How Prescription Medications Trigger False Positives
Urine drug tests typically begin with an immunoassay, a quick and inexpensive screening method. This test uses antibodies to detect the presence of certain drug metabolites in the urine. However, the immunoassay can sometimes be fooled by substances that have a similar chemical structure to illegal drugs, a phenomenon known as cross-reactivity. When this happens, a legally prescribed medication can trigger a "false positive" result, indicating the presence of an illicit drug that was never used.
Once an initial screen returns a positive result, especially in professional or clinical settings, a more advanced and highly accurate test, known as Gas Chromatography-Mass Spectrometry (GC-MS), is performed. The GC-MS test can differentiate between the chemical composition of different substances, verifying the result and clearing the individual.
Common medications that can cause false positives
Numerous types of medications, both prescription and over-the-counter, can potentially interfere with urine drug screens, leading to false positives for various illicit substances. These include certain antidepressants (like bupropion and sertraline which may show up as amphetamines or benzodiazepines), some pain medications (like tramadol for PCP or methadone), specific antibiotics (quinolones for opiates), and even over-the-counter drugs containing pseudoephedrine or diphenhydramine. Additionally, consuming poppy seeds can sometimes lead to a false positive for opiates due to trace amounts of codeine and morphine. Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen have also been rarely associated with false positives for THC or barbiturates, while some proton pump inhibitors (PPIs) have been linked to false positives for THC.
The role of the Medical Review Officer (MRO)
In professional drug testing, an initial positive result from the screening test is not immediately reported to the employer. Instead, it goes to a Medical Review Officer (MRO), a physician trained to interpret drug test results. The MRO contacts the individual to discuss any medications that could explain the positive screen, a crucial step in distinguishing legal prescriptions from illicit drug use.
Comparison of Screening and Confirmatory Tests
| Feature | Immunoassay (Screening Test) | Gas Chromatography-Mass Spectrometry (GC-MS - Confirmatory Test) |
|---|---|---|
| Accuracy | Lower accuracy; prone to false positives. | High accuracy; identifies specific chemical signatures. |
| Cost | Inexpensive and cost-effective for initial screening. | More expensive and used as a second step. |
| Turnaround Time | Rapid results, often same-day. | Slower; requires more complex lab analysis. |
| Detection Method | Uses antibodies to detect drug classes. | Identifies precise molecular structure. |
| Purpose | Broad, initial screening for a panel of drugs. | Confirms identity and quantity of a specific substance. |
What to do if you get a false positive
If you receive a positive drug test result and believe it is due to a prescribed medication, follow these steps:
- Do not panic: Understand that initial screenings are not definitive. The MRO is there to verify your legal medication use.
- Disclose your medications: When contacted by the MRO, be prepared to provide a full list of all prescription and over-the-counter drugs you have taken. Some testing facilities provide a form for this purpose before the test is even conducted.
- Provide verification: The MRO will need to verify your prescription. They can do this by contacting your prescribing physician or pharmacy. Having pharmacy receipts or the original prescription bottle can assist in this process.
- Request a confirmatory test: If an MRO is not involved or you need further verification, you have the right to request a more accurate confirmatory test, such as GC-MS.
- Inform your employer discreetly: You do not have to disclose your specific medical condition to your employer. The MRO will report a "negative" result back to the employer once your prescription is verified. For safety-sensitive jobs, you may need to discuss potential work restrictions with your doctor and employer if the medication could impair your ability to perform tasks.
Conclusion
It is entirely possible for prescription medications to show up on a urine drug test, particularly during the initial immunoassay screen. This happens because some medications share similar chemical structures with illicit drugs, causing cross-reactivity. However, a false positive from a legally prescribed medication is not a final verdict. The system includes crucial safeguards, like the Medical Review Officer (MRO) and confirmatory testing, to ensure accurate results and protect individuals from undue consequences. By understanding the process and being prepared to provide verification, you can navigate a positive drug screen with confidence and prevent any misunderstanding of your medical treatment. For more detailed information on specific cross-reactions, consulting with a pharmacist or reading authoritative literature from sources like the National Institutes of Health (NIH) is recommended.
Potential Cross-Reactions of Prescription Medications
Certain prescribed and over-the-counter medications are known to cause false-positive results for specific illicit drugs. These include bupropion (Wellbutrin) for amphetamines, dextromethorphan (Robitussin) for PCP or opiates, diphenhydramine (Benadryl) for opiates or PCP, fluoroquinolone antibiotics like levofloxacin for opiates, ibuprofen/naproxen (Advil/Aleve) for barbiturates or THC in rare cases, phentermine (Adipex-P) for amphetamines, pantoprazole (Protonix) for THC, quetiapine (Seroquel) for methadone, sertraline (Zoloft) for benzodiazepines or LSD, and tramadol for PCP or methadone.
