In the new study, Liu and her colleagues looked at data on more than one million women who had participated in the Million Women Study in England and Scotland. The mean age of participants was 56, and they were followed for an average of six years.
Women currently using HRT were 64 percent more likely to be hospitalized for gallbladder disease, but they were only 17 percent more likely to be admitted if they were using gels or patches.
Higher doses of estrogen were associated with a higher risk than lower doses. The risk also decreased the longer the woman had been off of HRT, although there was still some risk a decade after discontinuing.
The risk seemed mainly confined to estrogen. Adding progesterone did not have a significant impact on the risk of gallbladder disease.
Estrogen implants involved a level of risk that fell between those faced by users of oral HRT and those faced by users of patches or gels.
Among women using HRT, one cholecystectomy (removal of the gallbladder) could be avoided for every 140 women using transdermal therapy rather than oral therapy over a five-year period, the team found.
According to the study, about 1.1 percent of middle-aged women in the U.K. who have never used HRT will end up having a cholecystectomy. That proportion rises to 1.3 percent among women using patches or gel and to 2 percent for women taking HRT in pill form.
"Most people will do fine with pills, but some patients who are prone to gallbladder disease or even blood clots may do better with the patch," Sulak said. "Not all women can tolerate the patch, however. They may have skin reactions, or the patch won't
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