When local anaesthetic does not work, it is almost always a real and explainable problem with the tooth or the anatomy, not a personal failing and not a sign the dentist is bad with needles. For a settled lower molar, the standard block fails somewhere in the range of 5 to 30 per cent of the time. For a tooth that is already throbbing, reported failure rates climb to between 30 and 80 per cent. Being hard to numb is common enough to have its own research literature.
Key takeaways
- Lower back teeth are the hardest to numb, because the nerve sits deep and the bone around it is dense.
- An inflamed tooth is far harder to numb than a healthy one, which is the strongest argument for booking early.
- Feeling pressure and vibration is normal and does not mean the anaesthetic failed.
- The red hair claim is real but widely misreported, and what it actually shows is mostly about anxiety.
- There are several things a dentist can try next, and running out of options is unusual.
- Agree a stop signal before treatment starts, and use it. Nobody should be asked to push through pain.
The failure rate, and why it is a range not a number
For routine work on a non-inflamed lower tooth, inferior alveolar nerve block failure is commonly reported at around 5 to 30 per cent. In a tooth with symptomatic irreversible pulpitis, the sort that keeps you awake, published ranges include 30 to 80 per cent, 43 to 83 per cent and 44 to 81 per cent. One review of an alternative technique cited 12 to 65 per cent for the standard block.
Those ranges look sloppy because the studies define success differently. Some count no pain on drilling, some require two consecutive negative pulp tests, some use patient-reported comfort. Pooling them gives a range rather than a figure, and a single tidy percentage has flattened that away. Failure is uncommon in a calm tooth and common in an angry one.
Why a painful tooth is so much harder to numb
The explanation most people have heard is acidity. Inflamed tissue is more acidic, which shifts the anaesthetic into a charged form that cannot cross the nerve membrane as easily. It is taught everywhere, it is partly true, and it is not the main story. Experimental work has concluded that tissue acidity is not essentially responsible for anaesthetic failure in inflammation, so treat it as one contributing factor rather than the answer.
The better-supported mechanism is a change in the nerve itself. Inflamed pulp increases its production of a particular class of sodium channel, tetrodotoxin-resistant channels, which are roughly four times less sensitive to lidocaine than the usual ones. At the same time, inflammatory chemicals such as prostaglandin E2 lower the threshold at which the nerve fires, and nerve endings sprout new branches inside the pulp. The nerve becomes harder to switch off and easier to set off, at once.
That is the practical argument for not waiting. A small twinge on cold is a far easier tooth to treat, and a far easier tooth to numb, than the same tooth three weeks later. Our article on reversible and irreversible pulpitis explains where that line sits, and what to do about an abscessed tooth covers the stage past it.
Anatomy: why the bottom jaw is different
Upper teeth are usually numbed by an infiltration into the gum beside the tooth, because the bone there is thin and porous. The lower jaw is a solid shell of dense bone, so infiltration alone often will not reach and the dentist has to block the whole nerve further back, a smaller and deeper target.
Then there is variation. The nerve to the mylohyoid muscle branches off the inferior alveolar nerve before the point where the block is given, so it can escape the injection completely. It is usually called a motor nerve, but studies suggest around 20 per cent of its fibres fit the description of pain fibres, and it reaches openings on the inner surface of the jaw in about 43 per cent of cases. A split mandibular canal is another variant, with reported prevalence running from under 2 per cent on flat radiographs to as high as 65 per cent on three-dimensional scans, which says more about how scans are read than about biology.
None of this means anything went wrong. One injection into a standard location does not fit every jaw, and a second approach is often just a different address.
The red hair question, answered properly
There is a real finding here, and it is not the one that circulates. A 2004 study found that people carrying MC1R gene variants associated with red hair needed more general anaesthetic, with the quoted figure of about 20 per cent more inhaled agent. That is about being asleep for surgery in a hospital, not about a dental injection, and repeating it as though it were is the most common error on this topic. For completeness, Lumi Dental does not provide general anaesthetic; IV sedation is a separate option and you can read about it on our IV sedation page.
A study published in the Journal of the American Dental Association found something more useful. People with those MC1R variants reported significantly more dental care anxiety and more fear of dental pain, and were more than twice as likely to avoid dental care, even after adjusting for general anxiety and sex. Evidence that redheads resist dental local anaesthetic specifically is much weaker, resting on small studies with inconsistent replication, and several anaesthesiology reviews file it under myths.
So both halves are true. If you have red hair and have always felt appointments go badly, that experience is documented. The likely mechanism is expectation and anxiety rather than a resistant nerve, and that is treatable in ways a resistant nerve would not be.
Three different things called it did not work
These get reported identically and need completely different responses.
Not numb at all. You still feel sharpness, temperature or a jolt. This is a genuine anaesthetic failure and the dentist should stop and change approach.
Numb, but feeling pressure. Pressure, vibration and pushing are carried by different nerve fibres from pain, and local anaesthetic is not meant to remove them. Feeling the tooth being worked on without any sharpness is normal and expected. Nobody tells patients this often enough, so it gets reported as failure.
Numb, but frightened. Highly anxious patients feel pain more intensely and are more likely to read any sensation as the start of pain. Some give early responses to numbness testing because they are braced for something unpleasant. That is not imagination, it is how the nervous system behaves under threat, and the injection is consistently identified as the most anxiety-provoking moment of an appointment. Working on that often beats more anaesthetic, and our guide to overcoming fear of the dentist is a reasonable place to start.
What the dentist can try next
The figures below come mostly from studies of acutely painful teeth, the hardest situation there is. For an ordinary filling on a settled tooth, things go better than these numbers suggest.
| Option | What it targets | Roughly how often it works | What it feels like to you |
|---|---|---|---|
| Wait longer, or repeat the block | Slow onset, or a slightly missed target | Often enough to try first | Another few minutes, or one more injection |
| Buccal infiltration with articaine | Accessory nerve supply from the cheek side | Around 42 to 48 per cent in molars with acute pulpitis | A small injection in the gum beside the tooth |
| Intraligamentary injection | The ligament space around the tooth root | 48 to 74 per cent | Brief firm pressure, sometimes a sore tooth afterwards |
| Intraosseous injection | The bone immediately around the root | About 80 per cent, rising to about 98 per cent with a second cartridge | A short buzzing sensation, occasionally a fast heartbeat |
| Gow-Gates block | The nerve higher up, closer to its origin | Not clearly better than the standard block in trials | The same as a normal block, aimed differently |
| Buffered anaesthetic | The acidity of inflamed tissue | A recognised option in acutely painful teeth | No different to you |
| Intrapulpal injection | Directly inside the nerve chamber | Reserved for the 5 to 10 per cent where nothing else works | A brief sharp moment, then relief |
| Reschedule and settle the infection first | Swelling and inflammation around the tooth | Case by case | Another visit, but a much easier one |
On the choice of drug, a meta-analysis found articaine had greater odds of success than lidocaine, with the difference largest for infiltrations, and one trial of lower first molar infiltration found 64.5 per cent success with articaine against 38.7 per cent with lidocaine. That is a real difference in a specific situation, not a claim that one drug is stronger or safer overall.
Tell them, and they will stop
Agree a stop signal before anything starts. A raised left hand is the standard, and it works when your mouth is full. Pain during treatment is information, not weakness, and there is no reason to push through it.
Detail helps. Useful: I have had trouble getting numb before, it was a lower back tooth, I could feel sharpness when the drill touched it. Less useful: just give me more, which does not tell anyone which technique to switch to. Say whether the sensation was sharp, hot, cold, or a jolt when water hit the tooth, because those point to different problems.
Things that do not make you harder to numb
Alcohol comes up constantly. The evidence concerns chronic heavy use, where animal work suggests reduced lidocaine effect and duration, and even that is contested, since people in recovery from alcohol dependence were not at increased risk of inadequate pain control. A drink the night before is not the reason. Smoking, a high pain threshold and a fast metabolism do not burn through anaesthetic, and your own adrenaline does not cancel out the injection.
Antibiotics come up because people ask for them by name. Treating an infection first can help in specific cases, but antibiotics do not reliably make a tooth numbable and are not a routine fix.
The opposite problem, numbness that hangs around for hours after you leave, is a separate topic covered in how long dental numbness lasts. And if the injection itself is the part you dread, some small cavities can be treated without one, which we cover in fillings without a needle.
Common questions
Why doesn't local anaesthetic work on me at the dentist?
The most common reasons are an inflamed tooth, which changes the nerve so it resists the anaesthetic, and anatomical variation in the lower jaw that lets part of the nerve supply escape a standard block. Anxiety amplifies sensation as well. It is rarely about the person being unusual, and several alternative techniques exist.
Why do bottom teeth take more injections than top teeth?
The bone around upper teeth is thin and porous, so an injection into the nearby gum soaks through. The lower jaw is dense, so the dentist blocks the main nerve deeper in the jaw, a smaller target with more variation. That is why lower back teeth account for most numbing difficulties.
Do redheads need more anaesthetic at the dentist?
The well supported finding is that people with the relevant gene variants need more anaesthetic when asleep for surgery, and separately report more dental anxiety, more fear of dental pain and more avoidance of care. Evidence that they resist dental local anaesthetic is weak and inconsistent. Mention it anyway, since it is worth allowing extra time.
Why is it harder to numb a tooth with an abscess?
Inflamed pulp produces sodium channels that are much less sensitive to lidocaine, and inflammatory chemicals lower the threshold at which the nerve fires. Local acidity contributes but is not the main mechanism. Supplemental techniques usually solve it, and treating a tooth earlier is far easier than treating it once it is throbbing.
Does anxiety stop numbing from working?
Anxiety does not neutralise the anaesthetic, but it does raise pain perception and makes it more likely that pressure or vibration is read as pain. Anxious patients also sometimes respond early to numbness tests because they are braced for something unpleasant. Addressing the anxiety often does more than another injection.
If you have been hard to numb before
Say so when you book, not when you sit down. It changes the appointment length, the technique chosen and how much time there is to let things work. The team at Lumi Dental would rather set aside a longer visit than rush one. New patients can book a first check-up and clean under the current new patient special on our current deals page, or ask a question through the contact page.
This article is general information only and is not a substitute for individual dental advice. If you are in pain now, please arrange an assessment.
