Esters
Once absorbed into the bloodstream, ester local anesthetics are distributed throughout the body. Highly perfused organs like the brain, liver, kidneys, and heart accumulate higher concentrations of the drug early on, increasing the potential for systemic effects. However, because esters are rapidly hydrolyzed by plasma cholinesterase, their duration of action is generally short. The rapid breakdown of esters limits the extent of their systemic distribution, thereby reducing the likelihood of toxicity, except in individuals with cholinesterase deficiencies.
Amides
Amide local anesthetics, due to their slower metabolism in the liver, are more widely distributed in the body after absorption. Like esters, they initially accumulate in highly perfused organs, but their prolonged circulation due to slower clearance means that amides can maintain effective tissue concentrations for longer periods. Amides also tend to bind more extensively to plasma proteins, which contributes to their prolonged duration of action. Additionally, because of this prolonged distribution, amides have a higher risk of central nervous system (CNS) and cardiovascular toxicity if systemic levels become too high, especially in patients with impaired hepatic function.
