The majority of cancer patients receiving chemotherapy will consider taking complementary and alternative medicine (CAM) during their treatment. Biologically-active CAM may interfere with chemotherapy. With recent developments and the expansion of the use of immunotherapies, newly-identified complications from previously-thought harmless supplements, such as probiotics, may reduce treatment effectiveness. Cannabis taken by patients receiving immunotherapy may negatively impact treatment effectiveness. Cannabis procurement and use may or may not be declared by patients, as legal access is complicated and expensive. Patients, particularly those who wish to treat their cancer with cannabis, may be accessing supply through alternative medicine sources that might influence patients to abandon timely conventional treatment.