There are two types of local anesthetics: esters and amides.
Esters: procaine, tetracaine, novacaine, cocaine. Produce metabolite PABA.
Amides: lidocaine, bupivacaine. Preserved in MPB (structurally similar to PABA).
Pure amides, without MPB preservative (dental cartridge, single-dose ampules from resuscitation cart), will not cross react with esters.
Hence, in patient with history of novacaine allergy, use lidocaine without preservative from resuscitation cart.
Source
Roberts: Clinical Procedures in Emergency Medicine, 4th ed.
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