A number of studies show bioidentical hormones are beneficial and of low risk. Let's start with progesterone. In 1993, the book Natural Progesterone: The Multiple Roles of a Remarkable Hormone was published. This book revealed to millions of women the benefits of natural bioidentical versus synthetic progesterone. This is where many readers first learned of its mood-supporting effects.  

Progesterone Alleviates Certain Cases of PMS

Progesterone, which is made by men and women, is the hormone that rises during pregnancy and during the second half of a women's menstrual cycle. In the decade prior to menopause, production of progesterone declines, which may contribute to premenstrual syndrome (PMS). Some women suffering from PMS have found relief by applying topical progesterone during the latter half of their menstrual cycle. However, PMS may have different roots, and a decline in progesterone may not always be a cause. (Some women may be progesterone-intolerant, making them unable to use it due to adverse effects.)

Progesterone Protects the Uterine Lining

Estrogen replacement therapy, taken without progesterone, may have its own risks. Estrogen stimulates the uterine lining to grow. Progesterone protects the uterine lining from excessive proliferation. Women who have undergone a hysterectomy have been traditionally prescribed hormones without progestin or progesterone since there is no need to protect the uterus. The Women's Health Initiative study of hysterectomized women who used estrogen alone showed no increase in the risk of breast cancer among those women. However, in a different branch of the study, women who used both estrogen and progesterone did have an increased risk.

Bioidentical Progesterone May Prevent Breast Cancer

Many hysterectomized women have been made aware of the benefits of bioidentical progesterone and are beginning to use it. Some of the benefits include a positive effect on mood, sleep, libido, and well-being. In addition, bioidentical progesterone is safer than synthetic progestin and may even protect against breast cancer and other diseases. In one study that compared women who were prescribed topical progesterone for benign breast disease to nonusers, no increase in the risk of breast cancer was observed, and progesterone users appeared, in fact, to have a significantly lower risk of breast cancer.¹ In another study, estrogen, bioidentical progesterone, both hormones, or a placebo were used topically by women for 10 to 13 days prior to breast lump surgery. While use of estrogen alone resulted in a 62% increase in breast cell proliferation in comparison with the placebo, women who received both hormones had less proliferation. Those who received progesterone alone experienced a 66% reduction in proliferation when compared with the placebo group.² In a study involving 5,963 premenopausal women, participants with the highest levels of progesterone had a 60% lower risk of breast cancer compared to those with the lowest levels. The risk was further reduced for women with regular periods.³ And in a long-term study of 1,083 women treated for infertility, low progesterone levels were associated with a 540% increase in the risk of premenopausal breast cancer as well as a greater risk of cancer death.⁴ Another long-term study found an adjusted 51% reduction in the risk of developing breast cancer among women with the highest serum progesterone levels during pregnancy compared to women with the lowest levels.⁵ Furthermore, having a higher progesterone level at the time of surgery for breast cancer has been associated with increased survival.^6,7 Not only has bioidentical progesterone been associated with protection against breast cancer, but it also may benefit the cardiovascular system and the brain. Science is only beginning to uncover the impact progesterone has on every area of the body.

References:

1. Cancer Detect Prev. 1999;23(4):290-6.
2. Fertil Steril. 1995 Apr;63(4):785-91.
3. Int J Cancer. 2004 Nov 1;112(2):312-8.
4. Am J Epidemiol. 1981 Aug;114(2):209-17.
5. Cancer Epidemiol Biomarkers Prev. 2002 Apr;11(4):361-8.
6. Eur J Cancer. 1994;30A(4):445-8.
7. Br J Cancer. 1996 Jun;73(12):1552-5.