Lidocaine is a common local anesthetic used in dental injections, minor surgical procedures, and topical creams for pain relief. Despite its widespread medical use, many people worry that lidocaine could trigger a positive result on mandated drug tests, such as those required for employment or legal purposes. This article clarifies the relationship between lidocaine use and drug screening results by examining the drug’s legal status, testing methods, and how the body metabolizes the compound.
Is Lidocaine a Controlled Substance
Lidocaine is an amino amide local anesthetic that works by temporarily blocking sodium channels in nerve cells, preventing pain signals from reaching the brain. It is not classified as a federally scheduled controlled substance under the United States Controlled Substances Act. Therefore, lidocaine does not fall into the categories of drugs, such as narcotics or stimulants, that are regulated due to their potential for abuse and dependence.
Because lidocaine is not a controlled substance, its presence in a biological sample is not the cause of a positive drug test result. Drug testing is designed to detect illicit substances or misused controlled prescription drugs. If a positive result occurs after lidocaine use, it is due to a phenomenon known as cross-reactivity. This potential for cross-reactivity arises because the drug’s chemical structure and its metabolites can sometimes resemble the compounds that drug tests are designed to detect.
Understanding Initial Drug Screening Methods
Drug testing involves a two-step process: a preliminary screen followed by a definitive confirmation test. The initial screen is typically performed using an immunoassay (IA) test, which is a fast and cost-effective method. This test uses antibodies designed to bind to specific drug molecules or their metabolites, providing a presumptive result.
The limitation of the immunoassay screen is its lack of chemical specificity. Since the antibodies look for a general molecular shape, they can mistakenly bind to other compounds with similar structures. This accidental binding is called cross-reactivity and is the mechanism responsible for nearly all initial false-positive results. If a sample produces a presumptive positive result on the initial IA screen, the testing facility must proceed to the second, highly specific step.
The definitive confirmation test uses techniques such as Gas Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography-Mass Spectrometry (LC-MS). These methods are precise because they separate the sample into individual chemical components. Each molecule is then identified based on its unique mass and fragmentation pattern. This provides an exact chemical fingerprint, eliminating ambiguity created by the less-specific immunoassay screen. This accurate confirmation method can definitively distinguish between a legitimate drug metabolite and an illicit substance, resolving any false-positive result caused by cross-reactivity.
Chemical Structure and Metabolite Confusion
The potential for lidocaine to cause a presumptive false positive is rooted in its chemical breakdown within the body. When metabolized, the body breaks the parent drug down into several compounds, primarily Monoethylglycinexylidide (MEGX), also known as norlidocaine, and Glycinexylidide (GX). These metabolites are present in a urine sample and could hypothetically trigger the antibody-based initial screen.
Lidocaine has an amide-type chemical structure, which is distinct from the tropane alkaloid structure of cocaine. However, the structural makeup of the MEGX metabolite could theoretically share a minor resemblance to molecular fragments of certain illicit substances, such as the cocaine metabolite benzoylecgonine. This slight structural similarity causes immunoassay antibodies to occasionally mistake the lidocaine metabolite for a target drug. Scientific studies generally find that lidocaine and its metabolites do not cause false positives for cocaine on standard urine immunoassays.
The concentration of metabolites in the urine is a factor, as high levels of MEGX could increase the chance of cross-reactivity with screening antibodies. The mechanism for a false positive is dependent on the non-specific nature of the initial screen detecting a structurally similar compound. This is why any presumptive positive result must always be confirmed by the specific GC/MS method. The confirmation test correctly identifies MEGX and rules out the presence of any illicit substance.
What to Tell the Medical Review Officer
If a drug test returns a presumptive positive result after lidocaine use, the next step involves an interview with a Medical Review Officer (MRO). The MRO is a licensed physician with specialized knowledge of drug testing protocols. They act as an impartial gatekeeper between the laboratory and the employer. Their primary role is to investigate and verify any legitimate medical explanation for a non-negative test result.
During this confidential interview, you should disclose all legal medications and substances used, including any recent form of lidocaine administered. It is beneficial to have documentation ready, such as a prescription, a physician’s note, or medical records confirming the date and reason for the lidocaine use. The MRO uses this information to determine if the positive screening result is attributable to a legally used, non-controlled substance.
Once the MRO receives the confirmatory GC/MS or LC-MS test results, they will correctly identify the presence of lidocaine or its metabolites, not an illicit drug. The MRO will then report the final result as negative to the employer. Providing the MRO with prompt documentation of your legitimate lidocaine use is the most effective action to ensure a presumptive positive screening result is correctly reported as negative.