Archives for November 2014

5 More Reasons You Can’t Get Numb at the Dentist

In my recent post titled 5 Reasons You Can’t Get Numb at the Dentist, I laid out five reasons why this phenomenon occurs. This was based on years of experience as well as my own research.

In writing that post, it became clear that there were more than five reasons to explain the “I couldn’t get numb” occurrence.  So here are five more reasons why some patients experience this difficulty.

6. You have a Hot Tooth

Flames representing a hot tooth

Hot Teeth are tough to get numb!

If you are in severe pain in a specific tooth, it can be much more challenging to get you numb. This phenomenon, known by dentists as a hot tooth, produces these challenges in a different way than having an infection (infection was Reason #1 in the the first post on this subject.)

So why does this happen? There are two major reasons. When the nerve of a tooth becomes inflamed (such as from a large cavity), the nerve actually changes and becomes more excitable. Stated another way, the nerves are much more easily stimulated, and thus way more local anesthetic is needed.

The other reason is that the cavity can also cause the nerve to develop even more “receptors.” While the specific details are not that important for this summary, it basically means that there are more nerves that need to be taken out by the local anesthetic.

The end result in these cases is that it can take several injections to get a hot tooth numb. And in rare cases, sometimes antibiotics and two days off are needed for the tooth to calm down enough to be properly anesthetized!

7. Your metabolism

The term metabolism is technically inaccurate. But it is the best term to describe that in some individuals, your vascular system removes the local anesthetic from the area of the tooth nerve much more quickly than in other individuals. This results in you being adequately numb for only a short period of time, or simply not being numb enough at all. In either case, you end up feeling pain during the dental procedure.

Modern dental anesthetics are primarily metabolized in the liver. So any dentist who says you are metabolizing the local anesthetic too quickly is technically incorrect. But that will be addressed in another blog post.

8. I just used expired local anesthetic!

Dental lidocaine photo about to expire

Box and carpule of lidocaine. Yes, it can expire!

I can personally vouch that this has never happened to me. My assistants and I check the expiration date of each and every cartridge before injection. However, local anesthetic can and will expire. The average shelf life of local anesthetic is 12 to 18 months.

What happens if it expires? I really don’t know – but like any expired medication, it will likely begin to break down and result in a decrease in efficacy.

9. I missed

In many cases, in order to achieve adequate numbness, a dentist has to do a nerve block.

Femoral nerve block with an ultrasound

Ultrasound guided nerve block. Courtesy wikipedia.

Nerve blocks are not unique to dentistry. The photo to the left shows a femoral nerve block being performed by an anesthesiologist. In this case, the anesthesiologist is using an ultrasound to help guide the needle. This is done because inserting the needle and guiding it to the nerve is difficult.

Dentists do not have the luxury of using an ultrasound. Nor is it practical. For many nerve blocks, we have to insert a needle one and a half inches deep and rely on several anatomic indicators to help guide us.

Ted Williams was the last baseball hitter to hit above .400. That means 60% of the time, he didn’t get a hit. Most dentists have a batting average of around .900 for nerve blocks. So that means, just like Ted Williams, we sometime  swing and miss! So we have to inject again. And sometimes again. It’s part of doing nerve blocks… you don’t always hit a home run on the first at bat.

10. You’re Nervous

It is not a surprise that many people hate coming to the dentist. That it is a fact. Many people get very nervous coming to the dentist. That is a fact as well.

nervous people are tough to get numb

Dentistry is not a walk on the beach for most!

There is very little research to support my observation (and similar observations of other dentists) that patients who are very nervous tend to be more difficult to get numb.

Over the years, I’ve developed many theories for this. Many patients will forget to breathe and/or hold their breath. They may also clench their jaw muscles. Any of these will change the blood chemistry in a way that makes the local anesthetic less effective (similar to the blood chemistry changes I outlined in Point 1).

Since I do IV and Oral sedation dentistry, I’ve observed that patients who are sedated are much more easily numbed. This also suggests that nervousness and anxiety can make you more likely to experience pain.

The end result is that a nervous patient = a patient who may be difficult to get numb.

An eleventh and important reason – Ehlers Danlos – has its own post here

Do any patients or dental professionals reading this have other reasons? I have another list going but it is not at 5 yet. Leave a comment or email me at