A combination of estrogen and progestin increases breast tissue density which could, in turn, increase breast cancer risk.

Hormone replacement therapy (HRT) given to relieve menopausal symptoms comes in several formulations - some contain estrogen alone, while others use estrogen with a form of progesterone (known as combination therapy). The long term health effects of HRT are still a subject of controversy and much research.

In a new study, researchers at the University of California, Los Angeles, looked at how various forms of HRT affected the density of breast tissue, using digitized mammography. They find that the use of combination therapy increases the density of breast tissue by three to five per cent, while estrogen alone causes no such increase. Breast tissue density is a risk factor for breast cancer - the greater the density the higher the risk. However, this study does not tell us that the increases found with HRT use would increase cancer risk. Evidently more research is needed to clarify this issue.

Source

Journal of the National Cancer Institute 1st January 2003