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