Pharmaceutical Drugs That Can Cause False Positive Hair Tests for Methamphetamines
Several pharmaceutical medications can cause false positive results for methamphetamines in hair tests, with phentermine being the most common due to its structural similarity to methamphetamine. 1
Common Medications Causing False Positives for Methamphetamines
Sympathomimetic Medications
Phentermine: As a sympathomimetic amine anorectic with the chemical structure α,α,-dimethylphenethylamine hydrochloride, phentermine is chemically and pharmacologically related to amphetamines 1. Its structural similarity to methamphetamine makes it a common cause of false positives in hair testing.
Over-the-counter cold medications containing pseudoephedrine: These can cause false-positive screening results for amphetamines 2.
Other Medications
Atomoxetine: Used for ADHD treatment, it has been reported to cause false-positive results for amphetamines in urine tests, which may potentially affect hair testing as well 3.
Metoprolol: This beta-blocker has been documented to cross-react with amphetamine and MDMA immunoassays in urine tests at certain concentrations, which could potentially affect hair testing 4.
Limitations of Hair Testing for Methamphetamines
Hair testing for methamphetamines has several important limitations:
Validity concerns: The American Academy of Pediatrics notes that the validity of hair testing has not been firmly established 2.
Passive exposure: Hair tests may be affected by passive exposure to drugs, potentially leading to false positives 2.
Demographic variations: Differences among races and sexes can affect hair testing results 2.
Historical vs. current use: Hair testing is more likely to detect historical drug use rather than current use 2.
Confirmatory Testing Recommendations
When a hair test shows positive for methamphetamines:
Gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS/MS) should be used for confirmation, as these methods can definitively distinguish between similar compounds like phentermine and methamphetamine 5, 6.
The sensitivity of GC-MS for hair analysis of amphetamines can detect concentrations as low as 0.03-0.08 ng/mg of hair 6.
Patient Management Considerations
Patients taking medications like phentermine should inform healthcare providers about their medication use before undergoing drug testing 5.
Healthcare providers should obtain a complete medication history, including prescription and OTC medications, when interpreting drug test results 5.
Unexpected positive results should prompt a conversation with the patient rather than immediate punitive action 5.
Important Caveats
Hair samples can detect drug use over a longer period compared to urine or blood tests, making them more likely to identify historical rather than recent use 2.
Confirmatory testing is essential as immunoassay-based tests lack specificity to distinguish between structurally similar compounds 5.
Even with confirmatory testing, interpretation should consider the possibility of pharmaceutical drug use rather than illicit drug use when positive results occur 5.