Metformin, a first-line anti-diabetic medication, is used in the management of type 2 diabetes worldwide. Defects in insulin secretion, insulin activity, or both in diabetic patients affect the metabolism of carbohydrates, fat, and protein (which causes high blood glucose levels and obesity). Metformin decreases glucose production, increases insulin sensitivity, and enhances peripheral glucose uptake. Its effectiveness in managing blood glucose levels is well documented; however, the association between Metformin treatment and blood glucose levels and body weight remains a subject of investigation. This study aims to assess the association between the dosage and duration of metformin treatment with fasting blood glucose (FBS) level and body mass index (BMI) of diabetic patients. A cross-sectional study was conducted with 162 type 2 diabetic patients on routine checkup at Cape Coast Teaching Hospital in Ghana. Information on FBS level and BMI, metformin treatment, and demographic characteristics were obtained from records and face-to-face-interviews using a structured questionnaire. Pearson’s correlation was used to analyze the association between dosage and duration of metformin treatment with FBS level and BMI. The dosage of metformin treatment showed a significant positive correlation with the FBS level (r = 0.165, p = 0.036*) and the duration of metformin treatment showed a highly significant negative correlation with the FBS level (r = -0.204, p = 0.01**). However, there was no significant correlation between the dosage of metformin treatment and BMI (r = 0.109, p = 0.168) and between the duration of metformin treatment and BMI (r = 0.0790, p = 0.319). These results indicate that the dosage and duration of metformin treatment significantly affects fasting blood glucose level. Monitoring metformin treatment based on fasting glucose levels is more effective for managing diabetes than using body mass index as a guide.