How local anesthesia works
What you need to know
- The nerve sends its messages as a tiny electrical current, and to do that it needs sodium to enter the fiber. The anesthetic blocks the sodium channels (the gates sodium enters through) and the signal is cut off.
- There are 2 families: esters (like benzocaine and tetracaine) and amides (lidocaine, mepivacaine, prilocaine, articaine and bupivacaine). The anesthetics injected in dentistry are amides.
- Benzocaine (an ester) is used mainly as a topical gel: it numbs only the surface, about 2 to 3 mm, so the needle prick is less uncomfortable.
- Many contain epinephrine (adrenaline), which is a vasoconstrictor: it slightly narrows the blood vessels in the area. That way the anesthetic passes into the blood more slowly: it lasts longer and the risk of toxicity goes down.
- For some patients, for example those with heart or thyroid disease, their physician is consulted before using an anesthetic with a vasoconstrictor.
- With all anesthetics, the gums, lip and tongue stay numb longer than the tooth.
- In an infected area (pus), anesthesia doesn't work as well: the infection makes the tissue acidic and the anesthetic doesn't get into the nerve well.
- No dental anesthetic works 100% of the time: sometimes it needs reinforcing.
Injectable anesthetics (AAPD)
| Anesthetic | Common formulation | Duration (minutes) |
|---|---|---|
| Lidocaine | 2% with 1:100,000 epinephrine | 90–200 |
| Articaine | 4% with 1:100,000 epinephrine | 60–230 |
| Mepivacaine | 3% without vasoconstrictor | 120–240 |
| Bupivacaine | 0.5% with 1:200,000 epinephrine | 180–600 |
Terms in plain words
Local anesthesiaLoss of sensation in just one part of the body, without putting the person to sleep.
Sodium channelGate in the nerve fiber where sodium enters so the signal can travel.
VasoconstrictorSubstance that slightly narrows the blood vessels, like epinephrine.
Topical anesthesiaAnesthetic gel or spray applied to the mucosa before the needle prick.
For the exam
Why is epinephrine added to the anesthetic?
Show answerHide answer
To slightly narrow the blood vessels in the area: the anesthetic passes into the blood more slowly, lasts longer and the risk of toxicity goes down.
Sources
- American Academy of Pediatric Dentistry (AAPD). Use of Local Anesthesia for Pediatric Dental Patients. Reference Manual of Pediatric Dentistry, 2025:407–14. View source
- Badr N, Aps J. Efficacy of dental local anesthetics: A review. J Dent Anesth Pain Med. 2018;18(6):319–332. View source
More from Anesthesia
Anesthesia
Inferior alveolar nerve block
It numbs the lower teeth on that side, the lip and chin, and almost always the tongue too.
Nerves
Which nerve do you numb for each tooth?
To numb a tooth, you need to know which nerve gives it sensation: for the upper teeth it's the superior alveolar nerves, and for the lower teeth, the inferior alveolar nerve.
Safety
Safe anesthesia: aspiration, dosage and complications
Before injecting, you aspirate so you don't put the anesthetic into a blood vessel, you inject slowly, and you never go over the maximum dose based on weight.