Scientists have suspected for some time that chronic stress promotes cancer growth in humans. New research suggests that this may well be true. Erica Sloan, a researcher at UCLAs Semel Institute for Neuroscience and Human Behavior, and colleagues studied the effect of stress on a mouse model of breast cancer. One group was confined in a small area for a short period of time every day for two weeks, while the other group was not. Results showed that primary tumors grew similarly in both groups of mice, suggesting that their growth did not seem to be affected by stress. However, the stressed mice developed 30-times more secondary tumors (metastases) than control mice, thus showing that cancer growth was significantly more aggressive in the stressed mice. Further investigations revealed that stress appears to turn on a “metastatic switch” that biologically reprograms macrophages, the immune cells that are trying to eliminate the disease, transforming them to conspire against the host and aid the spread of the cancer. On a more promising note, the researchers were actually able to prevent the acceleration of cancer growth by treating stressed animals with beta blockers, which block the reprogramming of the immune cells.