B-337 Case Report: Fatal Opioid Overdose Induced by Over-the-Counter Cold Medication Diphenhydramine

Abstract

Abstract Background Accidental drug-related deaths frequently arise following ingestion of multiple co-intoxicants, including opioid analgesics. Hydrocodone, the most prescribed opioid for pain management, is metabolized by hepatic enzymes CYP2D6 and CYP3A4 to the pharmacologically active metabolite hydromorphone and the inactive metabolite norhydrocodone, respectively. As a result, inhibition of CYP2D6 by other pharmacologic agents can impair the analgesic properties of hydrocodone and prolong its half-life in circulation. One such example is diphenhydramine, which is a potent inhibitor of CYP2D6 activity. This case highlights a drug-drug interaction likely resulting in an accidental fatal opioid overdose. Case Presentation A 58-year-old female had longstanding prescriptions for hydrocodone/acetaminophen (Norco® 10/325), alprazolam, carisoprodol, and diltiazem. The decedent had recently taken an over-the-counter cold medication, diphenhydramine (Benadryl), shortly preceding death. A toxicology report (NMS labs, Horsham, PA) from a post-mortem femoral blood sample detected the presence of the following therapeutic compounds by High Performance Liquid Chromatography/Time of Flight-Mass Spectrometry (LC/TOF-MS) analysis: acetaminophen (14 μg/mL), hydrocodone (410 ng/mL), dihydrocodeine (hydrocodol, 24 ng/mL), alprazolam (42 ng/mL), carisoprodol (88 ng/mL), diltiazem (110 ng/mL), and diphenhydramine (150 ng/mL). Hydromorphone, the active metabolite of hydrocodone, was not detected (<2 ng/mL). With the exception of hydrocodone, which was present at fatal concentrations, all other compounds were within therapeutic range. Together, these observations are strongly suggestive of a drug-drug interaction leading to impaired hydrocodone metabolism. Conclusions This case report highlights a fatal opioid overdose as a result of CYP2D6 inhibition by diphenhydramine, a readily available over-the-counter medication. Post-mortem toxicological analysis revealed hydrocodone was present in lethal concentrations, whereas its pharmacologically active metabolite, hydromorphone, was undetectable at time of autopsy. Further, the patient was prescribed a formulation of hydrocodone in combination with acetaminophen, which was present at therapeutic concentrations despite the two drugs having similar elimination half-lives. These toxicological findings suggest administration of diphenhydramine precipitated a deadly accumulation of hydrocodone in the deceased. Overall, this case report underscores the importance of understanding and preventing harmful drug-drug interactions in patients taking opioid medications for pain management.