Top 8 Complications of Local Anesthesia

Local Anesthetics (LA) drugs are those used to reversibly numb a specific or generalized area in the mouth so that the patient will not feel any pain or discomfort during oral treatment. It is used most widely during root canal treatment where the pulp of the tooth with all its nerves and vascular supply is extirpated to alleviate its pain and inflammation.

There are a variety of LA drugs available in the market and all of them produce a similar function in producing numbness however, each of them may show differences in their onset, duration of numbness, and side effects.

In this article, I will talking mainly about the complications or side effects where a patient should know.

The number 1 common complication that is most frequently seen in children is a big ulcer in the lower lip. This happens after anesthesia where the patient is not instructed to not bite on his/her lips for 2-3 hours to compensate for duration of the anesthesia as the patient will not have any kind of sensation in his/her lips and might unintentionally bite on it for a long period of time leading to an ulcer.Picture1

The mistake here lies back to the dentist of not informing the patient or instructing him to not bite in his/her lips. If this incident occurs however, the ulcer will heal by itself in 10-15 days. It might take longer depending on the severity of the ulcer but, in general, it heals by itself. Analgesics might be prescribed to alleviate pain and inflammation.

The number 2 most common complication is trismus (a.k.a. limited mouth opening). This happens when the needle is inserted unintentionally through the muscles resulting in its spasm. This can happen to almost anyone regardless of their age because dentists will have to depend on imagination to get to the nerve correctly and start administering local anesthesia. Due to different location of bony landmarks and nerves in individuals, a needle might be inserted through a muscle accidentally.

Management of trismus will require the patient to place a hot or moist towels at the site for approximately 20 minutes every hour and should gradually open and close the mouth as means of physiotherapy. Analgesics might be prescribed depending on the severity of pain.


The number 3 rare complication is needle breakage which might happen due to unexpected sudden movement of the patient. The patient should always remain still when the dentist is injecting a local anesthesia. When this happens, never close your mouth, your dentist will be removing the needle using a special forceps if its visible. In case the needle is not visible, a referral to a maxillofacial surgeon is highly recommended. A radiographs might help in locating the needle fragment.


Most of the time, surgery will not be needed to remove it because the fragment will undergo fibrosis by the body and will remain still in its position as surgery might cause a lot more extensive complications rather than leaving it.


The number 4th common complication is Hematoma which is a localized mass of extravasated blood that may become clinically noticeable following injection. This occurs due to unintentional puncturing of a blood vessel during the process of penetration or withdrawal of the needle.JDPS-2-109-g001

When this occurs, your dentist will be applying a direct pressure on the bleeding site. Once bleeding has stopped, the patient is instructed to apply ice intermittently to the site for the first 6 hours.


Heat should never be applied for at least 6 hours as this will stimulate bleeding again. Analgesics might be prescribed depending on the severity of the case.

The number 5th complication is tachyphylaxis which is an increased tolerance to a drug that is administered repeatedly and will result in less anesthetic effect after reinjection. It is usually related to certain factors including edema (swelling with fluids), localized bleeding, clot formation, hypernatremia (elevated sodium levels in blood), and decreased pH of the tissues due to the presence of inflammation. Managing this complication will be entirely on the dentist part by eliminating theses factors.

The number 6th complication is the presence of inflammation around the injection site. This will reduce the onset of local anesthesia along with its duration and might result in failure. Managing of this scenario again is mainly dependent on the dentist on choosing a different location for injection.

The number 7th complication is the injury to a nerve where the patient experiences a sudden sharp and electrical pain. This happens most frequently where the injection was very accurate and the needle has touched the nerve. If the nerve is damaged in any case, the anesthetic effect might persist for weeks or even months or permanent loss of sensation if the impact has resulted in a permanent damage to the nerve.

Managing of such case is by time. The dentist will have to reassure the patient that condition will subside by itself and sensation will come back again normally. A vitamin B12 complexes might be prescribed to fasten the time of healing.


The number 8th frequent complication is facial nerve paralysis. This occurs when the dentists is trying to anesthetize the lower teeth by an injection technique known as Inferior Alveolar Nerve Block. The dentist might unintentionally misdirect the needle and inject the solution into an area where the facial nerve is located resulting in its anesthesia rather than injecting into the inferior alveolar nerve.


The patient might not be able to smile, or show the teeth, or close the eyelid, when they are asked to do so, on the affected side.

B: Before / A: After / Notice how the patient can't close his eyes or smile in his left side when he was instructed to do so.
B: Before / A: After / Notice how the patient can’t close his eyes or smile in his left side when he was instructed to do so.

The eye on the affected side should therefore be protected and covered by an eye-shield or eye-pad.

Adult Medical Eye Patch
Adult Medical Eye Patch

A drops of sterile paraffin might be administered to help protect the cornea.


Recovery from this scenario takes place within 2-3 hours after injection.

By the end of this article, I hope I have concluded and simplified the common complications that any patient might experience after injection with a local anesthetic drugs.

Make sure to inform your dentist about any side effects or complications you have experienced from a LA in the past so that the dentist will take his/her precautions to avoid any possible complications.

Have you ever experienced any side effect? Why don’t you mention it here and tell us what did you do to manage it.

I read each and every comment.

Haider Maitham, DDS / Bondistry


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