Local anaesthetic toxicity and methaemoglobinaemia
- Drugs
Too much anaesthetic, or an injection into a vessel, first excites the brain and can then stop the heart. Prilocaine and benzocaine can turn blood brown.
What it is
Local anaesthetic systemic toxicity (LAST) happens when blood levels rise too high, from an intravascular injection, a total amount too large for body weight, or reduced clearance. Early signs are neurological: numbness around the mouth, a metallic taste, tinnitus, dizziness, slurred speech, agitation and muscle twitching, then seizures and coma. Cardiovascular toxicity follows or, with bupivacaine, can come first: hypertension and tachycardia, then bradycardia, arrhythmias and cardiac arrest. Methaemoglobinaemia is a separate problem: prilocaine and benzocaine oxidise haemoglobin to methaemoglobin, which cannot carry oxygen, causing cyanosis that does not improve with oxygen.
What to know
- LAST: circumoral numbness, metallic taste, tinnitus, agitation, then seizures and cardiac arrest.
- Bupivacaine is the most cardiotoxic; children and older adults are more vulnerable.
- Treatment: stop the injection, call for help, airway and oxygen, a benzodiazepine for seizures, intravenous lipid emulsion for severe toxicity, prolonged resuscitation.
- Methaemoglobinaemia: cyanosis and pulse oximetry around the mid-80s despite oxygen, chocolate-brown blood.
- Methylene blue treats methaemoglobinaemia but is contraindicated in G6PD deficiency.
Risks and precautions
- Aspirating before injecting and injecting slowly prevent most intravascular injections.
- Mistaking early LAST for anxiety delays treatment.
- Infants are especially susceptible to methaemoglobinaemia from topical benzocaine; avoid it in children under two years.
Next step / Initial management
- Calculate the safe maximum for weight before treating small children and use the fewest cartridges needed.
- Suspected LAST or methaemoglobinaemia: stop the drug, oxygen, call emergency services and refer to hospital; lipid emulsion and methylene blue are given there by protocol.
- After any toxic reaction, document it and refer to review the cause before the next treatment.
Test yourself
Think of the answer, then tap the question to see it.
What are the early signs of local anaesthetic systemic toxicity?
Circumoral numbness, metallic taste, tinnitus, dizziness and agitation, before seizures.
What specific treatment is used for severe local anaesthetic systemic toxicity?
Intravenous lipid emulsion, with airway support, seizure control and prolonged resuscitation.
What is the antidote for methaemoglobinaemia and when is it contraindicated?
Methylene blue; it is contraindicated in G6PD deficiency.
Connections
Educational content; it does not replace assessment by a dentist or doctor and is not a clinical instruction. For facial or neck swelling with difficulty swallowing or breathing, a knocked-out tooth or bleeding that will not stop, go to an emergency department at once.
Sources: Local Anesthetic Toxicity (StatPearls Publishing (NCBI Bookshelf), 2026); Methemoglobinemia (StatPearls Publishing (NCBI Bookshelf), 2025); Methemoglobinemia - acquired (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.