The drug works by inhibiting the activity of estrogen, a female hormone that can drive the growth of breast cancer tumors. The drug is most often prescribed as a once-a-day pill for younger women with breast cancer.
Older, postmenopausal women are now often prescribed a newer class of drugs called aromatase inhibitors, which block estrogen released in body fat, experts said. Aromatase inhibitors are most easily used in women who no longer have ovaries that are producing estrogen, Davies said.
One reason for the switch to aromatase inhibitors: Prior research, as well as the current study, found that tamoxifen raises the risk of cancer of the lining of the uterus (endometrial cancer) and life-threatening blood clots in the lungs. The analysis found the added risk from tamoxifen to older women was small, and in younger women it was "almost non-existent," Davies said.
"The benefits greatly outweigh the risks," Davies said.
Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society, said studies such as this that look at death rates over the long-term are valuable.
"The study shows that tamoxifen as an adjuvant [additional] therapy for breast cancer has had a very successful track record that has been sustained over 30 years of use in the clinic, even though it's used less now," he said.
The analysis found tamoxifen worked equally well in women who underwent chemotherapy and radiation in addition to surgery, Davies noted. Another benefit: tamoxifen is inexpensive. Davies estimated a five-year course of the drug costs about $150, of particular importance in developing nations where breast cancer rates have risen dramatically, she said.
Many of the women in the analysis, she noted, failed to take the full five-year course of the drug, so it's possible the protective effect from fully taking the medication as prescribed mig
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