Background: Polycystic ovary syndrome (PCOS) is the most common endocrine disorder in premenopausal women, characterized by insulin resistance and hyperandrogenism. Previous studies have investigated the use of metformin, a wellknown insulin sensitizer, on lean PCOS patients, but it remains unclear as to how effective the treatment is. This literature review was carried out to better assess the effects of metformin on lean PCOS patients. Methods: Related literature was fully searched using MEDLINE, EMBASE, Web of Science, and COCHRANE for metformin therapy on lean women with PCOS (last updated May 2022). A review was performed to evaluate the effects of continued metformin treatment on relevant hormonal and metabolic indicators in these women. Results: A total of fourteen studies among 300 related articles were included in the review. Significant changes in total testosterone, sex hormone binding globulin (SHBG), blood insulin, and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) were reported in majority of the studies. Several studies observed significant changes in Body Mass Index (BMI), luteinizing hormone (LH), and blood glucose. Nearly none of the studies assessed reported significant changes in follicle-stimulating hormone (FSH) and androstenedione. Discussion: Current literature around the efficacy of metformin treatment on lean PCOS patients remains unclear and uncertain. The majority of studies reviewed indicate that metformin treatment significantly ameliorated total testosterone, SHBG levels, and insulin resistance in lean patients with PCOS. However, the mechanisms and effects of metformin on other treatment outcomes in lean PCOS women is unclear. Conclusion: Metformin may be a putative treatment for lean PCOS women with significant insulin resistance and/or hyperandrogenism. More research involving larger sample sizes and a greater number of clinical outcomes is required.