Can Allergy Medications Cause False Positive Results for Methamphetamines?
Yes, certain allergy medications, particularly those containing pseudoephedrine, can cause false positive results for methamphetamines on immunoassay-based urine drug screens due to structural similarities between these compounds. 1
How False Positives Occur
Immunoassay-based drug tests, which are commonly used for initial screening, have limited specificity and are prone to cross-reactions with medications that have similar chemical structures:
- Pseudoephedrine is structurally similar to amphetamine and methamphetamine 2
- OTC allergy and cold medications containing pseudoephedrine are particularly likely to trigger false positives for methamphetamines 1, 3
- Other antihistamines that can cause false positives include:
- Brompheniramine
- Diphenhydramine
- Doxylamine
- Promethazine 3
Factors Affecting Test Results
Several factors influence the likelihood of false positive results:
- Timing of medication use relative to testing
- Dosage of the allergy medication taken
- Specific immunoassay technology used for screening
- Individual metabolic differences
Confirmation Testing
When a positive result occurs on an initial screening test:
- Confirmatory testing using gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) should be performed 1
- These confirmatory methods can differentiate between actual methamphetamine use and false positives from allergy medications
- Confirmation testing is considered the gold standard and provides more accurate results with higher specificity 1
Clinical Implications
For healthcare providers interpreting drug test results:
- Always document any medications the patient is currently taking, including OTC allergy medications 1
- Unexpected positive results should prompt a conversation with the patient rather than immediate punitive action 1
- Consider the limitations of immunoassay screening tests when interpreting initial positive results 1
- Be aware that pseudoephedrine can be extracted from OTC products and used as a precursor in methamphetamine production, which may complicate interpretation in some contexts 4, 5
Recommendations for Testing
To minimize false positive results and ensure accurate interpretation:
- Use confirmatory testing for all positive screening results before making clinical decisions 1
- Consider medication history when interpreting drug test results
- Recognize that qualitative immunoassay tests have inherent limitations in specificity 1
- For legal or employment testing, always confirm positive screening results with more specific methods
Remember that a negative result on confirmatory testing following a positive screen strongly suggests that the initial result was a false positive, potentially due to cross-reactivity with allergy medications or other substances.