[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Dentistry](https://durgical.com/en/dentistry/) › [Drugs in dentistry](https://durgical.com/en/dentistry/areas/drugs/) › Local anaesthetic toxicity and methaemoglobinaemia # 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. Updated Nov 12, 2026 ## Key points - LAST: brain first (taste, tinnitus, seizures), then heart. - Lipid emulsion for severe LAST. - Cyanosis unresponsive to oxygen after prilocaine or benzocaine: methaemoglobinaemia. ## 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. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/dentistry/local-anaesthetic-toxicity-and-methaemoglobinaemia/) ## Connections ## Related 3 - [Local anaesthetics in dentistry](https://durgical.com/en/dentistry/local-anaesthetics-in-dentistry/) - [Medical emergencies in the dental chair](https://durgical.com/en/dentistry/medical-emergencies-in-the-dental-chair/) - [Dental nerve blocks and infiltration: the anatomy behind them](https://durgical.com/en/dentistry/dental-nerve-blocks-and-infiltration-the-anatomy-behind-them/) ## In Pharmacology 7 - [Lidocaine](https://durgical.com/en/pharmacology/lidocaine/) - [Bupivacaine](https://durgical.com/en/pharmacology/bupivacaine/) - [Prilocaine](https://durgical.com/en/pharmacology/prilocaine/) - [Benzocaine](https://durgical.com/en/pharmacology/benzocaine/) - [Methylthioninium chloride (methylene blue)](https://durgical.com/en/pharmacology/methylthioninium-chloride/) - [Benzodiazepines](https://durgical.com/en/pharmacology/classes/benzodiazepines/) - [Midazolam](https://durgical.com/en/pharmacology/midazolam/) [← All of dentistry](https://durgical.com/en/dentistry/) 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](https://www.ncbi.nlm.nih.gov/books/NBK499964/) (StatPearls Publishing (NCBI Bookshelf), 2026); [Methemoglobinemia](https://www.ncbi.nlm.nih.gov/books/NBK537317/) (StatPearls Publishing (NCBI Bookshelf), 2025); [Methemoglobinemia - acquired](https://medlineplus.gov/ency/article/000561.htm) (MedlinePlus, US National Library of Medicine, 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/dentistry/local-anaesthetic-toxicity-and-methaemoglobinaemia/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/dentistry/local-anaesthetic-toxicity-and-methaemoglobinaemia/