Benzocaine, a local anesthetic in the ester class, can cause an overdose primarily through topical or oral exposure, leading to systemic absorption and methemoglobinemia—a condition where the iron in hemoglobin is oxidized from Fe²⁺ to Fe³⁺, forming methemoglobin that impairs oxygen binding and delivery to tissues. 47 14 38 This results in tissue hypoxia despite normal oxygen saturation readings on standard pulse oximetry. Additional effects may include central nervous system depression (e.g., seizures, coma) and cardiovascular instability (e.g., hypotension, bradycardia, arrhythmias) if absorption is high enough to trigger local anesthetic systemic toxicity (LAST). 47 7 Overdose typically occurs from excessive topical application (e.g., sprays or gels for oral/throat pain, endoscopy premedication), accidental ingestion of products like teething gels or lozenges, or intentional misuse. 37 36 Symptoms usually appear within minutes to hours and include cyanosis (bluish skin/lips), headache, dizziness, fatigue, tachycardia, dyspnea, confusion, seizures, coma, and potentially death if methemoglobin levels exceed 70%. 47 22 Risk factors include young age (infants/children), elderly status, enzyme deficiencies (e.g., G6PD or methemoglobin reductase), concurrent use of other oxidants (e.g., nitrates, dapsone), and application to damaged skin/mucosa. 47 17 The response can be idiosyncratic, with some individuals affected at low doses while others tolerate higher amounts. 16
Direct human LD50 data for benzocaine is unavailable due to ethical limitations, with estimates derived from animal studies, case reports, and toxicological databases. 9 47 Oral LD50 in rats is consistently reported as 3042 mg/kg, with similar values in mice (2500 mg/kg) and rabbits (3042 mg/kg in some sources, though one outlier notes 1150 mg/kg). 1 3 8 9 0 Given the lack of human-specific LD50, animal data provides a rough extrapolation, though human toxicity (e.g., methemoglobinemia) often manifests at lower doses due to metabolic differences and idiosyncrasy. 5 60 Toxicological references like Poisindex suggest a threshold toxic dose of 22–40 mg/kg for methemoglobinemia onset, but lethal outcomes are rare and typically require higher exposures or vulnerability. 5 16 Case reports show severe effects (including fatalities) in infants at 15–25 mg/kg or higher, but adult deaths are uncommon and doses are often not quantified. 60 15 36 52 Using the rat oral LD50 as a conservative estimate for human extrapolation:
- Calculation: 3042 mg/kg × 110 kg ≈ 334,620 mg (or ~335 g).
This is a broad approximation, as actual lethality varies; symptoms may emerge at doses far below this, and prompt treatment (e.g., methylene blue) often prevents death. 47 18 This is not medical advice—consult toxicology experts or poison control for precise evaluations.
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