Can Allegra Cause A False-Positive Drug Test?

Answer

Thanks for reaching out to us! First and foremost, I'm so sorry to hear about the trouble you're going through. It must be incredibly frustrating and challenging to navigate.

Allegra (fexofenadine), a second-generation antihistamine, hasn't been mentioned, as far as I'm aware, in studies as being a responsible agent 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 (a combination product containing Allegra and pseudoephedrine), has definitively been shown to cause false positives for amphetamines.

I discuss this in more detail in the sections below.

So, overall, it's unlikely that Allegra caused your false positive result for cocaine, but just because something isn't yet reported doesn't mean it can't happen. Be sure to let your tester/doctor know about all the medications you take, and I'm hoping you can get another shot at a screening for your employment.

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.

Antihistamine False-Positives