With no sharp cure, breast cancer still be the major and the most serious life-threatening disease worldwide. Colorectal is the third most commonly occurring cancer in men and the second most commonly occurring cancer in women. In the present investigation, colon cancer cells (CaCo-2) and breast cancer cells (MCF-7) were treated with elevated doses of metformin (MET) for 48h. Cell count was assessed using trypan blue test, and the cytotoxicity was evaluated using MTT assay. Methylation-specific PCR was performed on the bisulfite-treated DNA against two tumor suppressor genes; RASSF1A and RB. Results indicated that: in breast cancer, the cell count was decreased significantly (P> 0.005) after being treated with 5, 10, 20, 50, and 100 mM of MET. The elevated concentration had increased reduction percentages on the MCF-7 cells, as 5 mM and 100 mM have yielded 35% and 93.3% reduction in cell viability, respectively. Colon cancer cells have responded to the doses of MET differently, as for the 5 mM and the 100 mM, it gave 88% and 60% reduction in cells viability, respectively. Cytotoxicity assay revealed that 5 mM and 100 mM of MET caused breast cancer cells to loss 61.53% and 85.16% of its viability, respectively, whereas colon cancer cells have responded to the 5 mM and 100 mM of MET by reducing the cells viability with 96.91% and 96.24%, respectively. No RB promoter methylation was detected in colon cells, while RASSF1A was partially methylated. In the MCF-7 breast cancer cells, both RASSF1A and RB were partially methylated.