Tamoxifen is a member of the selective estrogen receptor modulator (SERM) class of drugs. Tamoxifen binds to tissues that use estrogen. This binding blocks the action of estrogen in the breast but mimics the action of estrogen in the bones and uterus. Tamoxifen is used to treat estrogen positive breast cancer in men and women and to prevent breast cancer in women at high risk.
Estrogen-positive cancers require estrogen to grow. Tamoxifen binds to the cancer cells in these tumors but does not stimulate them. This prevents estrogen from binding to the cancer cells. Tamoxifen does not kill cancer cells, but it keeps them from reproducing.
In the STAR study, tamoxifen reduced the risk of both invasive and noninvasive breast cancers. Noninvasive (also known as "in-situ") breast cancers are those that are confined to a breast lobule or duct. Invasive cancers are those that have started to spread to surrounding breast tissue.
Effect on Bone Strength
Even though tamoxifen blocks the estrogen activity in breast cancer cells, it mimics estrogen activity in the bones. Tamoxifen, like estrogen, inhibits the activity of osteoclasts---the cells that break down bone tissue. When the breakdown of bone tissue outpaces the build-up of bone tissue, osteoporosis occurs. By mimicking the action of estrogen in bones, tamoxifen slows down the bone loss that usually occurs after menopause in women.
Use in Males
Male breast cancers are 81 to 90 percent estrogen receptor positive. Tamoxifen is used as an adjuvant therapy (auxiliary or add-on therapy) in male breast cancer. The anti-estrogenic effect of tamoxifen in males also stimulates the production of testosterone, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This makes it useful for treating male infertility and andropause (the drop in testosterone that occurs as men age). Tamoxifen is also used to mitigate the erectile dysfunction and breast-gland enlargement that sometimes occur as side effects from prostate cancer medications.
Use in Bipolar Disorder
Researchers have begun studying the usefulness of tamoxifen in bipolar disorder. In a study published in the March 2007 issue of Bipolar Disorders, tamoxifen improved symptoms of mania in bipolar patients in as little as five days. Tamoxifen blocks the effects of the enzyme protein kinase C. Protein kinase C regulates the action of neurotransmitters in the brain and is more active in people with bipolar disorder.
Side Effects
The most common side effect with tamoxifen is hot flashes. Tamoxifen can reduce cholesterol levels, but it does not reduce the risk of heart attack. It increases the risk of blood clots, which can lead to pulmonary embolism (blood clot in the lungs) and stroke.
Tamoxifen increases the risk of uterine cancer. The American Cancer Society says that tamoxifen should only be used in women when the risk of breast cancer is higher than the risk of developing uterine cancer.
Drug Interactions
Some SSRIs, like Paxil, Zoloft and Prozac, block the effect of tamoxifen. Those who need to take tamoxifen and an SSRI should switch to an SSRI that does not have this effect, like Celexa, Luvox or Lexapro.
