Cancer is a cataclysmic disease that affects not only the target organ, but also the whole body. Metal-based nanoparticles (NPs) have recently emerged as a better option for the treatment of this deadly disease. Accordingly, the present work describes a means to control the growth of cancer cells by using colloidal silver nanoparticles (AgNPs) processed via homemade solutions and the characterization of these materials. The AgNPs may become an instantaneous solution for the treatment of these deadly diseases and to minimize or remove these problems. The AgNPs exhibit excellent control of the growth rate of human liver (HepG2) and breast (MCF-7) cancer cells, even at a very low concentrations. The cytotoxic effects of AgNPs on HepG2 and MCF-7 cancer cells were dose dependent (2–200 μg/mL), as evaluated using MTT and NRU assays. The production of reactive oxygen species (ROS) was increased by 136% and 142% in HepG2 and MCF-7 cells treated with AgNPs, respectively. The quantitative polymerase chain reaction (qPCR) data for both cell types (HepG2 and MCF-7) after exposure to AgNPs showed up- and downregulation of the expression of apoptotic (p53, Bax, caspase-3) and anti-apoptotic (BCl2) genes; moreover, their roles were described. This work shows that NPs were successfully prepared and controlled the growth of both types of cancer cells.