The most worrisome side effect of taking estrogen without progesterone is an increased chance of endometrial cancer, which affects the lining of the uterus. If you have had a hysterectomy it is okay to take estrogen alone, but if you haven't, progesterone should be added to your regimen. Progesterone is given in pill form. It should be used in women who have a uterus and are receiving estrogen therapy to decrease the risk of endometrial cancer. A doctor should evaluate any woman receiving HRT who has abnormal vaginal bleeding.
Many women worry about the relationship between estrogen therapy and breast cancer.
The exact relationship between these is not clear, but women who have a very strong personal or family history of breast cancer might not be given estrogen therapy. This should be discussed with your doctor. Other conditions may be made worse by HRT and should be considered when deciding about therapy. These include endometriosis, gallbladder disease, and menstrual migraine headaches. The choice concerning HRT is an individual one.