What is the most common complication following an injection of local anesthetic?

Localized pain and bruising are the most common issues following local anesthetic injections. However, the most frequent, and often overlooked, complication is a psychogenic reaction such as syncope (fainting). Differentiating between common, minor side effects and rare but serious systemic reactions is crucial for patient care.

Quick Summary

The most frequent complication following a local anesthetic injection is a localized reaction, such as pain or bruising, or anxiety-related syncope. These are separate from the rare, but dangerous, systemic risk of Local Anesthetic Systemic Toxicity (LAST).

Key Points

• Most common issues are minor and local: Common complications include pain, soreness, and bruising at the injection site.

• Psychogenic reactions are common: Anxiety can cause reactions like syncope (fainting), which are physiological responses to stress.

• Serious complications are rare: Severe systemic issues, such as LAST, are rare but potentially life-threatening.

• Systemic toxicity affects CNS and CVS: LAST can lead to CNS symptoms (confusion, seizures) and/or cardiovascular issues (arrhythmias, hypotension).

• Prevention is key: Proper injection technique, including slow delivery and aspiration, is critical to prevent serious complications like LAST.

• Allergic reactions are extremely rare: True allergies to common local anesthetics are very infrequent, with many reactions being due to preservatives or patient anxiety.

• Good technique and communication minimize risk: Careful injection practice and patient communication are the best defense against complications.

The Spectrum of Complications Following Local Anesthetic Injection

Local anesthetic injections are frequently used in medicine and dentistry to provide temporary numbness. While known for safety and effectiveness, complications can occur. Complications are categorized as either local, affecting the injection site, or systemic, affecting the entire body. Recognizing the difference between a common, minor issue and a rare, serious one is essential for proper management.

Minor and Most Frequent Complications

The most common complications are minor, temporary, and localized issues that often resolve on their own. Anxiety-related reactions are also very frequent and must be distinguished from true adverse drug effects.

• Pain and Soreness: Discomfort or burning during injection and residual soreness at the site afterward are common. This is usually due to the rapid deposition of the solution, the needle penetrating sensitive tissue, or the slightly acidic pH of some anesthetic formulations.
• Bruising (Hematoma): Hematoma, or bruising, results from the needle piercing a blood vessel, causing blood to leak into the tissue. This resolves within a week or two and is managed with pressure and cold compresses.
• Psychogenic Reactions: Anxiety and fear are major causes of complications. The most common is syncope (fainting), caused by a drop in blood pressure and heart rate. Dizziness, hyperventilation, and nausea can also occur. These are not a result of the anesthetic itself but the psychological stress of the procedure.

Systemic Complications: Rare but Serious

While minor events are common, the most feared complications are systemic in nature and fortunately extremely rare. These typically occur when the anesthetic is accidentally injected directly into a blood vessel (intravascular injection) or an excessive dose is administered.

Local Anesthetic Systemic Toxicity (LAST)

LAST is a potentially life-threatening but rare emergency. It affects the central nervous system (CNS) and the cardiovascular system (CVS). Symptoms can begin within minutes of injection and escalate quickly.

CNS Symptoms:

• Early signs often include a metallic taste in the mouth, numbness around the lips (perioral paresthesia), tinnitus (ringing in the ears), and lightheadedness.
• These can progress to confusion, agitation, slurred speech, and muscle twitching.
• In more severe cases, seizures are the most common sign of LAST.

CVS Symptoms:

• Cardiovascular effects typically occur at higher blood concentrations than CNS symptoms and can include arrhythmias, conduction delays, and severe hypotension.
• Bradycardia (slow heart rate) or even cardiac arrest can occur in the most severe cases.

Other Complications and Considerations

• Allergic Reactions: True allergic reactions to amide-type local anesthetics (like lidocaine and bupivacaine) are exceptionally rare. Most suspected allergies are actually psychogenic reactions or a reaction to a preservative, such as sulfites, found in formulations containing a vasoconstrictor like epinephrine.
• Nerve Injury (Paresthesia): This is a rare, but potential, complication resulting from direct trauma to a nerve by the needle. It manifests as persistent numbness or altered sensation. The risk is extremely low, with most cases resolving over time.
• Trismus: A reduced ability to open the jaw, known as trismus, can sometimes follow an injection, especially in dental procedures. It often results from muscle trauma or hemorrhage into the masticatory muscles and usually resolves over several weeks with proper care.

Comparison of Common vs. Serious Complications

Feature	Common (Local & Psychogenic)	Serious (Systemic - LAST)
Incidence	Very Frequent	Very Rare
Typical Onset	Immediate during or shortly after injection	Typically within minutes, but can be delayed
Symptoms	Pain, soreness, bruising, dizziness, fainting	Metallic taste, tinnitus, confusion, seizures, cardiac arrhythmias
Severity	Mild to Moderate	Severe, potentially life-threatening
Cause	Injection site trauma, anxiety response	Inadvertent intravascular injection, overdose
Management	Reassurance, positional changes, compresses	Supportive care (airway management, oxygen), lipid emulsion therapy

Management and Prevention of Complications

The vast majority of minor complications are managed through proper technique and patient care. Prevention is key to avoiding serious systemic issues.

For Common Complications:

• Patient Communication: Addressing patient anxiety is a primary preventative measure for psychogenic reactions like syncope.
• Proper Technique: Injecting slowly and using sharp needles reduces pain and tissue trauma.
• Patient Education: Informing patients about what to expect, such as temporary soreness, helps manage expectations and reduces anxiety.

For Serious Complications (LAST):

• Aspiration: Before injection, a practitioner should aspirate to ensure the needle is not within a blood vessel.
• Incremental Dosing: Injecting in small, incremental doses is safer than administering a large dose at once.
• Safe Dosing: Always adhere to recommended maximum dosages based on patient weight and health.
• Observation: The patient should be monitored closely for any signs of toxicity following the injection.

Conclusion

The most common complication following local anesthetic injection is a minor, localized issue such as pain, soreness, or bruising at the injection site. A frequent, though often misattributed, complication is a psychogenic reaction, such as fainting. While rare, serious systemic toxicity (LAST) remains a risk, primarily associated with inadvertent intravascular injection or overdose. Through meticulous injection technique, careful patient monitoring, and clear communication, the risks associated with local anesthetics can be minimized, reinforcing their position as a cornerstone of modern medical practice.

For Further Information

Cleveland Clinic, Local Anesthetic Systemic Toxicity (LAST): An authoritative resource detailing the symptoms, causes, and treatment of this rare complication.

Reference

Cleveland Clinic. (2025). local anesthetic systemic toxicity (LAST). Retrieved from https://my.clevelandclinic.org/health/diseases/local-anesthetic-systemic-toxicity-last

Quizlet. (n.d.). LA Complications and Management - Chapter 17 Flashcards. Retrieved from https://quizlet.com/321725336/la-complications-and-management-chapter-17-flash-cards/

dentalcare.com. (2018). Complications of Local Anesthesia. Retrieved from https://www.dentalcare.com/en-us/ce-courses/ce325/complications-of-local-anesthesia

IntechOpen. (2019). Complications Associated with Local Anesthesia in Oral and Maxillofacial Surgery. Retrieved from https://www.intechopen.com/chapters/67979

Frequently Asked Questions

What's the difference between local and systemic complications?

A local complication occurs at the injection site, like pain or bruising. A systemic complication affects the whole body, such as LAST.

What is Local Anesthetic Systemic Toxicity (LAST)?

LAST is a rare but severe complication where too much anesthetic enters the bloodstream, potentially harming the central nervous system (confusion or seizures) and the cardiovascular system (arrhythmias).

Is it normal to feel dizzy or faint after an injection?

Yes, dizziness or fainting (syncope) is a common reaction caused by anxiety rather than the anesthetic. Patients should lie down with their legs elevated.

How can severe complications like LAST be prevented?

To prevent LAST, a healthcare provider should use careful technique, including aspirating before injecting and delivering the anesthetic slowly and in small doses.

Are allergic reactions to local anesthetics common?

No, true allergic reactions to modern amide-type local anesthetics are extremely rare. Many perceived allergies are often anxiety-related or a reaction to a preservative in the solution.

What should I do if I experience prolonged numbness after an injection?

Prolonged numbness (paresthesia) is a rare complication that may result from nerve trauma. Contact your healthcare provider, though most cases resolve over time.

Can an injection of local anesthetic cause bruising?

Yes, bruising (hematoma) is a common complication if a small blood vessel is nicked during the injection. Apply pressure and a cold pack.

How long do minor side effects usually last?

Minor side effects like soreness or bruising usually subside within a few days to a couple of weeks. Anxiety-related symptoms like dizziness typically resolve shortly after the injection.