Benzocaine and lidocaine are two common local anesthetics used to temporarily numb specific areas of the body. They are found in many over-the-counter and professional medical products to manage pain and discomfort. While both substances achieve a similar numbing result, they possess differences in their chemical structure, which affects their application, potency, and safety considerations.
Chemical Makeup and How They Work
The primary distinction between benzocaine and lidocaine resides in their chemical composition. All local anesthetics feature an intermediate chain that connects an aromatic ring to an amine group. The nature of this chain classifies the anesthetic: benzocaine is an “ester,” while lidocaine is an “amide.” This structural difference dictates how the body processes each substance.
This chemical classification determines the metabolic pathway for each drug. Ester-based anesthetics like benzocaine are broken down rapidly in the blood by enzymes called plasma pseudocholinesterases. This rapid breakdown in the bloodstream contributes to a shorter duration of action.
In contrast, amide-based anesthetics like lidocaine undergo metabolism in the liver. This process is more complex and slower than the breakdown of esters in the plasma. The liver’s involvement means that the numbing effect of lidocaine lasts longer. Both anesthetics share a core mechanism of action: they block sodium channels in nerve cell membranes to prevent the transmission of pain signals to the brain.
Comparing Clinical Characteristics
The chemical differences between benzocaine and lidocaine translate directly into their clinical performance characteristics, specifically their onset, potency, and duration of action. These factors are important in determining which anesthetic is suitable for a particular use, whether in a clinical setting or for at-home care.
Benzocaine is known for its rapid onset, especially in topical formulations like sprays. Some products can begin to numb an area within 15 to 30 seconds. Lidocaine’s onset is generally slower, with topical creams taking between three to five minutes to take effect. However, this can vary based on the formulation and concentration.
Lidocaine is a more potent anesthetic than benzocaine. This increased potency is linked to its higher lipid solubility, which allows it to permeate nerve membranes more easily. Lidocaine also has a longer duration of action, providing relief for approximately 35 to 40 minutes, compared to benzocaine’s duration of about 15 minutes.
Common Uses and Formulations
The distinct properties of benzocaine and lidocaine have led to their use in different products and settings. Benzocaine is most frequently found in over-the-counter (OTC) preparations due to its rapid action on surface tissues. Consumers commonly encounter it in sore throat lozenges and sprays, such as those sold under the brand name Cepacol. It is also the active ingredient in many topical creams for toothache and gum irritation, like Orajel.
Lidocaine is available in both OTC and prescription-strength formulations, reflecting its higher potency and longer-lasting effects. As an OTC product, it is often found in pain-relieving patches and creams designed to alleviate muscle and nerve pain, such as Lidoderm patches. These products deliver the anesthetic directly to the painful area over an extended period.
In professional medical and dental environments, lidocaine is a frequently used injectable anesthetic. Its reliability and longer duration make it suitable for minor surgical procedures, such as suturing wounds or performing dental work.
Safety Profiles and Potential Risks
Both benzocaine and lidocaine are generally safe when used as directed, but they carry potential risks and side effects. Common, minor side effects for both can include temporary redness, stinging, or a burning sensation at the application site. However, each anesthetic is also associated with distinct and more serious safety concerns that users should be aware of.
The most significant risk associated with benzocaine is a rare but serious blood disorder called methemoglobinemia. This condition reduces the amount of oxygen carried in the bloodstream and can be life-threatening. The U.S. Food and Drug Administration (FDA) has issued specific warnings against the use of OTC oral benzocaine products for teething pain in infants and children under two years of age due to this risk. FDA analysis has shown that benzocaine is significantly more likely to cause methemoglobinemia than lidocaine.
For lidocaine, the primary concern is the potential for systemic toxicity if excessive amounts are applied or absorbed into the bloodstream. This can affect the central nervous system and cardiovascular system, leading to symptoms like dizziness, muscle twitching, or, in severe cases, seizures and cardiac arrhythmias. The higher risk of allergic reaction with benzocaine is because its breakdown produces para-aminobenzoic acid (PABA), a known allergen.