Editor’s Perspective What We Already Know about This Topic What This Article Tells Us That Is New Background Several factors within the perioperative period may influence postoperative metastatic spread. Dexmedetomidine and midazolam are widely used general anesthetics during surgery. The authors assessed their effects on human lung carcinoma (A549) and neuroglioma (H4) cell lines in vitro and in vivo. Methods Cell proliferation and migration were measured after dexmedetomidine (0.001 to 10 nM) or midazolam (0.01 to 400 μM) treatment. Expression of cell cycle and apoptosis markers were assessed by immunofluorescence. Mitochondrial membrane potential and reactive oxygen species were measured by JC-1 staining and flow cytometry. Antagonists atipamezole and flumazenil were used to study anesthetic mechanisms of action. Tumor burden after anesthetic treatment was investigated with a mouse xenograft model of lung carcinoma. Results Dexmedetomidine (1 nM) promoted cell proliferation (2.9-fold in A549 and 2-fold in H4 cells vs. vehicle, P < 0.0001; n = 6), migration (2.2-fold in A549 and 1.9-fold in H4 cells vs. vehicle, P < 0.0001; n = 6), and upregulated antiapoptotic proteins in vitro. In contrast, midazolam (400 μM) suppressed cancer cell migration (2.6-fold in A549 cells, P < 0.0001; n = 4), induced apoptosis via the intrinsic mitochondrial pathway, decreased mitochondrial membrane potential, and increased reactive oxygen species expression in vitro—effects partly attributable to peripheral benzodiazepine receptor activation. Furthermore, midazolam significantly reduced tumor burden in mice (1.7-fold vs. control; P < 0.05; n = 6 per group). Conclusions Midazolam possesses antitumorigenic properties partly mediated by the peripheral benzodiazepine receptor, whereas dexmedetomidine promotes cancer cell survival through signaling via the α2-adrenoceptor in lung carcinoma and neuroglioma cells.