Hyoscyamine Overdose Range
Any dose exceeding 12 mL (1.5 mg) in 24 hours for adults or 6 mL (0.75 mg) in 24 hours for children aged 2-12 years constitutes an overdose, with pediatric overdose thresholds varying by weight for children under 2 years. 1
Specific Overdose Thresholds by Age Group
Adults and Children ≥12 Years
- Maximum safe dose: 12 mL (1.5 mg) in 24 hours
- Overdose: Any amount exceeding this threshold 1
Children 2 to <12 Years
- Maximum safe dose: 6 mL (0.75 mg) in 24 hours
- Overdose: Any amount exceeding this threshold 1
Infants and Children <2 Years (Weight-Based)
The overdose threshold is defined by exceeding the following 24-hour maximums based on body weight 1:
- 3.4 kg (7.5 lb): Maximum 24 drops (0.03 mg) - overdose if exceeded
- 5 kg (11 lb): Maximum 30 drops (0.0375 mg) - overdose if exceeded
- 7 kg (15 lb): Maximum 36 drops (0.045 mg) - overdose if exceeded
- 10 kg (22 lb): Maximum 48 drops (0.06 mg) - overdose if exceeded
Clinical Context from Reported Cases
A documented pediatric case involved 108 μg/kg of hyoscine hydrobromide (a related anticholinergic) in a 4-year-old, resulting in confusion, ataxia, hallucinations, hypotension, tachycardia, and QTc prolongation 2. This provides real-world evidence that doses substantially exceeding therapeutic ranges produce significant anticholinergic toxicity.
Important Clinical Considerations
The margin of safety is particularly narrow in young children. Even modest excesses above the weight-based maximums can produce antimuscarinic toxicity including:
- Agitation and hallucinations (most common)
- Mydriasis and tachycardia
- Seizures in severe cases
- Acute kidney injury (possibly from reduced renal perfusion secondary to tachycardia) 3
Critical pitfall: Hyoscyamine products are often attractively packaged and easily accessible over-the-counter, making unintentional pediatric ingestions common 2. The dropper markings require careful attention, as small volume errors translate to significant dose deviations in infants.