In this work, we studied the effect of common calcium channel blockers (Verapamil Hydrochloride and Nifedipine, usually prescribed for cardiovascular diseases) on the viscoelastic property of both tumorigenic (MCF-7) and vascular endothelial (HUVEC) cell-line. Specifically, an optical-tweezer was used to locally "press-down" on the cell membrane. Fluctuations of optically-trapped dielectric beads attached to the cell membrane were measured to study the effect of drugs on the mechanical properties of the membrane and the cytoskeleton. We observed that MCF-7 cells exhibit a significantly higher mean squared displacement (MSD) in response to these drugs as compared to HUVEC cells, under no-compression condition. Furthermore, optically induced compression of cell-membrane induced a significant drop in MSD of MCF-7 cells, whereas HUVEC cells showed a significant rise in MSD. To further test these results, we depolymerized actin-cytoskeleton using latrunculin-B and measured fluctuations of beads on the surface of cells. Immunofluorescence imaging of the actin cytoskeleton correlates with the variation in the viscoelastic response of MCF-7 and HUVEC under the influence of these drugs. Our power-spectrum analysis at 10 Hz shows that actin-depolymerization decreased the activity of both cell types; however, cells treated with nifedipine or verapamil showed contrasting effects. Together, these results show that clinically approved calcium-ion blocking cardiovascular drugs have a significant effect on the viscoelastic properties of circulating tumor cells in the blood. © 2020 ACM.