Numbing Creams

Dosage for Lidocaine is highly individualized based on the procedure, the area to be anesthetized, and the patient's physical status.

• Topical (5% Patch): Apply up to 3 patches to the most painful area. Patches should be worn for no more than 12 hours within a 24-hour period.
• Infiltration Anesthesia: Maximum single dose should not exceed 4.5 mg/kg (not to exceed 300 mg) without epinephrine.
• Antiarrhythmic (IV): Typically starts with a bolus of 1 to 1.5 mg/kg, followed by a continuous infusion of 1 to 4 mg/minute.

Lidocaine must be used with extreme caution in children. Dosing is strictly weight-based (e.g., 3 mg/kg to 5 mg/kg for local infiltration). The FDA issued a Boxed Warning against using 2% viscous lidocaine for treating teething pain in infants due to the risk of seizures and death.

Renal Impairment

While the kidneys excrete only a small amount of unchanged Lidocaine, metabolites can accumulate in patients with severe renal failure. Monitoring for toxicity is recommended.

Hepatic Impairment

Since Lidocaine is primarily metabolized by the liver, patients with hepatic dysfunction (e.g., cirrhosis) require significant dose reductions (often 50% or more) to avoid systemic toxicity.

Elderly Patients

Lower doses are generally recommended for geriatric patients due to age-related declines in hepatic blood flow and potential cardiac sensitivities.

• Topical Forms: Apply only to intact skin unless otherwise directed. Wash hands immediately after application. Do not cover patches with heating pads.
• Injectable Forms: Administered only by trained healthcare professionals in a clinical setting.
• Viscous Form: If used for the mouth or throat, avoid eating or chewing gum for 60 minutes after use to prevent choking or tongue injury.

If you miss an application of a topical Lidocaine patch, apply it as soon as you remember. If it is almost time for the next application, skip the missed dose. Do not apply extra patches to make up for a missed dose.

Signs of systemic Lidocaine toxicity (LAST) include perioral numbness (tingling around the mouth), metallic taste, tinnitus (ringing in the ears), tremors, seizures, and cardiac arrest. Seek emergency medical attention immediately if these symptoms occur.

> Important: Follow your healthcare provider's dosing instructions. Do not adjust your dose without medical guidance.

Common side effects are usually localized to the site of administration:

• Site Reactions: Redness, swelling, or a pale appearance (blanching) where the medication was applied or injected.
• Sensory Changes: Temporary numbness, tingling, or a 'pins and needles' sensation.
• Drowsiness: A mild sedative effect is common following systemic absorption.

• Nausea and Vomiting: Particularly common when used in systemic or epidural applications.
• Dizziness: Feeling lightheaded or unsteady.
• Hypotension: Low blood pressure, often occurring with spinal or epidural anesthesia.

• Methemoglobinemia: A rare blood disorder where the blood cannot effectively carry oxygen, characterized by bluish skin (cyanosis) and shortness of breath.
• Severe Bradycardia: An abnormally slow heart rate.

> Warning: Stop using Lidocaine and call your doctor immediately if you experience any of these:

• Central Nervous System Toxicity: Confusion, blurred vision, tremors, or seizures.
• Anaphylaxis: Severe allergic reaction including hives, swelling of the face or throat, and difficulty breathing.
• Cardiac Arrhythmias: Feeling like your heart is skipping beats or racing uncontrollably.
• Respiratory Depression: Significantly slowed or shallow breathing.

Prolonged use of topical Lidocaine may lead to contact dermatitis (skin rash) or localized hypersensitivity. Systemic accumulation is rare with topical use but can occur if applied to broken skin or used excessively over long periods.

FDA Boxed Warning (Viscous Lidocaine 2%): This formulation should not be used to treat teething pain in infants and young children. Improper use has resulted in seizures, brain injury, and death due to accidental ingestion and rapid systemic absorption.

Report any unusual symptoms to your healthcare provider.

Lidocaine should only be used under the supervision of a healthcare professional or exactly as prescribed for topical use. Accidental ingestion or excessive application can lead to life-threatening systemic toxicity.

Viscous Lidocaine 2%: The FDA warns that this product is not approved for teething pain in infants. In 2014, the FDA reviewed 22 case reports of severe adverse events, including deaths, in infants and young children given viscous lidocaine for mouth pain.

• Methemoglobinemia Risk: Patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency or those taking certain medications (like sulfonamides) are at higher risk for this life-threatening condition.
• Cardiovascular Disease: Use with caution in patients with heart block, severe heart failure, or bradycardia, as Lidocaine can depress cardiac conduction.
• Central Nervous System (CNS) Disorders: Patients with a history of seizures must be monitored closely, as Lidocaine can lower the seizure threshold.
• Liver Disease: Because the liver clears Lidocaine, patients with cirrhosis or hepatitis are at a significantly higher risk of toxicity.

For patients receiving IV Lidocaine, continuous ECG (electrocardiogram) monitoring and frequent blood pressure checks are mandatory. Serum Lidocaine levels may be measured, with a therapeutic range typically between 1.5 and 5.0 mcg/mL.

Lidocaine may cause temporary dizziness or drowsiness. Do not drive or operate heavy machinery until you know how the medication affects you, especially after receiving an injection.

Alcohol may enhance the sedative effects of Lidocaine. Avoid alcohol consumption if you are using Lidocaine systemically or in large topical doses.

There is no known withdrawal syndrome for Lidocaine; however, stopping the medication will result in a rapid return of sensation or the recurrence of arrhythmias if used for that purpose.

> Important: Discuss all your medical conditions with your healthcare provider before starting Lidocaine.

• Bupivacaine Liposome: Concurrent use can lead to the immediate release of bupivacaine from its liposomal carrier, resulting in systemic toxicity.
• Certain Class III Antiarrhythmics: Using Lidocaine with drugs like Dofetilide can significantly increase the risk of dangerous heart rhythms.

• Amiodarone: This combination can increase Lidocaine levels and lead to additive effects on the heart's electrical system, potentially causing severe bradycardia.
• Beta-Blockers (e.g., Propranolol): These drugs reduce hepatic blood flow, which can slow the metabolism of Lidocaine and increase the risk of toxicity.
• Cimetidine: This H2-blocker inhibits CYP enzymes, leading to increased Lidocaine concentrations in the blood.

• Fluvoxamine: An antidepressant that inhibits CYP1A2, potentially raising Lidocaine levels.
• Other Local Anesthetics: Using multiple numbing agents simultaneously can have an additive effect on systemic toxicity risk.

There are no significant food interactions for topical or injectable Lidocaine. However, if using viscous Lidocaine for the throat, food should be avoided for one hour post-application to prevent aspiration (inhaling food into the lungs).

• St. John's Wort: May induce CYP3A4 enzymes, potentially reducing the efficacy of Lidocaine.
• Kava Kava and Valerian: May increase the sedative effects when used alongside systemic Lidocaine.

Lidocaine may interfere with certain diagnostic tests, such as the bentiromide test for pancreatic function. Always inform your lab technician if you have recently received Lidocaine.

> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking.

• Hypersensitivity: Known allergy to Lidocaine or other amide-type local anesthetics (e.g., prilocaine, mepivacaine).
• Stokes-Adams Syndrome: A condition involving sudden fainting spells caused by heart block.
• Wolff-Parkinson-White (WPW) Syndrome: Severe sinoatrial, atrioventricular, or intraventricular heart block in the absence of a functional pacemaker.

• Severe Liver Disease: Requires extreme caution and dose reduction.
• Hypovolemia: Low blood volume can increase the risk of hypotension during spinal or epidural use.
• Myasthenia Gravis: Patients may be more sensitive to the neuromuscular effects of local anesthetics.

There is no cross-sensitivity between amide-type anesthetics (like Lidocaine) and ester-type anesthetics (like Procaine or Benzocaine). If a patient is allergic to one class, the other may often be used safely, though skin testing is recommended.

> Important: Your healthcare provider will evaluate your complete medical history before prescribing Lidocaine.

Lidocaine is classified as Pregnancy Category B (historical). Animal studies have not shown harm to the fetus, and it is frequently used for epidural anesthesia during labor. However, it does cross the placenta, and large doses can cause fetal bradycardia. It should be used during pregnancy only if clearly needed.

Lidocaine is excreted into breast milk in very small amounts. The American Academy of Pediatrics considers Lidocaine to be generally compatible with breastfeeding, as the concentration is unlikely to cause adverse effects in the nursing infant.

Safety and effectiveness in pediatric patients depend on appropriate dosing. As noted, viscous lidocaine is contraindicated for teething. Pediatric patients are at a higher risk of systemic toxicity if doses are not strictly calculated by weight.

Elderly patients are more likely to have reduced hepatic function and decreased cardiac output. Lower doses and careful monitoring for CNS and cardiac toxicity are essential in this population.

No specific dose adjustment is usually required for a single dose, but for continuous infusions, monitoring for metabolite accumulation is necessary to prevent neurotoxicity.

Patients with significant hepatic impairment (Child-Pugh Class B or C) require a 50% reduction in maintenance infusion doses for antiarrhythmic therapy to avoid toxic accumulation.

> Important: Special populations require individualized medical assessment.

Lidocaine is a membrane-stabilizing agent. It binds to the S6 segment of domain IV of the alpha subunit of voltage-gated sodium channels. By stabilizing these channels in the inactivated state, it prevents the rapid depolarization required for nerve impulse transmission and cardiac action potential propagation.

• Onset of Action: Very rapid (2–5 minutes for infiltration; 10–15 minutes for epidural).
• Duration: Moderate (60–120 minutes without epinephrine; up to 3 hours with epinephrine).
• Tissue Effect: Epinephrine is often added to cause vasoconstriction, which slows Lidocaine's systemic absorption and prolongs its local effect.

| Parameter | Value |

|---|---|

| Bioavailability | 35% (Oral - due to first pass); 100% (IV) |

| Protein Binding | 60% - 80% (mainly to AAG) |

| Half-life | 1.5 - 2 hours (Adults) |

| Tmax | 30 - 45 minutes (Topical) |

| Metabolism | Hepatic (CYP1A2, CYP3A4) |

| Excretion | Renal (<10% unchanged) |

• Molecular Formula: C14H22N2O
• Molecular Weight: 234.34 g/mol
• Solubility: Very soluble in water (as hydrochloride salt).
• Structure: An aminoethylamide derivative with a diethylamino group and a dimethylphenyl ring.

Lidocaine is the prototypical Amide Local Anesthetic. It is also a Class Ib Antiarrhythmic according to the Vaughan Williams classification system.