Can topical lidocaine (local anesthetic) cause a false positive urine test for cocaine?

Topical Lidocaine Does Not Cause False Positive Urine Tests for Cocaine

Topical lidocaine does not cause false positive results on standard cocaine urine drug screens, as confirmed by recent high-quality research. 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 on cocaine urine drug screens:

• Study included 121 samples positive for lidocaine
• 114 samples were also positive for lidocaine's primary metabolite (MEGX)
• None of the samples tested positive for cocaine/benzoylecgonine on standard urine immunoassay testing
• 95% confidence interval for false positive rate was 0-3.9% 1

Important Distinctions Between Lidocaine and Cocaine

Chemical Structure and Testing

• Lidocaine and cocaine are both local anesthetics but have different chemical structures
• Standard urine drug screens test for benzoylecgonine (BE), cocaine's primary metabolite
• Lidocaine's metabolite (MEGX) is structurally different from BE 1

Clinical Applications

• Lidocaine is preferred over cocaine for topical anesthesia in dermatologic and other procedures due to:
  • Lower cost
  • Better safety profile
  • Fewer adverse effects 2

Situations That Can Cause Positive Cocaine Tests

While pure lidocaine does not cause false positives for cocaine, it's important to be aware of situations that can:

1. TAC solution use: Tetracaine-Adrenaline-Cocaine mixtures (which contain actual cocaine) will produce positive urine tests for cocaine for up to 36-48 hours after application 3

2. Medicinal cocaine use: Intranasal cocaine used as a vasoconstrictor in ENT procedures will cause positive urine tests for up to 72 hours 4

3. Adulterated street drugs: Cocaine is often mixed with lidocaine in illicit preparations, but the positive test would be from the cocaine itself, not the lidocaine 5, 6

Implications for Drug Testing

When interpreting urine drug screens:

• A positive cocaine test in a patient who received only lidocaine should not be attributed to the lidocaine 1
• Always document medications the patient is currently taking to help interpret results 7
• Consider confirmatory testing using GC-MS or LC-MS to verify positive screening results, as these methods can differentiate between various substances with high accuracy 7

Clinical Practice Recommendations

• When topical anesthesia is needed, non-cocaine formulations like lidocaine are preferred over cocaine-containing preparations 2
• If cocaine-containing preparations (like TAC) are used, inform patients that they may test positive for cocaine for up to 36-48 hours 3
• Unexpected positive results should prompt a conversation with the patient rather than immediate punitive action 7

In summary, healthcare providers can confidently use topical lidocaine without concern about causing false positive cocaine urine tests in their patients.