Side Effects of Local Anesthesia in Dental Treatment

Local anesthesia is made up of several components as the local anesthetic drug, i.e. ester or amide, vasoconstrictor, and antioxidant. In some people, any one or more components can cause systemic side effects and allergies, though rare.

A dentist uses local anesthesia for various reasons such as before extraction of a tooth, root canal therapy, gum surgery, etc. Local anesthesia is administered via injections majorly. It is available in gel form and also as nasal spray.


The symptoms of side effects of local anesthesia are very distinctive, and the signs allow the dentist to diagnose and treat the patient immediately. Mortality is rare since the dentist is quick to treat the symptoms.

In this article, we will discuss the systemic side effects of local anesthesia such as local anesthesia toxicity, methemoglobinemia, etc. along with the treatment of these side effects.

How does local anesthesia work?

Local anesthesia numbs the body part where it is administered and helps manage pain by decreasing the pain or by blocking the pain pathway. It is the most used form of anesthesia in dentistry and does not cause loss of consciousness.

Systemic side effects of local anesthesia

Local anesthesia toxicity

The dose of local anesthesia used in dentistry is minimal. At times when the patient is given repeated injections for pain control or in case of vascular injection cause the local anesthesia levels to rise in the blood causing local anesthetic toxicity.

  • Signs and symptoms include

Tremors, shivering, muscle twitch and clonic-tonic seizures. Post this there can be respiratory depression, loss of consciousness and lethargy. Hypoxia secondary to respiratory depression can cause brain damage and death.

  • Cause of local anesthetic toxicity includes

The dosage of local anesthesia depends on body weight, and when given more than the limit, causes systemic toxicity. Children are at a greater risk of local anesthetic toxicity than adults.

According to “the rule of 25” – for every 25 lbs of the weight of a patient, the dentist can safely use one cartridge of local anesthetic. Similarly, for a 75lbs patient, the dentist can use three cartridges.

  • Management of toxicity includes

Seizures are transient following local anesthetic toxicity. After an episode of seizure, there is usually loss of consciousness followed by prolonged respiratory depression.

Management of local anesthetic toxicity aims at addressing both these symptoms. The dentist will place you in a supine position, monitor your vital signs and maintain the airway.

Definitive treatment includes injection of 5-10 mg of diazepam in case seizures don’t stop.


Methemoglobinemia is a dose-dependent reaction caused due to injection of nitrates and certain amide containing medicines. Prilocaine and benzocaine can induce methemoglobinemia. Oxidation of iron within hemoglobin produces methemoglobin.

  • Signs and symptoms include –

Cyanosis when methemoglobin in blood is 10-20%. The patient will have dyspnea and tachycardia when the level is 35-40%.

Prilocaine contains a component toluidine which causes methemoglobinemia and symptoms often develop after the patient has left the dental office, almost 1-3 hours after the administration of prilocaine. At higher concentrations, coma and death can also occur.

  • The risk factors for this reaction are –

Age (old and infants), anemia, hereditary methemoglobinemia, etc. Hence, it is better to avid prilocaine in geriatric and pediatric patients and to follow the dosing guidelines strictly.

  • Management of this condition depends on the symptoms

The dentist will monitor the heart and respiratory rate of the patient. In the case of healthy patients, the metabolites are eliminated within a few hours.

In the case of patients with symptoms, oxygen is administered via a face mask and the patient is shifted to the emergency room.

In the case of cyanosis, hypoxia and respiratory distress, the doctor administers intravenous methylene blue which converts methemoglobin back to hemoglobin.

Vasoconstrictor reactions

Most of the local anesthetics cause some amount of vasodilation at the site of injection. Exceptions are mepivacaine and prilocaine.

Vasoconstrictors are added to local anesthetics to prolong the duration of anesthesia. Vasoconstrictors increase heart rate and cause vasoconstriction in skin and vasodilation in muscle tissue.

  • Signs and symptoms include –

A transient rise in the heart rate, high blood pressure, cardiac dysrhythmias, and palpitations. Fear, anxiety, nervousness are associated with palpitations. A severe overdose can result in a heart attack or, stroke.

  • Management of vasoconstrictor reactions include –

Monitoring vital signs and assuring the patients that they will be alright in some time. In case the blood pressure doesn’t come back to normal, the dentist will place sublingual nitroglycerin beneath the patient’s tongue and send him to the emergency room for further treatment.

Allergic reactions

Allergic reaction to local anesthetic is extremely rare, but they do occur at times. Procaine and tetracaine  (ester form) cause allergic reactions.

In dentistry, usually, amide anesthetics are used frequently. Amide anesthetics have limited ability to cause hypersensitivity reactions and hence are safer than ester counterparts.

Methylparaben, a constituent of local anesthetics used to cause allergic reactions, but ever since it’s removal in the 1980s, allergic responses due to local anesthetics have diminished.

  • Symptoms include

Rash, urticaria, and anaphylactic response such as dyspnoea and hypotension. But the majority of patients who complain of allergies do not have any such symptom. In majority cases the reaction is psychogenic.

  • Management depends on the severity of allergic reactions.

In case of mild skin rash, diphenhydramine injection can be given intramuscularly or taken orally. In the case of an anaphylactic reaction, the patient must be given basic life support, epinephrine (0.3-0.5 mg, intramuscular or subcutaneous) and taken to the emergency room.

Other complications of local anesthesia

However, there could be other complications as well such as –


You may feel pain when your dentist administers the injection, and this can happen due to many reasons such as

  • If your dentist injects the solution rapidly then there can be swelling and pain. The dentist can prevent this by injecting the solution slowly.
  • Low pH and temperature of the anesthetic solution can cause pain. Your dentist can warm the cartridge in his palms before injecting.
  • Aggressive injection of the needle can rupture blood vessels and tear soft tissue which will cause pain and complications. The only way to avoid this situation is by being gentle while injecting the anesthetic solution. (2)

Needle breakage

Needle breakage is a rare side effect these days. Nowadays the quality of material used to manufacture the needle have improved and the cases of needle breaking while depositing the anesthetic has come down. Other causes of needle breakage are

  • Re-usage of needle –while giving anesthesia to a patient multiple times in a single appointment the dentist usually uses the same syringe. In such cases, the needle may break at times due to fatigue. Being careful while using the same needle will help avoid this situation.
  • Aggressive injection and incorrect technique can cause needle breakage such as insertion of the needle deep into the tissue or abruptly changing the direction of the needle. In both conditions, the hub of the needle could break.

In case the needle breaks both the patient and the dentist need to stay calm and not panic. Your dentist will try to locate the broken part in the tissues and remove it using forceps.

You need to sit still and not move your jaw or muscles (moving your jaw or muscles could cause the needle to go deeper and cause pain and further complications).

In case removal of the needle is not possible then your dentist will take radiographs and refer you to the oral surgeon. The oral surgeon will remove the broken part of the needle surgically under general anesthesia.


Trauma to the blood vessel causes bleeding into the tissue which leads to the formation of a hematoma. Hematoma formation occurs in highly vascular regions.

A hematoma is a more common side effect during the inferior alveolar nerve block. Hematoma forms very quickly and they appear quite large.

The dentist can avoid hematoma formation by aspirating the solution before injecting the solution, knowing the anatomical landmarks and being gentle during removal of the needle after administration of the injection.


Trismus is the inability to open your mouth. Trismus is a side effect after inferior alveolar nerve block and usually occurs 2-5 days after the procedure.

There are various reasons for trismus such as multiple injections in the same region, the patient had to keep his mouth open for too long (complicated extraction cases), infection close to the injection area or infection after treatment (dry socket after extraction causes trismus).

This type of trismus is usually temporary and disappears in a few days. In case the trismus is caused by needle breakage then the patient will require surgery.

In other cases, treatment includes heat therapy, liquid or soft food diet, muscle relaxants or physiotherapy (in case trismus doesn’t resolve on its own).


Paresthesia is one of the postoperative complications of local anesthesia. The trigeminal nerve is the commonly affected nerve due to the inferior alveolar nerve block. Altered nerve sensation is known as paresthesia which can be temporary or permanent.

Nerve injury (during needle insertion or withdrawal) or, needle scratches on the surface of the nerve causes paresthesia. (3)

A hematoma can also cause altered nerve sensation. In majority cases, paresthesia is temporary, but if the paresthesia is permanent, then treatment protocol involves medications instead of surgery.


Surgery may worsen the situation at times. It is best to avoid surgery since total disruption of nerves doesn’t take place.

Prevention of side effects due to local anesthesia

You can prevent these side effects by cooperating with your dentist.

  • Give a detailed medical history and take all your prescriptions if you tend to forget the names of medicines. A complete medical history will make it easier for your dentist to avoid a mishap.
  • Do not panic or be anxious, an injection feels like a pinprick and post that you won’t feel any pain while your dentist works on your teeth.

If you cooperate with your dentist, then side effects can be avoided easily.

Over to you

Every drug comes with side effects, same with local anesthetics. Local anesthetics form an integral part of dentistry in tooth extraction, root canal treatment, implant procedure.

Before every procedure, your dentist will inject local anesthetics and they are very safe. Just in case you have any doubts, you may ask the dentist to perform a patch test before injecting the local anesthetic.


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