Rapid methadone metabolism in patients with opioid use disorder could complicate methadone treatment. Toxicology screenings to monitor methadone levels may show negative for methadone, despite adherence regular regimen. A patient receiving treatment for opioid use disorder tested negative for methadone in 11 out of 22 toxicology screenings (50.0%). We hypothesized that the patient was a rapid methadone metabolizer. After tapering doses to a maintenance level and using supervised urine collection, the patient was negative for methadone in 7 out of 7 tests (100.0%) but positive for cocaine in 5 out of 7 tests (71.4%). Chronic cocaine use and genetic factors, particularly CYP2B6 polymorphisms, have been found to cause rapid methadone metabolism. Clinicians should be vigilant for unusual metabolic reactions and modify dose and monitoring schedules accordingly. More investigation into the physiological and genetic aspects of methadone metabolism is needed.