Type and Timing of Menopausal Hormone Therapy and Breast Cancer Risk: Individual Participant Meta-Analysis of the Worldwide Epidemiological Evidence

The benefits and risks of the use of menopausal hormone therapy (MHT, also known as hormone replacement therapy [HRT]) comprise an area of considerable debate. The widespread use of MHT began in the 1960s with initial preparations containing only oestrogen. Subsequent observations that these preparations led to endometrial disorders in postmenopausal women with an intact uterus resulted in the addition of progestogens to MHT preparations to confer protection to the endometrial lining in these women. As the use of MHT became more widespread, concerns began to be raised about breast cancer risk related to the use of these preparations. Broadly, modern MHT preparations may include oestrogen and progestogen, in which the progestogen component is given continuously or intermittently (for women with an intact uterus), or they may be oestrogen-only preparations (for women without an intact uterus). The different types of MHT are usually administered orally or transdermally (as creams or patches). Topical vaginal preparations are used only to treat local vaginal symptoms and are minimally absorbed systemically.