Now, new research reveals that the combined hormones may also make breast cancer screening more difficult.
Researchers examined mammogram results among the more than 16,000 women participating in the government trial.
They found that those taking hormones were more likely to have an abnormal result on their mammogram, meaning additional follow-up testing was needed.
In addition, 10 percent of hormone users underwent a breast biopsy to test for possible cancer, compared to just 6 percent of those taking the placebo.
Doctors say that the hormones increase breast density, which can make mammograms difficult to interpret.
The vast majority of women using hormones today take just estrogen, not the combination pill tested in this study, and estrogen generally has fewer health risks.
But experts still recommend women take the lowest dose for the shortest time possible.
