Local anesthetics work by blocking the transmission of nerve signals. Medications used for local anesthesia differ in how strong they are (potency), how quickly they take effect (onset of action), and how long they last (duration of action).

Low concentrations of local anesthetics block only pain (sensory) signals but not the nerve signals that control movement (motor activity). This means you may still be able to move the part of your body that has been affected. Higher concentrations of local anesthetics block both sensory and motor nerve signals.

Local anesthetics are usually injected into the part of the body that needs to be anesthetized. They are not usually injected into the bloodstream (intravenously, IV).

Two groups of medications are commonly used for local anesthesia.

• Amino amides:
  • Procaine (Novocain)
  • Chloroprocaine (Nesacaine)
  • Tetracaine (Pontocaine)
• Amino esters:
  • Lidocaine (Xylocaine)
  • Bupivacaine (Marcaine)
  • Mepivacaine (Carbocaine)
  • Prilocaine
  • Ropivacaine

Amino esters may cause allergic reactions in some people. Amino amides rarely cause allergic reactions.

Local anesthetics are commonly mixed with medications that restrict blood flow through tissues by narrowing blood vessels (vasoconstrictors), such as epinephrine. This is done so the anesthetic may be more slowly absorbed from the tissues into the bloodstream and remain close to the site of injection. This prolongs the effect of the anesthetic at the site where it is injected and helps prevent toxic effects on the rest of the body. It also helps reduce bleeding at the surgical site. However, these vasoconstrictors should not be used in parts of the body where restricted blood flow may lead to a loss of blood supply and tissue damage (necrosis), such as in the fingers, toes, nose, ears, and penis.

A local anesthetic may also be applied directly to the skin or mucous membranes as a cream or ointment, such as an EMLA cream. This is called topical anesthesia. Topical local anesthetics are used for very minor surgery on the surface of the skin.