Clinical Effects:

  • USES: H2 histamine blockers (or H2 blockers) are available as prescription and non-prescription medications and are used to treat reflux esophagitis, heartburn, and peptic ulcers. PHARMACOLOGY: Antagonist of H2 histamine receptors. This effect decreases gastric acid secretion. TOXICOLOGY: Cimetidine inhibits the cytochrome P450 oxidase enzymes, 1A2, C219, and 2D6, and decreases the metabolism of many other medications. EPIDEMIOLOGY: Exposures to H2 blockers are common, but significant toxicity has not been described. Adverse effects during therapeutic use are rare. MILD TO MODERATE TOXICITY: Mild drowsiness, dizziness or confusion. SEVERE TOXICITY: Severe toxicity is exceedingly rare. CNS depression and hypotension have been reported. ADVERSE EFFECTS: Dry mouth, delirium, and bone marrow suppression have been reported. Bradycardia and conduction effects are rare effects occurring mostly after rapid IV administration.

Range of Toxicity:

  • TOXICITY: These medications have a large therapeutic window and no more than minimal toxicity is expected even with very large overdoses. THERAPEUTIC DOSE: CIMETIDINE: ADULT: 400 to 1600 mg/day orally; PEDIATRIC: 20 to 40 mg/kg/day orally. FAMOTIDINE: ADULT: 20 to 80 mg/day orally; PEDIATRIC: 0.5 to 2 mg/kg/day orally. RANITIDINE: ADULT: 150 to 300 mg/day orally; PEDIATRIC: 2 to 10 mg/kg/day orally, divided in 2 to 4 doses.


  • Support: MANAGEMENT OF MILD TO MODERATE TOXICITY: Treatment is symptomatic and supportive. MANAGEMENT OF SEVERE TOXICITY: Intravenous fluids for hypotension, supportive care for CNS depression.
  • Decontamination: PREHOSPITAL: No prehospital decontamination is recommended. HOSPITAL: No decontamination is warranted in most cases.
  • Airway management: Rare cases with significant CNS depression may require intubation for airway protection.
  • Antidote: None
  • Monitoring of patient: No specific laboratory testing is indicated in most patients. Obtain a basic metabolic screen and serum acetaminophen and salicylate concentrations after deliberate overdose.
  • Enhanced elimination procedure: Enhanced elimination is not indicated.
  • Patient disposition: HOME CRITERIA: Patients inadvertent ingestions can be managed at home. All patients with self-harm ingestions should be sent to a healthcare facility for mental health evaluation. OBSERVATION CRITERIA: Patients with deliberate overdose or more than mild symptoms should be referred to a healthcare facility and observed until symptoms resolve. ADMISSION CRITERIA: Patients with significant CNS depression or other symptoms that persist after 6 to 8 hours of observation should be admitted, but this is exceedingly rare. CONSULT CRITERIA: Consult a poison center or medical toxicologist for assistance if symptoms are more than mild or if the diagnosis is unclear.