Lidocaine is a widely used local anesthetic. It works by interrupting the transmission of pain signals along nerve fibers at the site of application. The drug achieves this effect by blocking the voltage-gated sodium channels within nerve cell membranes, preventing the nerves from firing an electrical signal to the brain. The time the numbing sensation lasts is highly dependent on the method of delivery, the total dose administered, and the specific formulation used.
Duration of Topical Applications
Lidocaine applied topically comes in various forms, including creams, gels, patches, and sprays, designed to numb the skin or mucous membranes. The onset of the numbing effect for topical creams and gels is relatively fast, often beginning within three to five minutes of application. Achieving optimal numbness typically requires 25 to 40 minutes for the drug to sufficiently penetrate the outer layers of the skin.
The duration of the effect for standard topical applications is generally short because the drug is absorbed slowly and only achieves a shallow, localized concentration. A typical lidocaine cream or gel provides pain relief that lasts approximately one to two hours.
For chronic pain management, transdermal patches deliver the drug slowly and continuously, providing sustained local relief for up to 12 hours. The concentration of the product also influences duration, with prescription-strength formulations having a more profound and longer-lasting effect than over-the-counter options.
Duration of Local Nerve Blocks
Lidocaine injections deliver the anesthetic directly to the targeted nerves for procedures like dental work, minor surgery, and nerve blocks, resulting in a much faster and more profound effect. When administered as “plain” lidocaine (without additional compounds), the numbing effect sets in within minutes but lasts for a relatively short period, often between 30 and 90 minutes. This short duration occurs because the drug is quickly picked up by the rich blood supply in the tissue and carried away into the systemic circulation.
The most significant factor in extending the duration is the addition of a vasoconstrictor, most commonly epinephrine. Epinephrine constricts the blood vessels surrounding the injection site, dramatically slowing the rate at which lidocaine is absorbed and keeping the drug concentrated at the nerve for a longer time. When combined with epinephrine, the duration of the anesthetic effect is often extended to two to six hours, which provides sufficient time for most medical and dental procedures.
Systemic Metabolism and Elimination
Once absorbed from the application site, lidocaine enters the bloodstream and begins systemic metabolism. The drug is primarily processed by the liver, which breaks it down into various metabolites. The rate at which the body clears the drug is defined by its half-life.
In healthy adults, the elimination half-life of lidocaine is typically between 1.5 and 2 hours. Complete systemic elimination, where the drug is no longer pharmacologically active, usually takes approximately eight to twelve hours in individuals with normal liver function.
Any condition that impairs liver function, such as severe congestive heart failure or hepatic disease, can significantly prolong the half-life. In patients with liver dysfunction, the elimination time can be extended two-fold or more, meaning the drug remains in the body for a much longer period.