Best Antiemetics for Anticholinergic and Acetaminophen Poisoning
For anticholinergic poisoning, 5-HT3 receptor antagonists (particularly ondansetron) are the preferred antiemetics, while for acetaminophen poisoning, ondansetron should be used as a second-line agent after conventional antiemetics unless N-acetylcysteine administration is approaching the 8-hour critical window.
Antiemetics for Anticholinergic Poisoning
First-line Options
- 5-HT3 receptor antagonists (ondansetron, granisetron, dolasetron, palonosetron) are preferred as they have minimal anticholinergic properties and will not worsen the anticholinergic toxidrome 1
- Ondansetron is particularly advantageous as it does not cause extrapyramidal side effects or sedation that could mask or be confused with symptoms of anticholinergic poisoning 2, 3
- Standard dosing: ondansetron 8 mg oral twice daily or 8 mg/0.15 mg/kg IV; palonosetron 0.50 mg oral or 0.25 mg IV; granisetron 2 mg oral or 1 mg/0.01 mg/kg IV 1
Second-line Options
- Dexamethasone 8 mg oral or IV can be used alone or in combination with 5-HT3 antagonists for enhanced efficacy 1
- Lorazepam (0.5-2.0 mg every 4-6 hours orally, IV, or sublingual) can be added to any antiemetic regimen, particularly for anxiety-related nausea 4
Antiemetics to Avoid
- Phenothiazines (e.g., prochlorperazine), antihistamines, and metoclopramide should be avoided as they have significant anticholinergic properties that would worsen the anticholinergic toxidrome 5
- These agents could potentially exacerbate symptoms such as tachycardia, hyperpyrexia, mydriasis, urinary retention, and decreased secretions 5
Antiemetics for Acetaminophen Poisoning
First-line Options
- Conventional non-ondansetron antiemetics can be effective in approximately two-thirds of acetaminophen toxic patients with vomiting 6
- Control of vomiting is critical to ensure oral N-acetylcysteine can be retained and effectively administered 7, 6
Second-line Options
- Ondansetron should be used as a second-line agent if first-line antiemetics fail 6
- Ondansetron should be considered as first-line when N-acetylcysteine administration is approaching the critical 8-hour window, due to its lower failure rate (16.7% vs 33.3% for non-ondansetron antiemetics) 6
- Dosing: ondansetron 0.15 mg/kg IV has been shown effective in preventing emesis following N-acetylcysteine administration 2
Special Considerations
- Dilution of oral N-acetylcysteine minimizes its propensity to aggravate vomiting 7
- Patients at risk of gastric hemorrhage should be evaluated for the risk of upper gastrointestinal bleeding versus hepatic toxicity before administering oral N-acetylcysteine 7
Clinical Pearls and Pitfalls
- Key Pitfall: Using antiemetics with anticholinergic properties (phenothiazines, antihistamines) in anticholinergic poisoning will worsen the toxidrome 5
- Key Pitfall: Delaying effective antiemetic therapy in acetaminophen poisoning may necessitate switching to IV N-acetylcysteine, which is more costly and may not be immediately available 6
- Clinical Pearl: Ondansetron has been successfully used in other toxicological emergencies such as theophylline poisoning when other antiemetics have failed 8
- Clinical Pearl: The combination of a 5-HT3 antagonist with dexamethasone provides superior antiemetic effect compared to either agent alone 9
Monitoring and Follow-up
- Monitor for resolution of nausea and vomiting to ensure successful administration of antidotes (physostigmine for anticholinergic poisoning, N-acetylcysteine for acetaminophen poisoning) 5, 7
- Assess for potential side effects of antiemetics, particularly headache and constipation with 5-HT3 antagonists 9
- For acetaminophen poisoning, if vomiting persists despite optimal antiemetic therapy, consider switching to intravenous N-acetylcysteine administration 7, 6