Can Your Allergy Medicine Cause A Failed Drug Test?

In our latest question and answer, the pharmacist discusses which allergy medications can cause false positives on drug tests.

May 10, 2018

Tink asked

I have been taking OTC allergy meds, but have to buy them & give ID when I do. I had a work urine drug screen the other day & it showed positive for amphetamines... How does it breakdown in the body to do that? I need to prove I am not on anything illegal. I am confused as to how this happened?


Drug Test BottleSeveral over the counter and prescription allergy medications have been reported and documented in studies to be responsible for false-positives on drug tests. Below is a list of commonly used antihistamines that could potentially cause a failed drug test.

  • Brompheniramine: A "first-generation" antihistamine, brompheniramine is one of the active ingredients in Dimetapp. Brompheniramine has the potential to cause a false positive for amphetamines on urine drug screenings.

  • Ranitidine: The active ingredient in Zantac, ranitidine is a H2 blocker, most commonly used for acid reflux. It can also be used as an adjunct treatment for allergies. Ranitidine has been known to cause false positives for amphetamine on drug tests.

  • Promethazine: A prescription antihistamine commonly used in combination with cough medication in anti-tussive products. Promethazine may cause false positives for amphetamine on drug tests.

  • Diphenhydramine: The active ingredient in Benadryl, diphenhydramine is one of the most used over the counter antihistamines and is used for allergies and as a sleep aide. Studies have shown that diphenhydramine can cause false positives for both opioids and PCP.

  • Doxylamine: One of the active ingredients in NyQuil, doxylamine is a sedating first generation antihistamine and has been reported to cause false positives for opiates on urine drug tests.

  • Antihistamine/Decongestant combinations: Any allergy medication that contains a decongestant, such as phenylephrine or pseudoephedrine, can potentially cause a false positive for amphetamine or methamphetamine. These drugs include Claritin-D, Zyrtec-D and Allegra-D. All pseudoephedrine products in the United States require your ID to be available in order to purchase. Pseudoephedrine is the active ingredient in Sudafed, which is well known to cause false-positives for amphetamine based on its chemical similarity to methamphetamine.
Why Do Antihistamines Cause False Positives On Drug Tests?

False-positives on standard urinalysis drug tests are fairly common unfortunately. The main problem stems from the detection methods they use. Most employers/medical offices use simple, urine based immunoassay tests, which work by utilizing antibodies to react to specific drug compounds.

When a specified drug (i.e. drug being tested for) is present in a test urine sample, an antibody binds to it, producing a reaction that is classified as a positive result. Unfortunately with these urine tests, there can be a significant problem with 'cross-reactivity'. Drugs that are not being tested for often react with the wrong antibody (i.e. 'cross react'), which results in a "positive" result, although incorrect. As mentioned above, many antihistamines are known to "cross-react" with the wrong antibodies which lead to false-positives.

What To Do If You Receive A False Positive Drug Test

Urine immunoassay drug tests should ideally only be used as an initial screening. As they are known to cause false-positives, positive results should be confirmed or refuted with more accurate tests. More accurate tests actually can identify specific drug compounds in a sample, as opposed to relying on a chemical reaction. These tests include gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC).

These tests do not use antibodies and can accurately determine the exact molecule in question. If you received a 'false positive', these tests will be able confirm whether or not the offending drug compound is actually present in the tested sample.


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