Lidocaine is a common local anesthetic used to numb specific areas of the body, available as gels, sprays, and liquids [1.2.3]. While generally safe for topical use as directed, ingesting it can lead to severe health consequences and even death [1.2.3].
How Lidocaine Works and Why Ingestion Is Different
Lidocaine functions by blocking sodium channels in nerve cells, which stops them from sending pain signals to the brain [1.2.3]. When applied topically to intact skin, absorption into the bloodstream is minimal. However, when lidocaine is ingested, it is absorbed rapidly through mucous membranes and the gastrointestinal tract [1.3.1, 1.3.3].
While the liver metabolizes much of the lidocaine that passes through it—a process called the 'first-pass effect' which gives oral lidocaine a bioavailability of about 35%—absorption through the mucous membranes of the mouth and throat allows the drug to enter the bloodstream more directly [1.9.2, 1.3.3]. This rapid systemic exposure can quickly lead to toxic levels in the blood, overwhelming the body's ability to safely process the drug [1.2.3].
The Immediate Effects: Symptoms of Lidocaine Toxicity
Lidocaine toxicity, also known as Local Anesthetic Systemic Toxicity (LAST), presents with symptoms that typically progress as blood concentration levels rise. These symptoms primarily affect the Central Nervous System (CNS) and the Cardiovascular System [1.2.1, 1.8.4].
Central Nervous System (CNS) Toxicity
Early and mild signs of toxicity often appear first and are neurological. As levels increase, these can become severe [1.2.1].
- Early Symptoms: Circumoral (around the mouth) numbness, tongue paresthesia (numbness or tingling), metallic taste in the mouth, lightheadedness, dizziness, and tinnitus (ringing in the ears) [1.2.1, 1.2.2].
- Moderate Symptoms: Visual disturbances like blurred vision, muscle twitches, restlessness, agitation, and confusion may develop [1.2.1, 1.2.5].
- Severe Symptoms: As toxicity worsens, it can lead to slurred speech, seizures, loss of consciousness, coma, and respiratory arrest [1.2.5, 1.8.5]. In some cases, these excitatory signs may be brief or absent, with the first sign being drowsiness that progresses to unconsciousness [1.2.5].
Cardiovascular Toxicity
Cardiovascular effects usually require a higher concentration of lidocaine in the blood than what causes initial CNS symptoms [1.2.1].
- Initial Signs: Hypotension (low blood pressure) and bradycardia (a slow heart rate) are common depressant effects [1.2.5].
- Severe Signs: With higher toxic levels, life-threatening arrhythmias, atrioventricular (AV) heart block, and complete cardiovascular collapse or cardiac arrest can occur [1.2.1, 1.2.5].
Comparison Table: Topical vs. Ingested Lidocaine
| Feature | Topical Application (Intact Skin) | Oral Ingestion |
|---|---|---|
| Primary Use | Localized pain and itching relief (e.g., sunburns, cuts) [1.2.3] | Numbing of mouth and throat mucous membranes (under medical supervision) [1.10.1] |
| Absorption | Poor and slow [1.9.2] | Rapid from mucous membranes and GI tract [1.3.1, 1.3.3] |
| Systemic Exposure | Minimal when used as directed on small areas [1.2.3] | High, due to bypassing first-pass metabolism and direct absorption [1.3.3, 1.9.2] |
| Primary Risk | Skin irritation; toxicity if used on large areas or broken skin [1.2.3] | Systemic toxicity (CNS and cardiac effects), choking hazard from numb throat [1.2.1, 1.2.3] |
Special Populations at High Risk
Infants and Children
Children are exceptionally vulnerable to lidocaine toxicity. The U.S. Food and Drug Administration (FDA) has issued its strongest warning, a Boxed Warning, against using prescription viscous lidocaine for teething pain [1.4.1]. Accidental ingestion or improper application for teething can lead to seizures, severe brain injury, heart problems, and death [1.4.2, 1.10.1]. The American Academy of Pediatrics recommends non-medicinal options like chilled teething rings or gently massaging the gums instead [1.4.1].
Patients with Underlying Health Conditions
Individuals with certain health issues are at a higher risk for toxicity:
- Liver Disease: Since lidocaine is metabolized in the liver, impaired liver function can slow its clearance, leading to higher blood levels [1.9.4, 1.2.5].
- Heart Failure: Patients with heart conditions may be more susceptible to the cardiotoxic effects of lidocaine [1.2.5].
Emergency Response: What to Do for a Suspected Ingestion
If you suspect someone has ingested lidocaine, especially a child, it is a medical emergency.
- Call for Help Immediately: Contact Poison Control at 1-800-222-1222 in the United States or call 911 [1.2.3].
- Gather Information: Be prepared to provide the product name, the amount ingested (if known), the time of ingestion, and the person's age and weight [1.6.2].
- Do Not Induce Vomiting: This can increase the risk of choking, especially since the throat may be numb [1.2.3].
Medical Treatment for Lidocaine Toxicity
In a hospital setting, treatment focuses on supportive care and reversing the toxic effects [1.6.4].
- Supportive Care: This includes managing the patient's airway with 100% oxygen and ensuring adequate ventilation [1.6.4].
- Seizure Control: Benzodiazepines are the first-line treatment for controlling seizures [1.7.1].
- Cardiovascular Support: Medications may be used to manage low blood pressure and arrhythmias. In cases of cardiac arrest, modified ACLS protocols are used, avoiding certain drugs that can worsen toxicity [1.6.4, 1.6.5].
- Lipid Emulsion Therapy: For severe systemic toxicity, especially cardiovascular collapse, intravenous (IV) lipid emulsion is a primary antidote. It is thought to work by creating a 'lipid sink' in the bloodstream, which traps the fat-soluble lidocaine molecules, pulling them away from heart and brain tissue [1.7.1, 1.7.2].
Conclusion
While a valuable tool for local anesthesia, lidocaine is dangerous when ingested. The rapid absorption into the bloodstream can lead to a cascade of toxic effects on the nervous system and heart, which can be fatal. It is critical to store all lidocaine products securely and out of reach of children and to never use oral viscous lidocaine for teething pain [1.10.1]. Understanding the symptoms and knowing to call for emergency help immediately can be life-saving. For more information on drug safety, refer to authoritative sources like the U.S. Food and Drug Administration [1.10.1].
