Can topical lidocaine (local anesthetic) cause a false positive for cocaine in drug screening tests?

Topical Lidocaine Does Not Cause False Positive Results for Cocaine in Drug Screening Tests

Topical lidocaine does not cause false positive results for cocaine in standard drug screening tests, as confirmed by high-quality research specifically examining this question. 1

Evidence Overview

The most recent and highest quality evidence directly addressing this question comes from a 2019 cross-sectional observational study that specifically investigated whether lidocaine exposure could cause false positive results for cocaine on urine drug screens:

• 121 samples positive for lidocaine were analyzed
• 114 of these samples were also positive for lidocaine's metabolite (MEGX)
• None of the samples tested positive for cocaine/benzoylecgonine on standard immunoassay testing
• The study concluded there is no evidence that lidocaine or its metabolite can produce false positives on standard cocaine urine immunoassays 1

Chemical Structure and Testing Considerations

While both lidocaine and cocaine are local anesthetics, they have different chemical structures and metabolic pathways:

• Cocaine is metabolized to benzoylecgonine (BE), which is the target compound in most cocaine immunoassay tests
• Lidocaine is metabolized to monoethylglycinexylidide (MEGX)
• Modern immunoassay tests are designed to be specific for benzoylecgonine 1

Important Distinctions Between Lidocaine and Cocaine

Clinical Use

• Lidocaine is a preferred local anesthetic in medical settings due to its favorable cardiovascular and systemic toxicity risk profile 2
• Guidelines specifically recommend non-cocaine formulations over cocaine for topical anesthesia in office-based procedures 2

Testing Considerations

• While lidocaine itself doesn't cause false positives for cocaine, it's important to note that certain medications can cause false positives on drug screens:
  • Pseudoephedrine can cause false-positive results for amphetamines 2
  • Poppy seeds can cause false-positive results for opioids 2
  • However, lidocaine is not among these cross-reactive substances for cocaine tests 3, 1

Special Case: TAC Solution

One important exception to be aware of:

• TAC solution (Tetracaine, Adrenaline, and Cocaine) used as a topical anesthetic does contain actual cocaine
• Patients who receive TAC solution may test positive for cocaine for up to 36-48 hours after administration 4
• This is not a false positive but a true positive due to actual cocaine exposure

Confirmatory Testing

If there is concern about a false positive result on an immunoassay screening test:

• Confirmatory testing using gas chromatography/mass spectrometry (GC-MS) or liquid chromatography/mass spectrometry (LC-MS/MS) should be performed 3
• These methods can definitively distinguish between cocaine metabolites and other compounds

Clinical Implications

When interpreting drug test results:

• Consider the timing of potential drug use relative to testing
• Document all current medications to help interpret results
• Discuss unexpected positive results with patients in a non-judgmental manner
• Consider confirmatory testing when clinical history doesn't support cocaine use 3

In summary, the evidence clearly shows that topical lidocaine administration does not cause false positive results for cocaine on standard drug screening tests. If a patient tests positive for cocaine after receiving only lidocaine, other explanations should be considered.