What pharmaceutical drugs can cause a false positive hair test for methamphetamines?

Pharmaceutical Drugs That Can Cause False Positive Hair Tests for Methamphetamines

Over-the-counter cold medications containing pseudoephedrine are the most common pharmaceutical drugs that can cause false positive hair tests for methamphetamines, though confirmatory testing with gas chromatography and mass spectrometry can reliably distinguish between these substances. 1

Understanding Hair Testing for Methamphetamines

Hair testing for methamphetamines has several important characteristics:

• Detects historical drug use rather than current use 1
• Cannot detect drug use in the previous 7-10 days 1
• Most reliable for heavy, frequent past use rather than occasional use 1
• Typically uses the first 3 cm of hair closest to the scalp (covering approximately 90 days) 1
• Subject to potential issues with interpretation due to:
  • Passive environmental exposure 1
  • Differences in results based on race and sex 1
  • Hair structure, melanin content, and cosmetic treatments 1

Medications That Can Cause False Positives

Confirmed Cross-Reactants

• Pseudoephedrine (found in many over-the-counter cold medications) 1, 2
• Phentermine (weight loss medication) - structurally similar to methamphetamine and can cross-react on immunoassay tests 2

Testing Limitations and Interpretation

Hair testing for methamphetamine faces several challenges:

• Standard immunoassays lack specificity to distinguish between structurally similar compounds 2
• Hair tests may detect environmental exposure rather than actual use, particularly in children 3
• The AP (amphetamine) to MA (methamphetamine) ratio in hair samples shows large variation 4
• Cosmetic treatments including shampoos, bleaches, or dyes can alter drug concentrations in hair 1

Confirmatory Testing Recommendations

When a hair test shows positive for methamphetamine, confirmatory testing is essential:

• Gas chromatography/mass spectrometry (GC-MS) or liquid chromatography/mass spectrometry (LC-MS/MS) should be used to confirm positive results 2
• These methods can definitively distinguish between pseudoephedrine, phentermine, and actual methamphetamine 2
• Confirmatory testing is highly specific and can reliably confirm the presence of amphetamine or methamphetamine 1

Clinical Considerations

• Hair testing is more sensitive than urine or oral fluid testing for detecting long-term methamphetamine exposure 3
• Hair analysis can provide information about drug use patterns over months rather than days 5
• Unexpected positive results should prompt a conversation with the patient rather than immediate punitive action 2
• Always obtain a complete medication history, including prescription and OTC medications, when interpreting positive results 2

Important Caveats

• Hair testing validity has not been firmly established compared to other testing methods 1
• False positive results may be obtained with volatile drugs that can adhere to hair 1
• Environmental contamination can occur and lead to false-positive results, particularly in environments where methamphetamine is manufactured 3
• Laboratory methods and cutoff values vary, affecting interpretation of results 2

When interpreting hair test results for methamphetamine, always consider the possibility of pharmaceutical drug use and obtain confirmatory testing before making any definitive conclusions about illicit drug use.