Pharmaceutical Drugs That Can Cause False Positive Results for Methamphetamines in Hair Tests
Several pharmaceutical medications can cause false positive results for methamphetamines in hair tests, with pseudoephedrine (found in many over-the-counter cold medications) being the most common culprit. 1
Common Medications That Can Cause False Positives for Methamphetamine
Decongestants and Sympathomimetics
- Pseudoephedrine (found in many OTC cold medications) 1
- Phenylephrine (nasal decongestant and vasopressor) 2
- Phentermine (weight loss medication) 3
Antidepressants and Other Medications
- Bupropion (Wellbutrin, Zyban) - most frequent cause of false positives in urine tests 4
- Metoprolol (beta-blocker) - can cross-react with both amphetamine and MDMA immunoassays 5
Hair Testing Considerations
Hair testing has specific characteristics that affect interpretation of results:
- Hair testing detects historical drug use rather than current use 1
- Cannot detect use within the previous 7-10 days 1
- Most reliable for heavy, frequent past use rather than occasional use 1
- Hair structure, growth rate, melanin content, hygiene, and cosmetic treatments can affect results 1
- False positives may occur with volatile drugs that can adhere to hair 1
Limitations of Testing Methods
Immunoassay Limitations
- Standard immunoassays lack specificity to distinguish between structurally similar compounds 3
- Cross-reactivity between similar chemical structures is common 3
- Metabolites of certain medications can also trigger false positives 5
Confirmatory Testing
When a positive hair test result for methamphetamine is obtained:
- Confirmatory testing with gas chromatography/mass spectrometry (GC-MS) or liquid chromatography/mass spectrometry (LC-MS/MS) should be performed 3
- These methods can definitively distinguish between pharmaceutical medications and actual methamphetamine 3
- Segmental hair analysis can provide valuable retrospective information on the history of drug exposure 6
Clinical Implications
- False positive results can have serious consequences for patients, including inappropriate treatment decisions, legal issues, or employment consequences
- Patients taking medications that may cause false positives should inform healthcare providers about their medication use 3
- Healthcare providers should obtain a complete medication history before interpreting drug test results 3
- Unexpected positive results should prompt a conversation with the patient rather than immediate punitive action 3
Avoiding Misinterpretation
- Always consider the possibility of false positives when interpreting hair test results
- Document all medications the person is taking, including OTC medications
- Request confirmatory testing when results are unexpected or disputed
- Recognize that hair testing is more likely to detect historical rather than recent drug use 1
- Be aware that passive exposure to drugs can affect hair testing results 1