A new meta-analysis of worldwide data published in the Lancet confirms the connection between menopausal hormone therapy (MHT) and breast cancer. Results suggest that of the 20 million breast cancer diagnoses in Western countries since 1990, about 1 million may have been caused by MHT.

While earlier studies have linked MHT and breast cancer, this new meta-analysis focuses for the first time on the effect of the timing (current or former use), duration, and type of MHT on breast cancer risk. It includes data gathered from 1992 to 2018, for a total of 58 studies involving 143,887 postmenopausal women with invasive breast cancer and 424,972 controls without breast cancer.

The researchers found that the longer women took MHT, the higher was their risk of invasive breast cancer, and that this relationship held for both current and former MHT users. An increased risk of breast cancer persisted even for women who had discontinued MHT more than a decade prior. The risk was higher in current users than in former users. Using MHT for less than one year was not associated with an increased risk of breast cancer, but after one year of use, risk began to rise.

Both estrogen plus progesterone therapy and estrogen alone were associated with breast cancer in current and former users, but the risk of breast cancer was greater with the combination therapy. The risk was similar regardless of whether the therapy was oral or transdermal. Only the use of topical vaginal estrogen preparations was not linked to an increase in breast cancer risk.

There are now about 12 million users of MHT in Western countries. Current recommendations are for women to use MHT for as short a time as possible. In a published comment on the study, Joanne Kotsopoulos, associate professor, Dalla Lana School of Public Health and Department of Nutritional Sciences, University of Toronto, advised clinicians to "heed the message of this study but also to take a rational and comprehensive approach to the management of menopausal symptoms, with careful consideration of the risks and benefits of initiating MHT for each woman," including the severity of menopausal symptoms and contraindications for MHT.-Joan Zolot, PA