Local anaesthetic at the dentist: types, how long it lasts, does it hurt and when it may not work

How does local anaesthesia at the dentist work?

Local anaesthesia at the dentist is a drug injected into the gum or near a nerve that temporarily blocks pain signals from the treated area while the patient stays fully conscious. Dentistry uses drugs from the amide group: lidocaine, articaine, mepivacaine, prilocaine and bupivacaine, which are listed by the American Academy of Pediatric Dentistry (AAPD) in its 2023 guidelines.

Many products contain an added vasoconstrictor, most often adrenaline. According to the AAPD, it slows the absorption of the drug into the blood, which lowers the risk of toxic effects and prolongs anaesthesia at the injection site. How long it lasts also depends on the concentration and dose, the injection site and the patient’s age.

Local anaesthesia is a guaranteed service: the list of general dental services funded by the NFZ (Poland’s National Health Fund) includes topical, infiltration and intraoral nerve block anaesthesia, provided together with other services. Legal position as of 12 October 2026.

What types of dental anaesthesia are there?

The types of dental anaesthesia differ in where the drug goes and how large an area it switches off. Infiltration and nerve block anaesthesia are used most often, while topical anaesthesia with a gel only prepares the injection site.

In children, according to the AAPD, infiltration in the lower jaw can be as effective as a nerve block for fillings in primary molars, because a child’s bone is less dense. The AAPD also lists alternative techniques, such as computer-controlled delivery, intraligamentary injection and needle-free systems, which can improve comfort by controlling the speed and pressure of delivery.

How long does dental anaesthetic last?

Dental anaesthetic usually numbs the tooth for about an hour with an infiltration and for about 1.5 to 2 hours with a nerve block, while the lip, cheek and tongue stay numb for longer. The figures in the table come from the US labels of two products with adrenaline, published in the DailyMed database; the Polish summaries of product characteristics may give them differently.

The AAPD stresses that with every drug, soft tissue anaesthesia lasts longer than anaesthesia of the tooth and bone. So the tooth may already respond to stimuli while the lip still has no feeling. The Bristol Dental School leaflet states that numbness after an injection into the gum usually lasts 3 to 4 hours, after which feeling returns gradually.

Does a dental anaesthetic injection hurt?

A dental anaesthetic injection can be felt as a brief prick and a feeling of pressure, but both sensations can be eased. The AAPD recommends a numbing gel before the injection, because it works to a depth of 2 to 3 mm and reduces the pain of the needle passing through the mucosa, and slow delivery of the drug, which limits the stretching of the tissues and the discomfort that comes with it.

A 2018 Cochrane review (123 studies, more than 19,000 participants) found no difference in the pain of the injection itself between articaine and lidocaine; after the injection, lidocaine caused slightly less pain. The AAPD also notes that products with adrenaline are acidic, which can increase stinging during delivery.

If the injection itself worries you, say so at the start of the visit, and the dentist will check that the anaesthetic has taken effect before starting work. Ways of managing severe anxiety, from inhalation sedation to general anaesthesia, are compared in the article on sedation and general anaesthesia at the dentist.

What to do after the anaesthetic while the lip is still numb

After the anaesthetic, the most important thing is not to bite your lip, cheek or tongue until feeling returns, because numb tissue gives no warning through pain. The Bristol Dental School leaflet advises care for the whole time the numbness lasts, and after a tooth extraction also avoiding alcohol and very hot drinks.

In children, biting a numb lip or cheek is a known complication of local anaesthesia. The AAPD notes that such injuries usually heal on their own, although bleeding and infection are possible, and that bilateral nerve block anaesthesia of the lower jaw raises the risk compared with one side; the parent should know how long the numbness is expected to last.

• Wait until feeling has returned to the lip and cheek before eating anything that needs chewing.
• Do not test the numb lip with your teeth or fingers; the feeling of a swollen lip passes on its own.
• Avoid very hot drinks, because the numb mucosa will not sense the temperature.
• Keep an eye on your child after the visit and explain that a numb lip must not be bitten or sucked.
• If the numbness does not wear off within the time the dentist gave you, call the surgery.

When can a dental anaesthetic fail to work?

A dental anaesthetic may work less well or not at all when the drug goes into tissue affected by infection or severe inflammation. The AAPD explains that infection lowers the pH of the tissues, so the drug penetrates the nerve less easily: the onset is delayed, and the anaesthetic is sometimes ineffective.

This is clearest with irreversible pulpitis in back teeth. In the 2018 Cochrane review, anaesthesia before root canal treatment of such teeth was successful in 49% of patients after articaine and in 31% after lidocaine, with low-quality evidence.

The dentist then has several options: additional anaesthesia with a different technique, bringing the inflammation under control first and carrying out the actual treatment at the next visit, or, when the inflammation is too acute, treatment under general anaesthesia after assessment by an anaesthesiologist. Intraligamentary anaesthesia is not used where there is inflammation at the injection site.

Can you be allergic to dental anaesthetic?

A true allergy to dental anaesthetic is very rare. In a review of 23 case series (British Journal of Anaesthesia, 2012), of 2,978 people investigated for suspected allergy to local anaesthetics, allergy was confirmed in 29, that is 0.97%, and that was in a group already referred with a suspected reaction, not among all patients.

According to the AAPD, patients often report an allergy when they actually reacted to the adrenaline, to the preservative (metabisulphite) in products with adrenaline, to too high a dose or to the drug accidentally entering a blood vessel. Records of the earlier reaction or allergy testing help in choosing a drug, and an allergy to one amide drug does not rule out another amide.

When an allergy to local anaesthetics is confirmed, treatment can be carried out under general anaesthesia. Since 1 January 2026, children and young people up to the age of 18 with such an allergy have been entitled to dental treatment under general anaesthesia funded by the NFZ.

Anaesthesia in pregnancy, with illnesses and medicines: what to tell the dentist

Dental anaesthesia in pregnancy is acceptable: the American Dental Association (ADA) states that anaesthetics with adrenaline, such as lidocaine, mepivacaine or bupivacaine, may be used during pregnancy. The details, including articaine and sedation, are covered in the article on dental treatment in pregnancy.

The AAPD lists conditions in which a medical consultation about a product without adrenaline may be needed: significant cardiovascular disease, thyroid disease, diabetes, sulphite sensitivity and taking certain antidepressants and antipsychotics. That is why, before the anaesthetic, you should tell the dentist about any illnesses and all medicines, including those bought without a prescription.

The dentist adjusts the dose of the drug to body weight, which matters particularly in children. A rare complication is prolonged numbness (paraesthesia), described mainly after nerve block anaesthesia in the lower jaw and usually temporary; the AAPD notes that it has been reported more often with 4% solutions, such as articaine.

How it works at DentalBlue

In the price list of DentalBlue in Białystok, the item “Znieczulenie” (anaesthetic) costs 60 zł, and “Znieczulenie w endodoncji” (anaesthetic in root canal treatment) 20 zł. The clinic also provides inhalation sedation with nitrous oxide (the item “Podtlenek azotu”, 170 zł). It has its own anaesthesiology department, so treatment under general anaesthesia is available, among others, to patients allergic to local anaesthetics and to those whose inflammation is too acute for local anaesthesia to work. Every such treatment is preceded by a joint dental and anaesthesiology consultation. The final scope and cost are agreed at the visit, and prices may change.