Physostigmine Dosing for Diphenhydramine (Benadryl) Overdose
For diphenhydramine overdose with anticholinergic delirium, administer physostigmine 0.5-2 mg IV slowly over 2-5 minutes (no faster than 1 mg/minute), with repeat doses of 1-2 mg every 5-10 minutes as needed until symptoms resolve or a maximum of 2 mg total is reached in children, or up to 4 mg total in adults. 1
Dosing Algorithm
Adults
- Initial dose: 1-2 mg IV administered slowly over at least 2 minutes (maximum rate: 1 mg/minute) 1
- Repeat dosing: If anticholinergic symptoms persist and no cholinergic effects appear, repeat 1-2 mg every 5-10 minutes 2
- Maximum: Most patients respond to less than 2 mg total; rarely need more than 4 mg 2
Pediatrics
- Initial dose: 0.02 mg/kg IV or IM, administered no faster than 0.5 mg/minute 1
- Repeat dosing: If toxic effects persist without cholinergic signs, repeat at 5-10 minute intervals 1
- Maximum: 2 mg total 1
Critical Administration Details
Slow IV push is mandatory - rapid administration causes bradycardia, hypersalivation with respiratory compromise, and possible seizures 1. The FDA label explicitly warns against exceeding 1 mg/minute 1.
Redosing Frequency
Most patients (69%) require only a single dose of physostigmine 3. When redosing is needed:
- 31% of patients require multiple doses 3
- Time between doses: typically less than 5.5 hours 3
- Redosing beyond 6.5 hours from initial dose is rarely necessary 3
If symptoms recur repeatedly, consider physostigmine continuous infusion at 2 mg/hour for severe, persistent cases 4, though this is uncommon.
Expected Response
- Onset: Improvement in delirium, hallucinations, and agitation within minutes 2, 5
- Success rate: 74-79% of patients achieve symptom control 2, 5
- Diphenhydramine-specific response: 64% complete response rate (lower than other anticholinergics at 100%) 2
Safety Profile
Physostigmine has an excellent safety profile when used appropriately 2, 5:
- 95% of patients experience no adverse effects 2
- Adverse events (when they occur): emesis (2.1%), QTc prolongation (1%), seizures (1%) 2
- Contraindication: Do NOT use in tricyclic antidepressant overdose due to risk of asystole and seizures 4
Signs to Stop or Reduce Dosing
Terminate physostigmine if cholinergic crisis develops 1:
- Excessive salivation
- Emesis
- Urination
- Defecation
- Bradycardia
Reduce dose if 1:
- Excessive sweating
- Nausea
Disposition Considerations
- 45% of patients receiving one dose can be discharged directly from the ED 3
- 36% receiving multiple doses can still be discharged from ED 3
- Number of physostigmine doses does not significantly impact total hospital length of stay 3
Alternative When Physostigmine Unavailable
During physostigmine shortages, transdermal rivastigmine 9.5 mg/24-hour patch has shown efficacy as an alternative centrally-acting acetylcholinesterase inhibitor 6, 7. However, this is off-label and should only be considered when physostigmine is truly unavailable. Rivastigmine has slower onset but longer duration of action without aspiration risk 7.
Common Pitfalls to Avoid
- Do not administer too rapidly - this is the primary cause of adverse effects 1
- Do not use benzodiazepines as first-line - physostigmine is superior to non-antidote therapy (6.6 times greater odds of delirium control) 5
- Do not assume all anticholinergic overdoses respond equally - diphenhydramine has lower response rates than other anticholinergics 2
- Do not continue redosing beyond 6.5 hours - if symptoms persist this long, reconsider the diagnosis or consider continuous infusion 3