Critical Cases - Diphenhydramine Overdose!


Chief Complaint - diphenhydramine overdose



  • Pt states she swallowed approximately 25 tabs of generic diphenhydramine about one hour prior to arrival in an attempt to commit suicide
  • She feels palpitations and dizziness
  • Endorses dry mouth
  • Pt denies coingestions

Past Medical History

  • Depression on paroxetine daily

Physical Exam


T 98.9 HR 145  BP 138/91  RR 18 Pox 99% RA

  • General: Tearful and anxious appearing but in no distress
  • HEENT: Dry MM
  • Cardio: Tachycardic, no murmurs
  • Pulm: Clear bilaterally
  • Abd: soft NTND
  • Neuro: Awake and alert, moving all extremities



ECG interpretation: Sinus tachycardia, QTc prolonged at 575 ms


Clinical Course

  • During initial interview, patient experienced a generalized seizure
  • Patient treated with 2 mg IV lorazepam and seizure abatred within ~90 seconds
  • Prolonged Qtc treated with 4 mg IV magnesium
  • Patient vomited and was treated with inhaled ispropyl alcohol and intravenous trimethobenzamide
  • Patient continued to vomit and was somnolent after seizure and treatment with benzodiazepene, leading to aspiration
  • Decision made to intubate for airway protection 
  • Induction agent = propfol, paralytic agent = succinylcholine
  • Patient admitted to ICU for further management and was successfully extubated 16 hours later and transferred to psychiatry service
  • Qtc prolongation resolved spontaneously


Clinical Pearls

  • Pure diphenhydramine overdose leads to classic anticholinergic syndrome: "blind as a bat" (dilated pupils unresponsive to light), "dry as a bone" (dry mucous membranes), "tachy as a leisure suit" (tachycardia), "mad as a hatter" (mental status change,  seizures), "hot as a hare" (hyperthermia)
  • Treatment is largely supportive and includes: cooling measures for hyperthermia, benzodiazepenes for agitation and seizures, and foley catheter placement for expected urinary retention
  • Intubation was undertaken as post-ictal state and treatment with benzodiazepenes led to somnolence and inability to protect the airway
  • Prolonged Qtc especially over 500 ms can lead to polymorphic ventricular tachycardia: treatment includes intravenous magnesium 4-6 g IV and 2-4 mg/hr IV infusion, avoiding Qtc prolonging medications (hence trimethobenzamide for nausea, and NOT ondansetron), and increasing HR to shorten Qtc with isoproterenol or pacing (not performed in this case as patient was already markedly tachycardic)