This systematic review summarizes research on the use of progestin and breast cancer risk. Although mainly used for contraception, progestin can help treat menstrual disorders, and benign breast, uterine, and ovarian diseases. Breast cancer is the leading site of new, non-skin, cancers in females in the United States, and possible factors that may modulate breast cancer risk need to be identified. ProQuest (Ann Arbor, MI) and PubMed-Medline (US National Library of Medicine, Bethesda MD, USA) databases were used to search for epidemiologic studies from 2000 to 2015 that examined the association between progestin and breast cancer. Search terms included epidemiologic studies + progesterone or progestin or progestogen or contraceptive or contraceptive agents + breast cancer or breast neoplasms. A total of six studies were included in the review. Five of the six studies reported no association between progestin-only formulations (including norethindrone oral contraceptives, depot medroxyprogesterone acetate, injectable, levonorgestrel system users, implantable and intrauterine devices) and breast cancer risk. Duration of use was examined in a few studies with heterogeneous results. Unlike studies of other oral contraceptives, studies indicate that progestin-only formulations do not increase the risk of breast cancer, although the literature is hampered by small sample sizes. Future research is needed to corroborate these findings, as further understanding of synthetic progesterone may initiate new prescription practices or guidelines for women's health.
Keywords: Breast cancer; Contraception; Pharmacoepidemiology; Progesterone; Progestin; Progestogen; Risk.