Allegra (fexofenadine), a second-generation antihistamine, hasn't been mentioned in studies as being responsible for causing a 'false-positive' result on a urine drug screening.
However, several other antihistamines, like Benadryl (diphenhydramine) and brompheniramine have been extensively reported to cause false-positives for several drugs, including amphetamines and PCP.
Additionally, pseudoephedrine, the nasal decongestant in Allegra-D, has definitively been shown to cause false-positives for amphetamines.
I discuss this in more detail in the sections below.
Why Do False-Positives Happen?
False-positives on standard urine drug screenings are, unfortunately, fairly common, simply due to their non-specific nature.
They work by utilizing antibodies, that are designed to react with certain drug compounds.
When a drug compound being tested for is present in a sample, an antibody binds to it, which produces a reaction. This reaction is recorded as a positive result.
A false-positive occurs when an antibody binds to the wrong compound, typically one that is structurally related to the drug that is being tested for.
It isn't known how often false-positive drug tests occur, but most studies suggest they can happen fairly regularly.
In fact, one study that evaluated over 491 positive urinalysis tests for PCP found that less than 80% were confirmed positive by more specific testing methods (specifically gas chromatography-mass spectrometry). This means that over 20% of 'positive' urinalysis tests (in this study at least) were 'false-positives'.
Section SummaryFalse-positive results on urine drug screenings happen when an antibody reacts to the wrong drug compound in a sample, which can happen fairly often.
As mentioned, Allegra specifically hasn't been shown to cause false-positives on drug screenings but other antihistamines have, including:
Even though Allegra hasn't been specifically mentioned in studies as potentially causing a false-positive, it certainly is possible based on its structural similarity to other antihistamines.
Section SummaryAllegra (fexofenadine) has not been reported to cause false-positives on drug tests but other antihistamines, like Benadryl (diphenhydramine), have.
Pseudoephedrine False Positives
Pseudoephedrine is so closely related to amphetamine products that many studies theorize that amphetamine drug tests are likely the most difficult to interpret. A large review study, 'Urine drug screening: Practical Guide for Clinicians', states:
"Unfortunately, other stimulants, anorexiants, and chemically related compounds (e.g, pseudoephedrine), have been shown to produce false-positive results, making the amphetamine assay one of the most difficult tests to interpret."
Another decongestant, phenylephrine (Sudafed PE) has also been reported to cause false positives for amphetamine drugs.
Section SummaryPseudoephedrine can cause false-positives for amphetamines on drug tests.
What To Do If You Have A False-Positive Result
If you feel like Allegra-D is responsible for causing a false-positive drug test, see if you can request a confirmation test. These are done using methods that can actually detect specific compounds in a sample and don't rely on chemical reactions. The most common confirmatory test is gas chromatography-mass spectrometry. Also, be sure to tell your doctor, or tester, all the medication you take.
- American Journal of Health-System Pharmacy Commonly prescribed medications and potential false-positive urine drug screens.
- PubMed How Often Do False-positive Phencyclidine (PCP) Urine Screens Occur with Use of Common Medications?
- PubMed Interferences with urine drug screens.
- PubMed Urine drug screening: Practical guide for clinicians.