Full spectrum CBD oil is widely available and touted as a safe and effective way to lower anxiety, promote restful sleep, and reduce inflammation. Given the anecdotal evidence that full spectrum CBD oil can also be used as an adjuvant to chemotherapy, an over-the-counter CBD oil product was tested to determine if it possessed cell cytotoxic properties. A “triple negative” breast cancer cell line was chosen as it represents a breast cancer subset which lacks targeted therapies and correlates with poor clinical outcomes. Cancer cells exposed to CBD oil had reduced cell viability, broad changes in gene expression, and reduced expression of a key growth factor receptor compared to control cells. Together, these data suggest that CBD oil is effective at reducing breast cancer cell viability in vitro. Future studies will focus on determining the range of cancer cell types similarly affected and on confirming mechanistic details.