What is the toxic dose of amitriptyline (in mg/kg) for adults and children?

Toxic Dose of Amitriptyline

Amitriptyline toxicity typically occurs at doses >5 mg/kg in children, with severe life-threatening toxicity (seizures, cardiac arrest) reported at doses >35 mg/kg, though rare cases of significant toxicity have occurred at doses as low as 0.8 mg/kg.

Dose Thresholds by Population

Children (≤6 years)

• Non-toxic threshold: <5 mg/kg 1, 2, 3

  • In a study of 48 children, none developed significant toxicity at doses <5 mg/kg 1
  • Of 44 children studied, doses ranged from 2-97.5 mg/kg, with most remaining asymptomatic at lower doses 2

• Severe toxicity threshold: ≥35 mg/kg

  • A 2-year-old who ingested 35 mg/kg developed coma, seizures, and cardiac arrest requiring 17 hours of CPR but ultimately recovered 4
  • One child who ingested 97.5 mg/kg died from status epilepticus 2
  • Another child who ingested 36 mg/kg died from cardiopulmonary arrest 2

Critical Caveat - Individual Variability

However, significant toxicity can occur at much lower doses in susceptible individuals. One 8-year-old developed depressed mental status and seizures after ingesting only 0.8 mg/kg, with therapeutic serum levels measured 18 hours post-ingestion 5. This challenges the previously accepted 5 mg/kg safety threshold and emphasizes that any amitriptyline ingestion warrants careful evaluation.

Adults

The FDA label does not specify a toxic dose threshold 6. Therapeutic dosing ranges from 75-300 mg/day (approximately 1-4 mg/kg/day for a 75 kg adult) 6. The wide variation in absorption and distribution makes plasma levels poorly correlated with therapeutic effect, and by extension, toxicity thresholds are similarly variable 6.

Clinical Manifestations of Toxicity

Most Common Findings (in order of frequency):

• Neurological: Lethargy (77%), coma (48%), seizures 2, 3
• Cardiac: Sinus tachycardia (58%), QRS prolongation, ventricular dysrhythmias 7, 3
• Laboratory: Hyperglycemia, leukocytosis, hyponatremia 2, 3

ECG Predictors of Severe Toxicity:

• QRS duration ≥100 ms: 100% positive predictive value for coma 3
• R wave in aVR ≥3 mm: 100% negative predictive value for seizures (i.e., if <3 mm, seizures unlikely) 3
• QTc prolongation occurs in 22% of cases 3

Management Principles

Sodium bicarbonate is the definitive treatment for life-threatening tricyclic antidepressant cardiotoxicity 7. The 2023 AHA guidelines provide Class 1, Level B-NR recommendation for sodium bicarbonate in TCA poisoning causing QRS prolongation, hypotension, seizures, or ventricular dysrhythmias 7.

Treatment Algorithm:

1. Immediate: Activated charcoal if presenting within 1-2 hours (though no outcome difference was found in one pediatric study 1)
2. For life-threatening toxicity: Hypertonic sodium bicarbonate boluses (1000 mEq/L in adults, 500 mEq/L in children) 7
3. Refractory shock: Consider VA-ECMO 7
4. Seizures: Benzodiazepines (diazepam first-line) 7

Key Pitfall

Do not use succinylcholine or mivacurium for intubation - these cholinesterase-metabolized neuromuscular blockers are contraindicated 7. The FDA label also warns that amitriptyline prescriptions should be written for the smallest quantity possible to reduce overdose risk 6.

Time Course

Toxicity typically manifests within 1 hour of ingestion 4, but symptoms can persist for days. Most children recover with supportive care within 4 days 2, though cardiac toxicity can be refractory for up to 17 hours even with aggressive management 4.