AA new study has reported that the antidepressant Prozac (fluoxetine) increased the metastasis of breast cancer cells to the brain in a mouse model of breast cancer. However, the growth of the metastases was not affected by Prozac once they were established within the brain. Breast cancer patients may develop neurological side effects such as depression, chemo brain, or epilepsy. Many drugs are prescribed to manage these side effects without a clear understanding of how these may affect metastasis. Prozac is commonly prescribed to breast cancer survivors to combat symptoms such as depression and hot flashes. While antidepressant use has been reported to be associated with primary breast tumor development, its influence on brain metastasis is unknown. Based on Prozac’s known effects on neurons, the authors hypothesized that Prozac might promote brain metastasis by increasing both the invasion of the tumor cells into the brain and facilitating their growth once in the brain.

To conduct the study, the authors injected breast cancer cells into mice. The number of brain metastases were quantified three weeks after numerous metastases first became evident. Pretreating animals for three weeks with Prozac in drinking water (0.2g/L) was found to increase the metastasis of breast cancer cells to the brain. On the other hand, the growth of metastatic tumors was not affected by Prozac once they were established within the brain. The authors conclude that more studies are necessary before Prozac can be considered completely safe for breast cancer patients.