context: Hormones that every woman has in her body -- estrogen and progesterone -- can be fuel for some types of breast cancer. They help the cells grow and spread. Hormone therapy, also called endocrine therapy, adds, blocks, or removes those chemicals to treat the disease. There are two types of hormone therapy for breast cancer: Drugs that stop estrogen and progesterone from helping breast cancer cells grow Drugs or surgery to keep the ovaries from making the hormones Hormone therapy is different from hormone replacement therapy ( HRT), a treatment that adds hormones to the body to counter the effects of menopause. When you're diagnosed with breast cancer, your doctor will test cells from your tumor to see if they have parts on their surfaces called receptors that use estrogen or progesterone. If they do, it means that they depend on these hormones to grow. In that case, your doctor will probably recommend hormone therapy as part of your treatment plan. If you've already been treated for breast cancer, you might use hormone therapy to help keep it from coming back. It also helps lower your odds of getting new cancers in the other breast. Also, if you don't have the disease but have a family history of it, or genes that raise your risk, your doctor may recommend hormone therapy to lower your chances of getting it. Common hormone therapy drugs for breast cancer treatment include: Anastrozole ( Arimidex) Exemestane ( Aromasin) Fulvestrant ( Faslodex) Goserelin ( Zoladex) Letrozole ( Femara) Leuprorelin, leuprolide acetate ( Lupron) Megestrol ( Megace) Tamoxifen ( Nolvadex, Soltamox) Toremifene ( Fareston) Tamoxifen is a pill that doctors have prescribed for more than 30 years to treat breast cancer. It works by keeping estrogen from attaching to the cancer cells. Doctors first used tamoxifen to treat women whose breast cancer had spread in their bodies because it slowed or stopped the growth of the disease. The drug also lowers the chance that some early stage breast cancers will come back. And it can lower the risk that a woman will get cancer in her other breast later on. Women who are at high risk for breast cancer can take tamoxifen to try to lower their chances of getting the disease. It's an alternative to watchful waiting or having surgery to remove a breast, called a mastectomy, before they get the disease. Tamoxifen is an option for: Treatment of the earliest form of breast cancer, ductal carcinoma in situ ( DCIS), along with surgery Treatment of abnormal cells in the glands that make milk, called lobular carcinoma in situ ( LCIS), to lessen the chance that they'll become more advanced breast cancer Treatment of breast cancer in men and women whose cancers use estrogen Treatment of breast cancer that has spread to other parts of the body or that comes back after treatment To prevent breast cancer in women at high risk for the disease Some people should not use tamoxifen: Pregnant women Women planning to get pregnant Men or women who've had blood clots or a stroke Talk to your doctor to see if tamoxifen is right for you. For women, the side effects of tamoxifen are similar to some of the symptoms of menopause. Two of the most common ones are hot flashes and vaginal discharge. Other problems for women may include: Vaginal dryness or itching Irregular periods Headache Nausea and vomiting Skin rash Fatigue Water retention and weight gain The side effects may be similar to menopause, but tamoxifen doesn't trigger menopause. For men, tamoxifen can cause: Headache Nausea and vomiting Skin rash Sexual side effects like erectile dysfunction or a lower sex drive Yes. The risks include: Fertility. Tamoxifen can increase your fertility for a short time. But it might harm a growing baby, so it's important to use some form of barrier birth control while you're taking it, like condoms or a diaphragm. Don't use birth control pills. They can change how the drug works and affect the breast cancer. Tell your doctor right away if you think you've become pregnant while you're taking tamoxifen. Blood clots. Women who take tamoxifen may have a slightly higher risk of blood clots in their lungs or large veins. It's an even bigger risk for smokers. Stroke. Uterine cancer or sarcoma. The drug may make a woman more likely to get these diseases. But this risk is small, and it may be outweighed by the benefits of tamoxifen for breast cancer treatment. Talk to your doctor to know for sure. Cataracts. Tamoxifen seems to give some women a higher chance of having this condition, which clouds the lens inside the eye. People have also reported eye problems such as corneal scarring or retinal changes. Medications. Tamoxifen may affect how other drugs work in your body. A large study by the National Cancer Institute looked at whether tamoxifen lowered cases of breast cancer in healthy women who were known to be likely to get the disease. The results of the trial showed a 50% reduction in breast cancer in the women who took the drug. Studies have also shown that tamoxifen lessens the risk of breast cancer returning in women who have had the earliest form of the disease, ductal carcinoma in situ ( DCIS). Another drug, raloxifene ( Evista), which prevents the bone-thinning disease osteoporosis, is similar to tamoxifen. Studies have found that it prevents breast cancer in women who are at high risk, but with fewer side effects. The FDA has approved it for breast cancer prevention. Aromatase inhibitors: This type of hormone therapy drug keeps your body from breaking down testosterone into estrogen. They include: Anastrozole ( Arimidex) Exemestane ( Aromasin) Letrozole ( Femara) Aromatase inhibitors keep breast cancer from getting worse for longer than tamoxifen in women who have the advanced disease whose tumors rely on estrogen to grow. For women who have gone through menopause, the drugs can fight cancer even after it has spread to other parts of the body. They are pills that you take once a day. Side effects of aromatase inhibitors include: Nausea and vomiting Diarrhea and constipation Hot flashes Headache Bone pain Severe fatigue Skin rash Swelling and water retention Flu-like symptoms Increased bone loss Fulvestrant ( Faslodex): This shot keeps estrogen from attaching to cancer cells. Common side effects include: Pain where you get the shot Nausea and vomiting Loss of appetite Weakness and fatigue Hot flashes Cough Muscle, joint, and bone pain Constipation Shortness of breath