Progestins and breast cancer risk.

There is still no clear account on the role that progestins play in human breast cancer, and the issue continues to be debated. The effects of progestins on breast cell proliferation have been investigated in a number of different experimental systems with conflicting results. Progesterone has been reported to both stimulate and inhibit the growth of experimental mammary tumors, depending upon the dose and the experimental models. Epidemiological studies on the effect of combined hormone replacement therapy (HRT) on breast cancer risk in menopausal women are limited because the use of a progestin in addition to estrogens was not widely adopted until 10 years ago. Taking meta-analysis into account estrogen-progestin use did conclude that there is not an excess risk in combined HRT users. Epidemiological studies have provided conflicting results, ranging from a protective effect to a deleterious effect of progestin addition on breast cancer risk. Progestins include a large family of molecules characterised by different binding capacities to androgen receptors. This implies that they should be considered as distinct yet related, therapeutic agents. The use of different progestins may account for the controversial results obtained in studies conducted in different geographic areas.