In this episode, I’ll discuss how I deal with allergic reactions to local anesthetics. I’ll also include a tip that will help you never forget how to tell whether a local anesthetic is ester or amide-based.
Local anesthetic agents such as procaine and lidocaine can rarely cause allergic reactions. The allergic reactions may be due to the drugs themselves or to excipients such as methylparaben.
Local anesthetics can be divided into two different structural groups – esters and amides.
The ester-based local anesthetics are: procaine, tetracaine, and chloroprocaine
The amide-based local anesthetics are: lidocaine, bupivacaine, mepivacaine, and ropivacaine
Most allergic reactions are due to the ester-group of local anesthetics. The reaction is most often due to para-aminobenzoic acid, which is a metabolite of ester-based local anesthetics. Para-aminobenzoic acid is also structurally similar to the preservative methylparaben.
A patient who is allergic to methylparaben can appear to be allergic to both ester and amide-based local anesthetics. This is because many local anesthetic formulations contain methylparaben.
For this reason, many manufacturers of local anesthetics now make methylparaben-free (MPF) formulations.
If I need to use a local anesthetic in a patient with an allergy to local anesthetics, I choose a methylparaben free agent in the opposite class. For example, if a patient is allergic to lidocaine then I will use a methylparaben free ester-based local anesthetic. If a patient is allergic to procaine, I will use a methylparaben free amide-based local anesthetic.
How to tell an amide from an ester
A pharmacist once taught me this trick to remember how to tell whether a local anesthetic is an ester or an amide:
Look at the generic name of the local anesthetic. If it contains 2 of the letter “i” then it is an amide. If it contains only 1 letter “i” then it is an ester. Lidocaine, bupivacaine, mepivacaine, and ropivacaine all contain two of the letter “i” and are the amide anesthetics. I have no idea if this is by coincidence or it is a rule that will carry on for future local anesthetics. So, if a new local anesthetic comes to market, verify if this rule still holds before applying it.
If you like this post, check out my book – A Pharmacist’s Guide to Inpatient Medical Emergencies: How to respond to code blue, rapid response calls, and other medical emergencies.