Concomitant medications and immune checkpoint inhibitor therapy for cancer: causation or association?

The majority of cancer patients assume concomitant medications for the treatment of cancer-related symptoms or co-morbidities. As immune checkpoint inhibitors expand in the treatment of a widening range of malignancies, drug–drug interactions have become an area of increasing interest due to the potential for some concomitant medications to exert immune-modulatory effects and influence outcomes from immunotherapy. Here, we review the evidence supporting this association across selected drug classes including antibiotics, proton pump inhibitors, metformin, and opioids.