Background: Several opioid analgesics have well-known germline PGx associations which may predict either inefficacy or exaggerated (toxic) responses, depending on the patient’s genotype. Despite this, germline PGx testing has not been routinely incorporated into oncology care. We hypothesized that CYP2D6 germline PGx profiling offers the potential to improve oncology patients’ pain control by identifying individuals at increased risk for inadequate analgesia with standard opioid dosing. Methods: We retrospectively analyzed the medication histories of over 81,000 adult oncology patients treated at the University of Chicago from 2012-2018 for exposure to opioids. CYP2D6 genotype (permitting assignment of metabolizer phenotype: normal metabolizer [NM], intermediate metabolizer [IM], or poor metabolizer [PM]) was determined post-hoc for 127 patients who were genotyped for other reasons unrelated to pain prescribing. The primary endpoint was the number of opioids required for pain control over the course of longitudinal care, comparing PM/IMs with NMs. The secondary endpoint was the number of hospitalizations for pain control. Results: Over 47,000 oncology patients were exposed to opioids, with an average of 2.67±1.6 different opioid medication exposures per patient. Thirteen percent of genotyped patients were IM/PM, who were at risk for inadequate analgesia. IM/PM patients demonstrated an increased number of different opioid exposures (4.5±2.1) compared to NM (2.7±2.1, P value = 0.002). IM/PM patients were also more likely to have a pain related hospitalization (OR 4.17, CI 1.3-13.2, P = 0.016). Conclusions: Based on population prevalence, we estimate that > 6000 oncology patients (1000 patients/year) who received opioids at our center were IM/PM and thus at risk for inadequate analgesia due to genetic predisposition. CYP2D6 germline PGx profiling offers the potential to improve oncology patients’ pain management.