I had a couple fillings done today at the dentist. One on the top and one on the bottom. The top filling went fine with no issues.

For the second filling, the first injection of local anesthesia didn't work at all. After I felt severe pain she did a second injection. This time I noticed some mild effect, but it still hurt severely. A third one didn't help.

We persevered and the filling was (painfully) completed. But I'm curious how common this is? and what are the possible causes?

I'd never experienced local anesthesia not working before, and I'd be lying if I said it hasn't made me more hesitant to go to the dentist in future due to the fear it wont work again.

Note: This is not in anyway me seeking medical advice. I'm just genuinely curious why this might have happened.

  • One possibility no one mentioned (but it was alluded to) is the pH of the tissue. In the presence of infection which alters local pH (e.g. abscess), lidocaine is often ineffective. However, your dentist would have told you if you had an abscess. Commented Jun 7, 2022 at 16:38

3 Answers 3


This depends on the type of anesthetic used, there are plenty:

Types of anesthetic

which include:

  • Esters (Benzocaine, Cyclomethycaine, Propoxycaine)
  • Amides (Bupivacaine, Mepivacaine, Trimecaine)
  • Naturals (Saxitoxin, Tetrodotoxin, Spilanthol).

This link:

How to overcome failed local anaethesia

is dental specific, and lists possible causes of local anesthetia failure:

  • Operator (wrong anaesthetic, poor technique)
  • Anatomical (inconsistent foramina, multiple nerve supply, cortial barrier)
  • Pathological (reduced jaw opening, inflammation)
  • Psychological (fear, anxiety, stress).

Any general statistics won't mean much as it depends very much on the patient's personal condition and the implementation and type of anesthetic used.


In addition to JonMark Perry's excellent answer it's also worth noting that many of us aren't wired the same as everyone one else.

When your dentist tries to numb a particular tooth, he or she is trying to numb the particular nerve, or group of nerves, that innervates that tooth. Those nerves may not be in the same location as the next patient. This blog by Nicholas Calcaterra DDS mentions this as the fourth of five reasons you may not be numbed. The most pertinent passage:

4. You’re wired differently
The human body is incredibly variable. People are double jointed. Remember the kid in grade school who could move his ears? Why is Usain Bolt faster then any other human? You get the picture.

Each person is different! Most people have what I might call “standard anatomy.” This means that the nerves going to your teeth are where you might expect them to be located. But just like Usain Bolt and the kid from fifth grade who could move his ears, some patients have extreme variability with the nerves going to their teeth. We see this most frequently with lower molars. Some people may have up to 4 nerves going to their lower molar teeth. This can mean 4 different injections to get them numb! This doesn’t mean your dentist is incompetent – it means you’re wired differently. So if that happens to you, just think about Usain Bolt and the kid from fifth grade who could move his ears.

I had the same thing happen to me a very long time ago and just grinned and bore it; it was temporary, after all, and it has never happened since. My dentist then said he just couldn't locate the nerve going to that particular tooth. And in the citation above, if the dentist doesn't get all four of those nerves going to the molar in question, you won't be completely numbed.

  • This should be the answer. I'd like to add that the fundamental problem is that nerve location/configuration and body chemistry may vary widely by person, therefore building a local anesthetic solution that works for everyone all the time is largely doomed to become an exercise in futility. Think of it as having to write a program that works on every possible architecture and operating system out there. There's always going to be the off-chance it fails on some oddball system you'd never have predicted that doesn't implement industry standards or lacks critical functionality. Commented Mar 31, 2019 at 3:29

In addition to the excellent answers, there's another, and that's regarding your metabolism. The most commonly used local anesthetics are metabolized by the liver, specifically the P450 system. If you're a fast metabolizer, your body degrades the medication, and its effects, quickly. The opposite is true for a slow metabolizer. As a physician who sutures wounds, I've learned that some patients require a more potent strength of a longer lasting anesthetic than others to get the same effect.

  • 1
    While this may apply to IV meds, it should not affect local anesthesia. The lidocaine (for example) used to block a nerve doesn’t get into circulation to get metabolized before the procedure is done… and certainly not if epi is used as well. (I’ve also sutured too many wounds to count and never encountered this problem. Commented Jun 7, 2022 at 16:33

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