Head and neck squamous cell carcinoma (HNSCC) is the sixth leading cancer in the world; the main risk factors are alcohol and tobacco use. Advancements in therapies have yet to improve the prognosis of HNSCC. The connection between diabetes and cancer is being recognized, and metformin has been shown to decrease cancer incidence in diabetic patients. Accordingly, here, for the first time, we investigated metformin's efficacy on the growth and viability of human HNSCC FaDU and Detroit cells. Our results show that metformin treatment (5-20 mM) dose-dependently inhibits the growth of both cell lines. In FaDU cells, metformin caused 18-57% and 35-81% growth inhibition after 48 and 72 h treatments, respectively. Similarly, in Detroit 562 cells, 48 and 72 h metformin treatment resulted in 20-57% and 33-82% inhibition, respectively. Mechanistically, metformin caused G 1 arrest, which coincided with a decrease in the protein levels of CDKs (2, 4 and 6), cyclins (D1 and E) and CDK inhibitors (p15, p16, p18 and p27), but no change in p19 and p21. Metformin also decreased the levels of oncogenic proteins Skp2 and β-Trcp. In other studies, metformin decreased the phosphorylation of 4E-BP1 at Ser65, Thr37/46 and Thr70 sites, but drastically increased the phosphorylation of EF2 at Thr56 and AMPK at Thr172, which results in global translational inhibition. In summary, the observed wide spectrum of mechanistic effects of metformin on HNSCC cells provides support for the anticancer capability of the drug and its potential use in future therapies.