Epinephrine is Contraindicated in Hydroxyzine Overdose Due to Paradoxical Hypotension
Do not use epinephrine in hydroxyzine overdose—it will worsen hypotension rather than correct it. The FDA drug label for hydroxyzine explicitly states: "Do not use epinephrine as hydroxyzine counteracts its pressor action" 1. This is a critical, potentially life-threatening drug interaction that distinguishes hydroxyzine overdose management from typical poisoning protocols.
Why Epinephrine Fails: The Pharmacologic Mechanism
Hydroxyzine blocks α-adrenergic receptors, which are the primary targets for epinephrine's vasoconstrictive effects. When you administer epinephrine in this setting:
- α-receptor blockade prevents vasoconstriction: Epinephrine cannot increase peripheral vascular resistance because hydroxyzine occupies these receptors
- Unopposed β-receptor stimulation occurs: Epinephrine's β2-adrenergic effects (vasodilation in skeletal muscle beds) proceed unchecked
- Paradoxical hypotension results: Instead of raising blood pressure, epinephrine may actually lower it further through unopposed β-mediated vasodilation
This is the same mechanism seen with other α-blocking agents and represents a fundamental pharmacologic antagonism.
Correct Management of Hydroxyzine Overdose
First-Line Hypotension Treatment
Use levarterenol (norepinephrine) or metaraminol instead 1. These agents are specifically recommended in the FDA label because they can overcome the α-blockade more effectively than epinephrine.
Initial Supportive Measures
- Intravenous fluid resuscitation: Normal saline boluses to address distributive shock
- Frequent vital sign monitoring: Hypotension is the primary cardiovascular manifestation
- ECG monitoring: Hydroxyzine overdose causes QT prolongation and risk of Torsades de Pointes 1
Clinical Presentation Differs from Other Antihistamines
Recent evidence shows hydroxyzine overdose presents differently than diphenhydramine poisoning 2:
- Less antimuscarinic toxicity: Fewer anticholinergic findings (dry mouth, urinary retention, mydriasis)
- More CNS depression: Hypersedation is the predominant feature, not agitation
- Lower seizure risk: Convulsions occur but are less common than with diphenhydramine
- Rare mortality: Only 0.02-0.8% mortality rate in large cohort studies 2
Specific Interventions
- If vomiting hasn't occurred: Induce emesis or perform gastric lavage if presenting early 1
- For seizures: Benzodiazepines are first-line; physostigmine has been used successfully in case reports 3
- For QT prolongation/arrhythmias: Avoid drugs that further prolong QT; magnesium sulfate for Torsades de Pointes
- General supportive care: Most patients recover with observation and symptomatic treatment within 72 hours 3
Critical Pitfall to Avoid
The most dangerous error is reflexively administering epinephrine for hypotension without considering the specific toxin. This stems from epinephrine's role as first-line therapy in anaphylaxis 4, but that context is entirely different. In anaphylaxis, epinephrine works through multiple beneficial mechanisms (α1-vasoconstriction, β1-inotropy, β2-bronchodilation, mast cell stabilization). In hydroxyzine overdose, the α-blockade eliminates the primary therapeutic benefit while preserving the risks.
Why This Differs from Anaphylaxis Management
The guidelines emphasizing epinephrine as first-line treatment 4, 5 apply to allergic reactions, not antihistamine overdose. In fact, these same guidelines note that antihistamines themselves are second-line agents that "should never be administered alone" in anaphylaxis 6 because they lack epinephrine's rapid vasoconstrictive effects. The irony is that in hydroxyzine overdose, the drug blocks the very receptors that would make epinephrine effective.
Bottom line: Hydroxyzine overdose requires alternative vasopressors (norepinephrine or metaraminol), supportive care, and cardiac monitoring—never epinephrine.