Endometriosis is a chronic illness defined by the presence of endometrial glands and stroma outside the uterine lining. While prevalence estimates vary depending on the population studied, endometriosis is thought to affect approximately one in ten reproductive aged women, up to 50% of women with infertility, and up to 70% of adolescents with chronic pelvic pain. Over 10 million women in the United States have endometriosis and incur significantly higher direct and indirect healthcare costs than those without. Annual health care costs are estimated at $100 billion in the United States alone. While the majority of patients report the onset of endometriosis symptoms during adolescence, delays in diagnosis are common and range from 7 to 12 years. During these delays, women may suffer from debilitating symptoms interfering with school and social activities, and commonly experience reduced health-related quality of life and work productivity. When left untreated, endometriosis can progress significantly in over 50% of patients and result in a higher stage of disease. Therefore, timely intervention and maintenance of therapy is critical for preventing the progression of disease and patient burden.

Therapeutic options for endometriosis include surgical and medical management. Because surgery is not curative, long-term medical therapy is often advised to prevent recurrence of symptoms and retard the progression of disease. Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used for endometriosis-related pain but do not remove or decrease deposits of ectopic endometrium. Hormonal therapies are the mainstay of medical therapy as they can inhibit prostaglandin production that contributes to pain, and cause decidualization and atrophy of ectopic endometrial tissue. Unfortunately, hormonal treatments have varied degrees of success and no data support one treatment over another. Additionally, the treatment choice may be limited to medication side effects, availability, costs, and contraceptive needs. Hormonal treatments do not cure endometriosis; 11-19% of women may not experience any relief with medical therapy, and up to a third of women experience recurrence of pain symptoms after treatment cessation. Additionally, as endometriosis is predominantly a disease of young reproductive-aged women, patients may desire a treatment with a different mechanism of action that does not impact ovarian function and fertility. There remains an unmet clinical need among women with endometriosis for effective and well-tolerated medical therapies.

Metformin, an insulin-sensitizing oral biguanide, is approved for the treatment of type 2 diabetes. Several published studies using in vitro and animal models have implicated metformin with a regression of endometriotic implants and reduction of inflammatory activity. Yet, the mechanisms of action of metformin remain to be further elucidated. Only one prospective study to date has investigated the use of metformin in humans as monotherapy for endometriosis; this study had substantial limitations. Further investigation will be improved with participant and provider blinding, use of validated pain measures, and inclusion of a broader population of patients affected by endometriosis. A randomized clinical trial is necessary to investigate the mechanisms and efficacy of metformin as an anti-endometriotic drug.