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 nick@directionsindentistry.net.

5 Reasons You Can’t Get Numb at the Dentist

As a dentist in private practice, I hear stories ALL the time about patients who “couldn’t get numb at the dentist.” Not surprisingly, many of the comments that readers post here on this site also deal with this very issue.

In response to this, I decided to compile a Top 5 list of reasons for why this happens. So here goes:

1. You have an infection
dental abscess on gums showing infection with pus

This tooth was difficult to get numb because of the infection in the gums above the tooth!

Sometimes, a patient comes in with an active infection and it can be difficult to get them completely numb. This is obviously very frustrating for both the dentist and the patient. Why does this happen?

An active dental infection will usually result in the presence of pus. In most cases, the pus is acidic. Conversely, dental anesthetics (lidocaine, novcaine, etc.) function best in slightly basic environments. The end result is that the unique chemistry of the infection “deactivates” the local anesthetic, making it so that more anesthetic is needed. And in cases of severe infections, sometimes you simply cannot get the patient 100% numb.

2. You moved during the injection

Let’s face it – some injections – but not all – hurt! And it just so happens that the one that hurts the most is the one that requires the most patient cooperation. I speak from experience having done this well over ten thousand times.

large dental needle for nerve blocks to get numb

We have to sink this needle deep into the tissue. If you move, it is easy to miss the target. Paper clip is for scale.

For lower back teeth, we nearly always need to do a nerve block.  This is where we have to go deep through muscle and other tissue and deposit the local anesthesia near the nerve. We can’t actually see the nerve – we have to use various anatomical landmarks to guide us to the area.

If you move suddenly, the needle will also move.  Most often, it will have moved away from the nerve! So we then end up depositing the anesthetic farther away from the nerve than we would like. What happens next? You don’t get numb. Fortunately, if we have to administer a second injection, it will rarely hurt, and then we can place it spot on.

On rare occasions – and this has never happened to me nor most dentists – you can move so much that the needle can actually break!

3. I am not using epinephrine

I’ve blogged about this before. Epinephrine is added to dental local anesthetics because it enhances the numbness. How does it do this? Epinephrine acts as a vasoconstrictor and reduces blood flow in the area of the injection. The end result is that the local anesthetic stays around much longer and gives a more profound feeling of numbness.

marcaine dental anesthetic with epinephrine

Marcaine with epinephrine. The epinephrine will allow for a more profound level of local anesthesia

In certain circumstances, we use a local anesthetic that does not contain epinephrine. Why? Patients with certain cardiac conditions or who take certain medications are best served with one that does not contain it. Others have experienced a mild adverse reaction (some mistakenly think they are allergic to epinephrine) and prefer we don’t use it. A small fraction of patients are allergic to the sulfite preservative so we can’t use it in those cases either.

If we can’t use epinephrine, there is a chance you won’t feel numb enough. Or we’ll have to re-inject multiple times.

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.

Do you think your nerves look like the drawing below?

trigeminal nerve anatomy variation can make dental numbing challenging

Nerves going to lower teeth. Each person is different! Image courtesy wikipedia commons.

If you answered yes, then you’re probably wrong!

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.

5. You have red hair

Joan from Mad Men has red hair and can't get numb at the dentist

Joan from Mad Men would have difficulty getting numb!

I’ve blogged about this on two separate occasions – here and here.

But to summarize, people with red hair have a built in resistance to local anesthetics. This means that more local anesthetic is required to achieve profound numbness in those people with red hair. The reason behind this is complex, but the genetics for red hair also confers resistance to local anesthetic.

I can say definitively that from personal experience, redheads nearly always require more local anesthetic. All of my red haired patients are aware of this – and we joke about it each and every visit!

Want to see reasons 6 through 10? Here they are!

Novocaine Allergy Part III – True Amide Allergies

This is the third installment in the Novocaine Allergy series. Part I talked about reactions to certain older style dental anesthetics which are basically not used any more in dentistry. Part II talked about allergic and sensitivity reactions to preservatives and other components found in some local anesthetics.

This now leaves us with what I call true allergies.  I use the term true to indicate it is a real allergic reaction to a dental local anesthetic – as opposed to an adverse reaction or other phenomenon. I stress this distinction because allergic reactions to modern local anesthetics are extremely rare.

Lidocaine – A Modern Dental Local Anesthetic

lidocaine local anesthetic with epinephrine used at the dentist

Lidocaine with Epinephrine is the most common formulation in the U.S.

Lidocaine was first synthesized in 1943 and became widely available in the United States in 1948. Lidocaine was based on a new chemical structure of local anesthetics called amides. This class is chemically different than the previous ones such as novocaine and cocaine.

Immediately after its introduction, lidocaine took off in popularity for many reasons. One of the reasons is because lidocaine did not cause allergic reactions the way older anesthetics did. Because of this, the older class of anesthetics – novocaine included – were phased out – and by the 1980s basically no dentists in the United States were using novocaine anymore.

Allergic to Lidocaine?

True allergies to lidocaine and other amide based anesthetics are exceedingly rare. There is conflicting evidence on the prevalence of these reactions to lidocaine and other anesthetics. A prominent 2009 article in the journal Anesthesiology says “allergic reactions to amide local anesthetics remain anecdotal.” A 2013 article published out of Saudi Arabia documents a case study of a true lidocaine allergy in a 12 year old. According to this 2002 paper, an individual with an allergy to one amide does not mean he/she will react to others. Any Google search will yield thousands of results – some from prominent medical journals with sterling reputations – and others from individuals posting their experiences and assumptions.

lidocaine and articaine both amide type local anesthetics

Most experts agree that in the cases of true allergies to lidocaine, other local anesthetics such as articaine can be used (assuming proper testing first).

The current consensus in the dental community is that true allergies to amide based local anesthetics are possible but very rare. As a patient, you have much better odds of being struck by lightning than experiencing an allergic reaction to lidocaine!

If you are injected with a local anesthetic and are allergic, what would happen? You would likely exhibit all the classic signs of an immediate type reaction: generalized swelling, itching, urticaria (hives), possible respiratory difficulty, and many other signs.

But remember, only a qualified allergist can diagnose you with an allergy.  Neither I nor any internet site can tell you definitively.

Events where you think you are allergic but are not!

I’ve witnessed firsthand hundreds of incidents where a patient thinks he or she is allergic when in reality something else is occurring. These incidents include:

photo of epinephrine which is used at the dentist in local anesthesia

A racing heart does not mean you are allergic to epinephrine!

  • A racing heartbeat (tachycardia) occurring immediately after a dental local anesthetic is administered does not mean you are allergic to either the local anesthetic or the epinephrine in it. This is a dental myth I busted here.
  • If you start to feel faint and break out in a cold sweat and nearly pass out immediately after an injection, this is most likely vasovagal syncope (and not an immediate allergic reaction).
  • If you do exhibit signs of an allergic reaction, you are far more likely to be reacting to a preservative in the local anesthetic than you are to the actual local anesthetic. The most common culprits are methlyparaben and metabisulfite.

Dental Treatment with a Documented Amide Allergy

If you are one of the rare individuals with an amide allergy confirmed by an allergist and you need dental treatment, what are your options?

  • Get the dental treatment done without any local anesthetic. I do this periodically for my own patients who don’t like the feeling of being numb. But this would only work for minor procedures, not for major ones such as extractions or root canals.
  • Have the allergist test to see your response to other amide local anesthetics. Many papers in the literature talk about individuals reacting to one amide but not another.
  • Benadryl (diphenhydramine) can be used as a local anesthetic when injected. While not as effective as amides, it can offer some degree of local anesthesia. But check with your dentist – very few dentists are prepared to do this and would need advance notice.
  • As a last resort, the dental procedures can be done under general anesthesia.

Final Thoughts and a Disclaimer

While true allergies are rare, they can and do occur. But don’t assume. An adverse or unexpected reaction does not mean you are allergic!

And as mentioned in both the sidebar and footer of this website, this article is for informational use only and is not intended for medical advice. Please consult your physician if you believe you have an allergy and always discuss this with your dentist prior to having any dental treatment performed.

Novocaine Allergy Part II – Methylparaben and Sulfites

In Part I of this series, I covered allergic reactions to ester based local anesthetics used in dentistry. These occur very rarely now because the entire class of ester local anesthetics have essentially been phased out in favor of amide based local anesthetics. Nevertheless, allergic reactions can and do occur after the “novocaine shot.” So the question is, what is/are producing the reactions?


chemical structure of methylparaben


Methylparaben is a preservative used in the pharmaceutical, personal care, and food industry. It is found in many cosmetics currently on the market in both the United States and elsewhere. Methylparaben was at one point included as a preservative in dental local anesthetics. Its main function was to inhibit the growth of bacteria and to help maintain the sterility of the anesthetic.

Methylparaben is chemically very similar to PABA – the metabolic by-product of many ester-type local anesthetics. As I outlined in Part I of this series, PABA can produce allergic reactions in some individuals. Because of this similarity to PABA, when methylparaben is injected as part of a local anesthetic, allergic reactions can occasionally occur.

Because of this, since the mid 1980s, the U.S. Food and Drug Administration mandated the removal of methylparaben from single use dental local anesthetic cartridges. As a result, unless the dentist is using local anesthetic from a multi-use container (which is incredibly unlikely in your typical private practice in the U.S.), you will not be exposed to methylparaben as part of the local anesthetic injection.

Some multi-use vials of local anesthetic still contain methylparaben. But those are typically seen in hospital settings and in individual physician offices.

Since I have never used a dental local anesthetic with methylparaben in it, I have never seen an allergic reaction firsthand.

Sulfite Sensitivity

Sodium metabisulfite used as a preservative in dental local anesthetics

The label from a box of lidocaine with epinephrine used in my office in Connecticut. Metabisulfite is a component.

Sulfites are a class of chemicals used a preservatives. Like methylparaben, sulfites are used in a variety of ways. They are most commonly used to preserve food and can frequently be found in wine, jams, some frozen seafood, and many other products.

In dentistry, sulfites are added to local anesthetics that contain epinephrine. The sulfite – most commonly seen as potassium metabisulfite – is used to prevent the breakdown of the epinephrine. This allows the local anesthetic to have a shelf life of more than a year.

Exposure to sulfites in food as well as a “novocaine shot” can provoke allergy-type symptoms in susceptible individuals. If you have asthma, you are much more likely to be sensitive to sulfites than non asthmatics.

So what would an allergy to sulfites look like? I personally have never seen one. In this article, a patient was injected repeatedly on one side with a dental local anesthetic containing metabisulfite. Within a day, she was experiencing mild swelling at the injection site. After a couple of days, she experienced severe facial swelling with pain and was admitted to the hospital. Allergy testing later concluded an allergy to bisulfite found in the local anesthetic.

Bupivacaine with epinephrine using metabisulfite as a preservative

Box of bupivacaine showing sodium metabisulfite as a component.

So, if you suspect you may have sulfite sensitivity, be sure to ask your dentist to use a local anesthetic that does not contain epinephrine. Dental local anesthetics that do not contain epinephrine do not have metabisulfite.

There is an important distinction between sensitivity to sulfites and allergic reactions to sulfites. The Cleveland Clinic has a nice summary located here.

Note that sulfite sensitivity and sulfa-drug allergies are totally different! So if you have an allergy to sulfa drugs – more formally known as sulfonamides and includes the brand name Bactrim – it does not mean you are allergic or sensitive to sulfites. And vice versa. The reference is located here.

So what’s next in Part III? We’ll cover true allergies to dental local anesthetics – which are extremely rare but have been found.