This inconsistency of effect has been suggested to imply a lack of meaningful impact of HRT on breast cancer risk.There is a non-statistically significant increased rate of breast cancer for hormone replacement therapy with synthetic progesterone. The risk may be reduced with bioidentical progesterone, though the only prospective study that suggested this was underpowered due to the rarity of breast cancer in the control population. There have been no randomized controlled trials as of 2018. The relative risk of breast cancer also varies depending on the interval between menopause and HRT and route of synthetic progestin administration.The most recent follow up of the Womens Health Initiative participants demonstrated a lower incidence of breast cancer in post-hysterectomy participants taking equine estrogen alone, though the relative risk was increased if estrogen was taken with medroxy-progesterone. Estrogen is usually only given alone in the setting of a hysterectomy due to the increased risk of vaginal bleeding and uterine cancer with unopposed estrogen.HRT has been more strongly associated with risk of breast cancer in women with lower body mass indices (BMIs). No breast cancer association has been found with BMIs of over 25.
Hormone replacement therapy (HRT), also known as menopausal hormone therapy or postmenopausal hormone therapy, is a form of hormone therapy used to treat symptoms associated with female menopause. These symptoms can include hot flashes, vaginal atrophy, accelerated skin aging, vaginal dryness, decreased muscle mass, sexual dysfunction, and bone loss. They are in large part related to the diminished levels of sex hormones that occur during menopause.Estrogens and progestogens are the main hormone drugs used in HRT. Progesterone is the main female sex hormone that occurs naturally and is also manufactured into a drug that is used in menopausal hormone therapy. Although both classes of hormones can have symptomatic benefit, progestogen is specifically added to estrogen regimens when the uterus is present to avoid the increased risk of endometrial cancer. Unopposed estrogen therapy promotes endometrial hyperplasia and increases the risk of cancer, while progestogen reduces this risk. Androgens like testosterone are sometimes used as well. HRT is available through a variety of different routes.The long-term effects of HRT on most organ systems vary by age and time since the last physiological exposure to hormones, and there can be large differences in individual regimens, factors which have made analyzing effects difficult.