The Double-Edged Sword of Oral Lidocaine
Lidocaine is a powerful local anesthetic used to numb areas of the body. While it is safely used for various applications, its effects depend heavily on the method of administration. Oral lidocaine exists in specific, prescription-only viscous solutions designed for conditions like mouth and throat sores. However, ingesting other lidocaine formulations, or improperly using the oral solution, can be extremely dangerous. The body processes orally ingested lidocaine in a way that can lead to systemic toxicity, affecting vital organs like the brain and heart.
The Role of First-Pass Metabolism
When lidocaine is swallowed, it is absorbed through the gastrointestinal tract and must first pass through the liver before reaching general circulation. This process is known as 'first-pass metabolism.' A significant portion of the drug is metabolized by the liver, which greatly reduces its bioavailability, or the amount of active drug that enters the bloodstream. This is why the dose of a prescribed oral lidocaine solution is much higher than what would be administered via injection. However, relying on this metabolic process for safety is unreliable, especially with improper dosing or in individuals with compromised liver function.
Symptoms of Systemic Lidocaine Toxicity
Accidentally or improperly ingesting a large amount of lidocaine can lead to local anesthetic systemic toxicity (LAST), a life-threatening condition. The symptoms of toxicity typically progress from mild to severe, affecting the central nervous system (CNS) and the cardiovascular system. Recognizing these symptoms early is crucial for emergency intervention.
Central Nervous System (CNS) Effects:
- Early Signs: Dizziness, lightheadedness, tinnitus (ringing in the ears), a metallic taste in the mouth, circumoral numbness, blurred or double vision, and nervousness.
- Progression: As toxicity worsens, symptoms can escalate to muscle twitching, tremors, confusion, slurred speech, seizures, and eventually, coma and respiratory arrest. Drowsiness can be an early sign of dangerously high blood levels.
Cardiovascular System Effects:
- Early Signs: In some cases, cardiovascular issues may present before or alongside CNS symptoms. These include sympathetic activation like tachycardia (fast heart rate) and hypertension.
- Progression: Increased lidocaine levels in the blood can lead to a depressant effect on the heart, causing bradycardia (slow heart rate), hypotension (low blood pressure), irregular heartbeats, and ultimately cardiovascular collapse or cardiac arrest.
Special Risks for Infants and Children
Infants and young children are particularly vulnerable to lidocaine toxicity because a smaller dose can more quickly lead to dangerously high plasma levels. Their lower body weight and differences in metabolism make them highly susceptible to overdose, even with quantities that might seem small to an adult. The FDA has mandated a boxed warning for oral viscous lidocaine, explicitly stating it should not be used for teething pain. Ingestion of lidocaine in this vulnerable population has resulted in seizures, cardiopulmonary arrest, and death.
The Immediate Local Effects
Regardless of systemic absorption, ingesting lidocaine will numb the mouth and throat. While this is the intended effect for painful oral sores, it creates a significant risk of aspiration and choking. The numbing can interfere with the ability to swallow effectively, and eating or drinking too soon after use can lead to serious complications. Patients are typically advised to avoid food and drink for at least an hour after administration for this reason.
Comparing Oral Lidocaine Use
| Feature | Prescribed Oral Viscous Lidocaine (Used as directed) | Non-Prescribed Oral Ingestion (e.g., topical gel) |
|---|---|---|
| Purpose | To numb and soothe painful mouth/throat sores or suppress gag reflex. | Pain relief not intended for internal use. |
| Absorption | Swishing and spitting limits systemic absorption, but swallowing the prescribed dose allows controlled absorption. | Significant and rapid absorption into the bloodstream, bypassing safety measures. |
| Metabolism | High first-pass metabolism by the liver controls the amount reaching the bloodstream. | High risk of overwhelming first-pass metabolism and causing high plasma levels. |
| Dosing | Administered in carefully measured, infrequent doses under medical supervision. | Unmeasured doses lead to unpredictable and potentially toxic levels. |
| Side Effects | Mild side effects are possible. Serious side effects typically only occur with overdose or misuse. | High risk of serious systemic toxicity affecting CNS and cardiovascular systems. |
| Safety Profile | Relatively safe for adults when strictly adhering to prescription guidelines. Extremely unsafe for infants. | Inherently dangerous and should be avoided. Call Poison Control immediately. |
What to Do in Case of Overdose
If you or someone you know has accidentally ingested a non-oral lidocaine product or taken too much of a prescribed oral solution, it is an emergency. The first priority is to contact a poison control center immediately or seek emergency medical help. Do not induce vomiting unless specifically instructed to do so by a healthcare professional. Be prepared to provide details about the type of product, the amount ingested, and the approximate time of ingestion.
Conclusion
Taking lidocaine orally carries significant risks, which vary greatly depending on the formulation and the dosage. While certain prescription oral viscous solutions are used safely for specific medical conditions under strict supervision, ingesting topical versions or overdosing on the viscous solution can cause severe and life-threatening systemic toxicity. Symptoms affecting the central nervous and cardiovascular systems, such as seizures and cardiac arrest, are the most serious concerns. The danger is especially high for infants and young children. Therefore, lidocaine should only be used as prescribed, with strict adherence to dosing instructions, and all lidocaine products must be stored securely out of the reach of children to prevent accidental ingestion.
For more information on local anesthetic toxicity, refer to resources from reputable health organizations like the Cleveland Clinic.
