Abstract
Background: The effectiveness of metformin in preventing Type-2 Diabetes Mellitus (T2DM) is examined. There are new available data. Currently, there are no available analyses classifying its effectiveness compared to placebo, standard care, or lifestyle interventions, and there is limited evidence on the combined action of metformin and lifestyle interventions in preventing T2DM. Objective: To calculate the updated overall effectiveness of metformin in preventing T2DM using all available and most recent data, and to explore the effectiveness of metformin and lifestyle interventions in preventing T2DM. Materials and Methods: A search was performed in PubMed and the Cochrane Library Central Register of Controlled Trials (CENTRAL) (from inception to 24 May 2025). A systematic review (SR) and meta-analysis (MA) of randomized controlled trials (RCTs) was carried out, including metformin-naïve adults with any identified diabetes risk factors. The overall effectiveness of metformin was estimated by combining studies that compare metformin against placebo, metformin and standard care against standard care, and metformin plus lifestyle interventions and the same lifestyle interventions. The combined action of metformin and lifestyle interventions was evaluated against standard care. We performed a GRADE assessment of the overall evidence. Results: Overall, metformin may reduce the incidence of T2DM by 23% in high-risk adults (OR 0.77, 95% CI 0.67, 0.88, p-value 0.0001) and 25% in patients with prediabetes (OR 0.75, 95%CI 0.66, 0.86, p-value < 0.0001). It is also effective in both obese and normal-weight patients, in Caucasians, in studies with female predominance, in studies with a mean age over 60 years, at 1700 mg daily, and after 18 months of administration. Effectiveness weakens after interruption of administration. Metformin is more effective compared to placebo and when combined with standard care than standard care alone, but not when combined with lifestyle interventions against lifestyle interventions alone. Metformin and lifestyle interventions reduce the incidence of diabetes in patients with prediabetes by 52% compared to standard care (OR 0.48, 95% CI 0.30, 0.77; p-value 0.002). There are effectiveness concerns in studies with more men than women, Asian Indians and Pakistanis, a mean age below 60 years, 500 mg of metformin daily, and after six months. The effect is reduced during post-intervention. Finally, metformin alone is more effective than standard care (OR 0.56, 95% CI 0.34, 0.90, p-value 0.02). The quality of evidence was moderate for the overall effectiveness of metformin and metformin combined with lifestyle interventions, and low for metformin against standard care. Conclusions: A 1700 mg dose of metformin daily is effective in preventing T2DM, especially in Caucasians, in women over 60 years, in prediabetes, and independent of obesity. Lifestyle interventions and 500 mg of metformin daily may prevent T2DM in patients with prediabetes, especially in men and Asian Indians or Pakistanis under 60 years. The effectiveness of complex interventions is more pronounced than that of metformin alone in patients with prediabetes. Further research is needed for post-intervention effectiveness, patients with any diabetes risk factors, patients from different regions, and women in complex interventions.