Prilocaine Methaemoglobinaemia — FRCA Primary MCQ

Correct answer: A — Prilocaine

Prilocaine is the local anaesthetic most strongly associated with methaemoglobinaemia. It is metabolised in the liver and kidneys to o-toluidine and related metabolites, which oxidise haemoglobin iron from the ferrous (Fe2+) to the ferric (Fe3+) state. Methaemoglobin cannot bind oxygen normally and also increases the oxygen affinity of remaining normal haem groups. Risk is dose-dependent but may occur after repeated smaller doses or at lower doses in infants, anaemia, haemoglobinopathies or with other oxidising drugs; therefore, 600 mg should not be treated as an absolute threshold. Lidocaine can rarely cause methaemoglobinaemia but is less strongly associated. Levobupivacaine, ropivacaine and bupivacaine are principally associated with neurological and cardiovascular systemic toxicity. Symptomatic acquired methaemoglobinaemia may be treated with intravenous methylthioninium chloride 1–2 mg/kg over 5 minutes, subject to contraindications such as G6PD deficiency.

Reference: Prilocaine hydrochloride 1% Solution for Injection, Summary of Product Characteristics, sections 4.4, 4.8 and 5.2, current UK SmPC accessed 2026-08-19. https://www.medicines.org.uk/emc/product/870/smpc