Abstract Introduction: Cannabidiol (CBD) exhibits multiple therapeutic properties, but its use in advanced cancer patients raises concerns about potential drug–drug interactions (DDIs) due to polypharmacy. This study aims to look for evidence of DDIs between concomitant medications and CBD oil in a randomized placebo-controlled trial of CBD oil for symptom control (MedCan-1 parent study). Materials and Methods: Surrogate measures were used to identify possible drug interactions: (1) the maximum mL of oil self-selected by patients in CBD or placebo groups in relation to opioids, specific drug groups, or individual agents; (2) the occurrence of any new or worse adverse effect in relation to the study arm...