A report in September 2009 from Health and Human Services' Agency for Healthcare Research and Quality suggests that tamoxifen, raloxifene, and tibolone used to treat breast cancer significantly reduce invasive breast cancer in midlife and older women, but also increase the risk of adverse side effects.
A beneficial side effect of tamoxifen is that it prevents bone loss by acting as an estrogen receptor agonist (i.e., mimicking the effects of estrogen) in this cell type. Therefore, by inhibiting osteoclasts, it prevents osteoporosis. When tamoxifen was launched as a drug, it was thought that tamoxifen would act as an estrogen receptor antagonist in all tissue, including bone, and therefore it was feared that it would contribute to osteoporosis. It was therefore very surprising that the opposite effect was observed clinically. Hence tamoxifen's tissue selective action directly led to the formulation of the concept of selective estrogen receptor modulators (SERMs). In contrast tamoxifen appears to be associated with bone loss in premenopausal women who continue to menstruate after adjuvant chemotherapy.
Tamoxifen is a selective estrogen receptor modulator. Even though it is an antagonist in breast tissue it acts as partial agonist on the endometrium and has been linked to endometrial cancer in some women. Therefore endometrial changes, including cancer, are among tamoxifen's side effects. With time, risk of endometrial cancer may be doubled to quadrupled, which is a reason tamoxifen is typically only used for 5 years.
The American Cancer Society lists tamoxifen as a known carcinogen, stating that it increases the risk of some types of uterine cancer while lowering the risk of breast cancer recurrence. The ACS states that its use should not be avoided in cases where the risk of breast cancer recurrence without the drug is higher than the risk of developing uterine cancer with the drug.
Cardiovascular and metabolic
Tamoxifen treatment of postmenopausal women is associated with beneficial effects on serum lipid profiles. However, long-term data from clinical trials have failed to demonstrate a cardioprotective effect. For some women, tamoxifen can cause a rapid increase in triglyceride concentration in the blood. In addition there is an increased risk of thromboembolism especially during and immediately after major surgery or periods of immobility. Tamoxifen is also a cause of fatty liver, otherwise known as steatorrhoeic hepatosis or steatosis hepatis.
Central nervous system
Tamoxifen treated breast cancer patients show evidence of reduced cognition and semantic memory scores. However memory impairment in patients treated with tamoxifen was less severe compared with those treated with anastrozole (an aromatase inhibitor).
A significant number of tamoxifen treated breast cancer patients experience a reduction of libido.
Pre-mature growth plate fusion
While tamoxifen has been shown to antagonize the actions of estrogen in tissue's such as the breast, it's effects in other tissues such as bones has not been documented fully. There have been studies done in mice showing tamoxifen mimic the effects of estrogen on bone metabolism and skeletal growth. Thus increasing the possibility of pre-mature bone fusion. This effect would be less of a concern in adults who have stopped growing.
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