How does reduced blood flow make a local anesthetic last longer?

The duration of a local anesthetic's effect is determined by how quickly the drug is dispersed, metabolized, and excreted from the injection site. Because the dispersal of the anesthetic agent from the site of administration depends on local blood flow, reducing that flow keeps the drug concentrated at the target area for a longer period.1

To achieve this, clinicians sometimes add epinephrine to the anesthetic solution. Epinephrine induces local vasoconstriction—the narrowing of blood vessels—which restricts blood flow in the region and effectively prolongs the action of the anesthetic.1