Background <p>Metformin is a commonly used antihyperglycemic drug to treat type 2 diabetes mellitus worldwide. However, its effectiveness was recently documented beyond diabetic treatment, such as in the case of cancer. Recent epidemiological evidence suggests that the use of metformin has shown benefits in breast and colorectal cancer; however, its effectiveness was inconclusive in the case of head and neck cancer. This umbrella review aimed to evaluate the effectiveness of metformin among head and neck cancer patients.</p> Methods <p>Three electronic databases and additional sources (Google Scholar and citation screening) were used to extract relevant research records. A multistage screening was performed by two independent reviewers, and the study selection process was reported according to PRISMA guidelines. Along with descriptive findings and overlap assessment, a meta-analysis was performed using a random-effects model with 95% confidence interval.</p> Result <p>A total of five studies were included. Analysis revealed that metformin increases overall survival probability among head and neck cancer patients by 0.74 times [95% C.I. (0.67–0.81)] compared to non-metformin user, indicating pharmacological effectiveness. Overall heterogeneity was negligible (I2 = 0%). Egger’s test suggested funnel plot asymmetry, indicating potential publication bias. All studies have shown effectiveness in lowering incidence risk.</p> Conclusion <p>Findings showed promising potential of metformin among HNC user. However, as there were very few studies available and quality assessment showed low confidence, a more comprehensive evaluation of metformin in diabetic and non-diabetic patients should be conducted to confirm these findings.</p>