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Janet M., a fifties-something woman, entered my office and said as she sat down, "I've read that if I take hormones I'll increase my breast cancer risk. I'm going crazy without sleep and with these mood swings, but I don't want to increase my breast cancer risk by taking hormones."
Like many women, Janet had heard that a recent study, Women's Health Initiative (WHI), definitively showed that hormone replacement therapy (HRT) increases breast cancer risk. Janet, like most people, didn't realize that this study found no statistically significant increase in breast cancer risk to women who took HRT.
When differences are not significant, an increase in risk may well be due to other factors, not one being studied, such as HRT use. As often happens when a medical story is reported, emphasis was on increase in risk, not whether increase was likely to be due to agent being studied or to size of risk.
The actual size of a risk is important in any woman's decision making process. In this case risk was exceedingly small -- only 8 in 10,000 women a year -- which is 0.08% or eight hundredths of one percent! Janet was amazed to learn actual size of increase, and said, "You mean I was getting all concerned for a risk that small!"
"And," I pointed out, "even this very small difference in risk may not be due to hormone use." I explained that breast cancers take an average of eight years to reach about half an inch in size. This means that breast cancers started in first year of study would not be detected for eight or more years. The study followed women for only about five years, so all or most of breast cancers found were probably present in an undetected state before study began.