Tamoxifen is available in 10 mg and 20 mg tablets. When used for early-stage breast cancer, tamoxifen is usually prescribed at 20 mg a day for 5 years. Tablets may be taken once or twice a day.
How It Works
Many women have breast cancer that tests positive for estrogen receptors (ER+). This means that estrogen promotes the growth of the breast cancer cells. Tamoxifen blocks the effects of estrogen on these cells. It is often called an "anti-estrogen."
Tamoxifen slows or stops the growth of cancer cells that are already present in the body. It helps keep the original breast cancer from coming back and helps prevent new cancer in the opposite breast. It also reduces the risk of breast cancer in women who have a high risk for this disease.
Why It Is Used
Tamoxifen is used to prevent and treat breast cancers that test positive for estrogen receptors (ER+). It blocks the effects that the hormone estrogen has on cancer cells and lowers the chance that breast cancer will grow. Tamoxifen:
In rare cases, tamoxifen is used to treat breast pain (mastalgia), because it reduces estrogen levels that cause breast swelling. This is an unlabeled use.
How Well It Works
Tamoxifen is highly effective in lowering the risk of breast cancer recurrence. In women who have already had breast cancer, tamoxifen also lowers the risk of breast cancer in the opposite breast (contralateral).
For postmenopausal women, a two-stage treatment using tamoxifen and then an aromatase inhibitor, such as anastrozole (Arimidex), exemestane (Aromasin), or letrozole (Femara) may work better than only taking tamoxifen.
The most common side effects caused by tamoxifen are hot flashes; vaginal dryness, discharge, or irritation; and decreased interest in sex. These side effects are not usually serious, but they can be bothersome.
Other side effects are rare but are more dangerous. These include:
See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)
What To Think About
Because tamoxifen can cause changes in the lining of the uterus, women who use it should have yearly pelvic exams and should be evaluated further if they experience any abnormal uterine bleeding.
Tamoxifen may not work as well if a woman is also taking some types of medicine to treat hot flashes or depression. If you take tamoxifen, talk with your doctor about any other medicines you are taking.
Tamoxifen should not be used if you are pregnant or breast-feeding. Discuss birth control with your doctor if you are taking tamoxifen and could become pregnant.
eMedicineHealth Medical Reference from Healthwise
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