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What is If a patient is absolutely allergic to LA, the specific drug used in place of it is: and explain in detail?
If the patient is allergic:

To ester local anesthetics, try an amide local anesthetic.
To amide local anesthetics, try an ester local anesthetic.
To methylparaben or is allergic to both amide and ester local anesthetics, try a MPF product.
To antioxidants or sulfite compounds, review the product ingredients.

Some  drugs  like pheniramine,  promethazine,  diphenhydramine have  strong  while others  have  weak  membrane stabilizing  property.
MPF= Methyl paraben Free 
Reference : KDT pharmacology 8th ed page no 180