For a study, researchers sought to determine if the formulation of menopausal hormone treatment (HT) utilized affected the increased risk of breast cancer. Using data from the UK Clinical Practice Research Datalink, investigators conducted a population-based case-control study of women aged 50 and up. Women with incident breast cancer were matched (1:10) to a control group of women with comparable follow-up time and no history of breast cancer. For the following menopausal HT formulations, exposures were categorized as ever or never: bioidentical estrogens, animal-derived estrogens, micronized progesterone, and synthetic progestin. The adjusted impact of menopausal HT formulation on breast cancer risk was estimated using logistic regression models.

Between 1995 and 2014, 43,183 breast cancer cases were detected and matched to 431,830 control women. In adjusted analyses, menopausal HT usage was related to an elevated risk of breast cancer (odds ratio [OR] 1.12, 95% CI 1.09–1.15) compared to those who never used it. Estrogens were not related with breast cancer when compared to never users (bioidentical estrogens: OR 1.04, 95% CI 1.00–1.09; animal-derived estrogens: OR 1.01, 95% CI 0.96–1.06; both: OR 0.96, 95% CI 0.89–1.03). Progestogens seemed to be related to breast cancer in distinct ways (micronized progesterone: OR 0.99, 95% CI 0.55–1.79; synthetic progestin: OR 1.28, 95% CI 1.22–1.35; both OR 1.31, 0.30–5.73). 

Although menopausal HT appeared to be associated with a higher risk of breast cancer overall, the danger appeared to be mediated mostly by formulations using synthetic progestins. Micronized progesterone may be the safer progestogen to utilize while treating menopausal HT.