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Living With Cancer

Does tamoxifen help?

WebMD Feature

Oct. 30, 2000 -- The day that forever changed Bonnie McDonald's life started out like any other. It was a typically hectic Monday morning last spring when the 37-year-old decided to grab a quick shower. She'd gone for a run after dropping her son off at school and was about to rush off to her part-time catering job when she felt a pea-sized lump in her left breast. Tests later that week showed that she had found a cancerous tumor. "It was so frightening," she says. "I'd always been athletic and healthy. That was my first glimpse of mortality."

McDonald underwent a lumpectomy and eight weeks of radiation. Then her oncologist suggested she begin taking tamoxifen, an anticancer drug that has been playing a substantial role in the fight against the return of breast cancer. Tamoxifen works by blocking the estrogen that breast cancer cells need to grow. Her doctor explained that, as a side effect of the treatment, she might go through a temporary menopause and could experience hot flashes, mood swings, and all of the other symptoms associated with the change of life. He also said that taking tamoxifen could increase her risk of developing endometrial cancer -- cancer of the lining of the uterus or womb.

His prescription, he said, was based on a balance of risks and benefits and, like many other medical decisions these days, the choice was ultimately up to the patient. McDonald got on the Web and did her own research, talked to women in similar circumstances, and finally decided that she would take tamoxifen.

McDonald's choice is one that many women are making. About one million women worldwide take tamoxifen, according to a review article in the May 27 British Medical Journal, and medical research backs up the choice. In May, a Lancet article reported that breast cancer death rates in the United States and Britain have dropped dramatically -- between one quarter and one third -- in the past decade. Sir Richard Peto, a leader of the international Early Breast Cancer Trialists' Collaborative that produced this new report, calls this news "the best decrease in national death rates that has ever been seen with any common cancer." Surely earlier diagnosis, better surgery, chemotherapy, and radiotherapy all play a part in improved survival rates, Peto says, but he is convinced that the widespread and prolonged use of tamoxifen may be the most important factor. However, women on tamoxifen must cope with the side effects of taking the drug -- some of them quite severe.

How It Works

Originally developed as a fertility agent, tamoxifen is a selective estrogen receptor modulator (SERM). It affects the way estrogen reacts with cells. Breast cells have estrogen receptors on their surfaces that allow the hormone to lock onto them. While this is a perfectly normal process, over time the buildup of estrogen can promote breast cancer in some women. About 70% of breast cancers are estrogen receptor positive, meaning that estrogen fuels their growth. Tamoxifen blocks the absorption of estrogen by the breast cancer cell by locking onto these receptors first, thereby preventing metastasis, the growth of the cancer and its deadly migration to other parts of the body.

In 1998, Peto and associates published a report in The Lancet analyzing data from 55 clinical studies involving 37,000 women. They concluded that women who had estrogen receptor positive or estrogen receptor unknown tumors and took tamoxifen for five years reduced their risk of breast cancer recurrence from 42% to 26%, while women with all estrogen receptor status types lowered their risk of developing a new tumor in the other breast by 50%.

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