Adding continuous progestin to estrogen lowers the risk of endometrial cancer in postmenopausal women, according to a study recently published in the JNCI: Journal of the National Cancer Institute (doi:10.1093/jnci/djv350).

The addition of progestin to an estrogen regimen is known to lower the risk of endometrial cancers, which are associated with estrogen alone use; but just how much the addition of progestin lowers that risk has remained unclear.

To determine the effects that continuous estrogen plus progestin use has on the risk of endometrial cancer, Rowan T. Chlebowski, MD, PhD, at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, and colleagues reviewed data from a Women’s Health Initiative (WHI) randomized clinical trial assessed 16 608 postmenopausal women age 50 to 79 years with intact uteri. All participants had normal findings on endometrial biopsy on entry. In the double-blind placebo-controlled trial, participants were randomly assigned to daily estrogen plus progestin given as one pill or placebo. The participants were followed for 13 years.

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Study findings demonstrated a statistically significant reduction in endometrial cancer incidence in the intervention group (33 cases, 0.06% yearly) compared with the placebo group (95 cases, 0.10% yearly) after 5.6 years’ median intervention and 13 years’ cumulative follow-up.

“Continuous combined estrogen plus progestin use for 5.6 years in postmenopausal women with normal endometrial biopsy at therapy initiation resulted in a statistically significant reduction in endometrial cancer incidence, with the difference becoming statistically significant during longer-term post intervention follow-up,” the authors reported, adding that, “In postmenopausal women, continuous combined estrogen plus progestin use reduces endometrial cancer incidence by 35%.”