Orlando Health Cancer Institute in Florida, told Health. Dr. Shah had not treated Goodacre but reported prescribing tamoxifen to patients. Still, tamoxifen can be lifesaving, according to Dr. Shah. And for many people, the benefits of tamoxifen outweigh its potential side effects. Tamoxifen Can Treat or Prevent Breast Cancer Breast tumors need estrogen to grow. Tamoxifen is in a class of medications known as antiestrogens. Tamoxifen blocks the activity of the hormone estrogen in the breast, possibly stopping the growth of some breast tumors. Healthcare providers often prescribe the drug—taken as a pill or liquid—to pre-menopausal people after surgery for early-stage breast cancer. Surgeons can't always be sure they have removed all cancerous cells. So, tamoxifen can reduce the risk that those leftover cells multiply, causing cancer to return. Healthcare providers may also prescribe tamoxifen to pre- or post-menopausal people who do not have breast cancer but have a high risk of developing the illness. Those people are usually 35 or older and have risk factors such as: Family history of cancerAbnormal biopsiesHistory of thoracic radiation given before 30 years oldHistory of lobular carcinoma in situ (LCIS)History of atypical hyperplasiaA positive test for high-risk genes, like mutations of BRCA genes Tamoxifen Is Only Effective for Some Types of Cancer Tamoxifen may reduce the risk of cancer in high-risk people by more than 30%. But because the drug affects estrogen receptors in the body, it only works against estrogen-receptor-positive or progesterone-receptor-positive cancers. Together, those make up about two-thirds of all breast cancers, according to Breastcancer.org. "The way tamoxifen works is you're depriving your cancer cells of estrogen, which is their nutrition," explained Dr. Shah. "So that will only work for the cancers that are estrogen-fed or progesterone-fed." Factors Affecting the Effectiveness of Treatment Some evidence suggests that some people have an abnormal version of an enzyme called CYP2D6. That enzyme may make tamoxifen less effective. However, a study published in the Journal of the National Cancer Institute could not find a link between the enzyme and tamoxifen. Certain medications, including Benadryl (diphenhydramine), Tagamet (cimetidine), and some antidepressants, can also block the enzyme's activity. "That's why it's very important to make sure your oncologist knows every medication you're taking, including supplements," said Dr. Shah. People May Take Tamoxifen for Ten Years After breast cancer treatment, most people who take tamoxifen take it for five years. But according to a study published in 2013 in the Lancet, tamoxifen may reduce a person's risk for breast cancer even further if they take the drug for 10 years. But healthcare providers only recommend tamoxifen after breast cancer treatment for people who haven't gone through menopause yet. After menopause, other drugs—called aromatase inhibitors—may work better than tamoxifen. "If a [person] becomes post-menopausal during those five or 10 years [they're] on tamoxifen, we will switch [them] to this other group of drugs that is 20% to 25% more effective," said Dr. Shah. Side Effects of Tamoxifen Despite its protective effects against cancer, tamoxifen has some uncomfortable side effects. According to the National Library of Medicine, those side effects may include the following: Hot flashesMood swingsNauseaVomitingWeight gain or lossMenstrual irregularities Changes in sex drive Sleep disturbances Tamoxifen also raises the risk for more severe health issues, like blood clots and uterine cancer. One study published in 2018 in Medicine (Baltimore) found that tamoxifen use was associated with nearly a two-fold increase in the odds of deep vein thrombosis or pulmonary embolism among older women with breast cancer. Though, for most people, the overall risk for those problems is low. According to the American Cancer Society, the risk of developing endometrial cancer from tamoxifen is less than 1% per year. Many people who take tamoxifen are perimenopausal. And according to a study published in 2017 in the Journal of Clinical Oncology, some people mistake menopause symptoms with tamoxifen side effects. The researchers reported that symptom-related drop-out rates were similar across a nearly five-year period for people who took tamoxifen and for those who took a placebo pill. "Patients will say I started taking tamoxifen, and I gained 40 pounds," said Dr. Shah. "But that's probably a combination of hormonal and lifestyle changes they're going through. Tamoxifen can cause some fluid retention, and it could maybe cause a two- or three-pound weight gain. But beyond that, there are probably other things going on, as well." It's essential to talk to a healthcare provider if you're experiencing side effects rather than stopping a recommended course of tamoxifen, added Dr. Shah. "There are ways to manage the side effects, and most of them are not permanent. They get better with time," said Dr. Shah. A Quick Review Tamoxifen is a drug that may stop the growth of some breast tumors that need estrogen to grow. And while the drug can reduce the risk of cancer in high-risk people by more than 30%, it is only recommended after breast cancer treatment for people who haven't gone through menopause yet. Despite its protective effects against cancer, tamoxifen may cause side effects such as pain, hot flashes, tiredness, depression, or headache, among others. Tell a healthcare provider if any of your symptoms are severe or do not go away. Metastatic Breast Cancer Treatments Was this page helpful? Thanks for your feedback! Tell us why! Other Submit Sources Health.com uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. MedlinePlus. Tamoxifen. BreastCancer.org. Tamoxifen uses, side effects, and medicine facts. BreastCancer.org. Breast cancer hormone receptor status. Goetz MP, Sun JX, Suman VJ, et al. 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