Type 2 Diabetes Mellitus (T2DM) is a prevalent metabolic disorder characterized by chronic hyperglycemia and associated with significant morbidity and mortality worldwide. Metformin is the recommended first-line oral antidiabetic agent owing to its efficacy, safety, cost-effectiveness, and favorable metabolic profile. To enhance therapeutic outcomes and patient convenience, metformin is available as Immediate-Release (IR) and Extended-Release (ER) formulations. Although both formulations contain the same active drug, they differ in release characteristics, dosing frequency, gastrointestinal tolerability, and adherence patterns. This review critically compares IR and ER metformin formulations with respect to pharmacokinetic properties, clinical efficacy, safety, and patient adherence. Evidence from clinical studies demonstrates that both formulations provide comparable glycemic control, including reductions in glycated hemoglobin (HbA1c) and fasting plasma glucose levels. However, ER metformin offers a controlled drug-release profile that minimizes peak plasma concentrations and improves gastrointestinal tolerability. Consequently, ER formulations are associated with fewer adverse gastrointestinal events such as nausea, diarrhea, and abdominal discomfort. The reduced dosing frequency of ER metformin also contributes to improved patient adherence, treatment satisfaction, and long-term persistence with therapy. Despite its higher cost, ER metformin may be particularly beneficial for patients experiencing intolerance or poor compliance with IR formulations. Future research should focus on personalized treatment approaches, innovative delivery technologies, real-world studies evaluating long-term adherence and clinical outcomes, and optimization of formulation selection strategies. Overall, both IR and ER metformin remain effective therapeutic options, with formulation selection depending on individual patient requirements and treatment goals.