Estrogen comes in 3 main forms in women’s bodies, estrone, estradiol and estriol. I discuss this in detail in my book, Dr. Liz’s Easy Guide to Menopause. Estriol is a form of estrogen not currently used in pharmaceutically-made hormone therapy products, but is used commonly in compounded hormone therapy products. Because there are no patented products containing estriol, it is considered to be on the margin of medical use in the United States.
I am not sure the controversy over estriol will ever be definitively settled. For now, I want to list here some of the points in the scientific literature that support the safety of estriol.
Safety of estriol in rats: “Inhibition of radiogenic mammary carcinoma in rats by estriol or tamoxifen.” Cancer 1989 May 1;63(9):1685-92. Rats were radiated to cause mammary (breast tissue) cancer; the rats given estriol had a sginificant decrease in the development of cancer in this tissue. Also, some rats were given tamoxifen which also (more dramatically) lowered the incidence of mammary cancers in the radiated rats.
Safety of estriol in women: “Safety and Efficacy of Oestriol for Symptoms of Natural or Surgically Induced Menopause.” Human Reproduction 2000 May;15(5):1028-36. Fifty-three Japanese women were given oral estriol daily for one year. They had good symptom relief and no adverse effects noted at the one-year mark (short follow-up).