Which hormone therapy is associated with an increased risk of breast cancer?

Combined hormone therapy with estrogen and progestin is associated with an increased risk of breast cancer due to the effects of both hormones on breast tissue. Estrogen alone can stimulate the growth of certain types of breast cells, and when progestin is added, it can exacerbate this growth due to its own actions on hormone receptors within breast tissue. Research, particularly randomized controlled trials like the Women's Health Initiative, has shown a clear link between the use of combined hormone therapy and an elevated risk of developing breast cancer compared to women who do not use hormone therapy or who use estrogen alone.

In contrast, estrogen therapy on its own does carry some risks but the data regarding its increased association with breast cancer is less pronounced. Progestin alone typically does not show the same level of risk as the combination with estrogen in studies focused on breast cancer. Hormonal contraceptives, while they can have various health effects, have not been definitively linked to an increased risk of breast cancer in the same manner as combined hormone therapy, particularly in younger women. Thus, the distinction lies in the combined effect of estrogen and progestin, which compounds the risks associated with hormone therapy.