What medications can cause false positives for cocaine on drug screens?

Medications That Can Cause False Positive Results for Cocaine on Drug Screens

Several medications and substances can cause false positive results for cocaine on urine drug screens, but most standard immunoassay panels have high specificity for cocaine metabolites with few documented cross-reactions. 1

Common Causes of False Positive Cocaine Results

Confirmed Non-Cross-Reactive Medications

• Lidocaine: Despite common claims, research shows that lidocaine and its metabolite norlidocaine (MEGX) do not cause false positive results for cocaine metabolite (benzoylecgonine) on standard immunoassay urine drug screens 2
• Amoxicillin: Despite mentions in lay literature, research has demonstrated that amoxicillin does not cause false positive urine screens for cocaine metabolites 3

Potential Cross-Reactive Substances

• Coca leaf tea: Contains small amounts of cocaine that can be detected
• Topical anesthetics: Some may have structural similarities to cocaine
• Certain over-the-counter medications: May interfere with test results in rare cases

Factors Affecting Drug Screen Accuracy

Immunoassay Limitations

• Standard immunoassay panels have known detection gaps and limitations 1
• Detection window for cocaine metabolites is typically only 2-4 days
• Dilute urine specimens may cause false negative results 1
• Laboratory error or improper specimen collection can affect results

Concentration Thresholds

• Screening cutoff concentrations (typically 300 ng/mL for benzoylecgonine) may miss low-level cocaine use 4
• In one documented case, a patient with cocaine intoxication tested negative on immunoassay screening because benzoylecgonine levels were below the cutoff threshold (75 ng/mL vs 300 ng/mL cutoff) 4

Best Practices for Drug Screen Interpretation

When Interpreting Positive Cocaine Results

• Always obtain a complete medication history before interpreting results 1
• Consider that preliminary screening tests yield more false positives than confirmatory tests 5
• Request confirmatory testing using gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) when results are unexpected 1

Clinical Implications

• Dismissing patients based on UDS results alone could constitute patient abandonment 1
• False positive results can have serious consequences, including inappropriate treatment, legal issues, or loss of employment 1
• Drug testing should be used for medical reasons with patient consent, not as a punitive measure 5

Other Notable Cross-Reactions in Drug Screens (Not Cocaine)

While not related to cocaine false positives, be aware of these common cross-reactions:

• Labetalol: Can cause false positive results for amphetamines and methamphetamines 6
• Metoprolol: Can cross-react with both amphetamine and MDMA (ecstasy) immunoassays 7
• Pseudoephedrine and dextromethorphan: Can cause false positive results on drug screens 5

Summary of Key Points

• Most medications do not cause false positive results for cocaine on standard immunoassay urine drug screens
• Despite common beliefs, research specifically shows that lidocaine and amoxicillin do not cause false positive cocaine results
• When unexpected positive results occur, confirmatory testing using GC-MS or LC-MS should be performed
• Always consider the clinical context and obtain a complete medication history when interpreting drug screen results