In a patient with diphenhydramine overdose presenting with anticholinergic toxicity and hallucinations, should an antipsychotic be administered?

Antipsychotics Should NOT Be Given for Hallucinations in Diphenhydramine Overdose

In a patient with diphenhydramine overdose presenting with anticholinergic toxicity and hallucinations, antipsychotics are contraindicated and should be avoided. Antipsychotics will worsen the anticholinergic syndrome due to their own anticholinergic properties, potentially exacerbating delirium, hyperthermia, and cardiovascular instability 1.

Why Antipsychotics Are Dangerous in This Context

Diphenhydramine overdose causes anticholinergic toxicity—a syndrome characterized by hallucinations, delirium, hyperthermia, tachycardia, mydriasis, dry mucous membranes, and urinary retention 2, 3, 4. The hallucinations are a direct result of central anticholinergic effects, not primary psychosis.

The critical pitfall: Most antipsychotics (both typical and atypical) possess significant anticholinergic properties themselves 1. Administering these agents will:

• Worsen the existing anticholinergic delirium
• Intensify hyperthermia and cardiovascular instability
• Prolong the toxic syndrome
• Increase risk of seizures and cardiac dysrhythmias

The pediatric emergency medicine guidelines explicitly warn: "Because of their anticholinergic properties, antipsychotics may worsen the condition of patients who present with intoxication from drugs with anticholinergic properties (eg, hallucinogens) or with an anticholinergic delirium" 1.

The Correct Treatment Approach

First-Line Management: Supportive Care

• Airway protection and respiratory support
• Intravenous fluids for hypotension
• Cooling measures for hyperthermia
• Benzodiazepines (lorazepam or midazolam) for severe agitation or seizures—these do NOT worsen anticholinergic toxicity 1, 5
• Cardiac monitoring for dysrhythmias (diphenhydramine causes sodium channel blockade) 3

Antidotal Therapy: Physostigmine

Physostigmine is the specific antidote for anticholinergic delirium and hallucinations 6, 7, 4. It is a reversible acetylcholinesterase inhibitor that crosses the blood-brain barrier and directly reverses central anticholinergic effects.

Dosing:

• Adults: 1-2 mg IV slowly over 5 minutes
• Pediatric: 0.02 mg/kg IV slowly
• May repeat if symptoms recur 4

When to use physostigmine:

• Severe anticholinergic delirium with hallucinations
• Agitation requiring chemical restraint
• To avoid intubation in patients with altered mental status 6

Alternative During Physostigmine Shortage: Rivastigmine

If physostigmine is unavailable (national shortages occur), rivastigmine is an alternative acetylcholinesterase inhibitor 6, 7:

• Oral/NG: 3 mg every hour until symptom resolution
• Transdermal patch: 9.5 mg patch for sustained effect over 24 hours
• Provides consistent drug absorption without aspiration risk
• Does not cause muscarinic toxicity when used appropriately 6

Key Clinical Distinctions

Anticholinergic Delirium vs. Primary Psychosis

The hallucinations in diphenhydramine toxicity are:

• Part of a constellation of anticholinergic signs (fever, dry skin, mydriasis, tachycardia, urinary retention)
• Typically visual and tactile rather than auditory
• Associated with disorientation and fluctuating consciousness
• Resolve with anticholinergic reversal, not antipsychotics

When Antipsychotics ARE Appropriate for Hallucinations

Antipsychotics are indicated for hallucinations in these contexts 8, 9:

• Primary psychiatric disorders (schizophrenia, bipolar disorder with psychosis)
• Delirium from non-anticholinergic causes (cancer, metabolic derangements)
• Alzheimer's disease with problematic delusions/hallucinations
• Alcohol or benzodiazepine withdrawal (though benzodiazepines are first-line)

Common Pitfalls to Avoid

1. Assuming all hallucinations require antipsychotics: Always identify the underlying cause first
2. Missing anticholinergic toxidrome: Look for the complete picture—fever, dry skin, mydriasis, tachycardia, urinary retention
3. Using diphenhydramine as a sedative in agitated patients: This is explicitly contraindicated in anticholinergic toxicity 1
4. Combining multiple anticholinergic agents: Many medications have anticholinergic properties (antihistamines, antipsychotics, tricyclic antidepressants)
5. Delaying physostigmine due to unfamiliarity: It is safe and effective when anticholinergic toxicity is confirmed 6, 7, 4

Monitoring Requirements

Patients with diphenhydramine overdose require:

• Continuous cardiac monitoring (QTc prolongation risk) 1, 3
• Serial vital signs (temperature, heart rate, blood pressure)
• Neurological assessments (mental status, pupil size)
• Urinary output monitoring
• Observation for seizures (sodium bicarbonate for wide QRS) 3

Bottom line: Antipsychotics are not a blanket treatment for all hallucinations. In anticholinergic toxicity from diphenhydramine overdose, they are contraindicated. Use benzodiazepines for agitation and physostigmine (or rivastigmine) for definitive reversal of anticholinergic delirium and hallucinations.