After 5 years of follow-up of women taking estrogen plus progestin, the WHI Estrogen-plus-Progestin study reported a 37 percent reduction in colorectal cancer cases compared with women taking the placebo. On average, the researchers found that if a group of 10,000 women takes estrogen plus progestin for a year, 6 fewer cases of colon cancer will occur than in a group of nonusers. These findings are consistent with observational studies, which have suggested that the use of postmenopausal hormones may reduce the risk of colorectal cancer. The WHI Estrogen-Alone study concluded that estrogen alone had no significant effect on colorectal cancer risk.
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