Can Mucinex DM Cause False Positive Drug Tests?

In our latest question and answer, the pharmacist discusses whether or not Mucinex DM can cause a false-positive drug screening.


I took Two Mucinex DM this morning without thinking and i have a drug test on Thursday for a new job. Will Mucinex show up and give me a false positive or will I be okay?

Asked by caddy On Oct 30, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Nov 01, 2018
Drug Screening Title


Mucinex DM contains two active ingredients:

  • Guaifenesin
  • Dextromethorphan

Guaifenesin has not been linked to causing false-positives on drug tests. Dextromethorphan however, has been (1, 2). Nevertheless, dextromethorphan causing a false-positive is thought to be very uncommon and if it does occur, there are confirmatory tests available.

Dextromethorphan False-Positives

Dextromethorphan is an over the counter cough medication and a common ingredient in various cough, cold and flu products. It is useful in treating chronic, nonproductive coughs, but it has no expectorant activity.

Dextromethorphan is chemically related to opioid medications but does not exhibit their typical characteristics such as analgesia and respiratory depression.

Several studies, such as the one published by Mayo Clinic Proceedings, have listed dextromethorphan as being a possible agent responsible for causing false-positive results for:

  • Opiates (codeine and morphine)
  • Phencyclidine (PCP)

Therefore, taking Mucinex DM prior to a urine drug screening has the possibility of causing a 'false-positive', although this would be considered rare.

Why False-Positives Occur

Standard drug urinalysis tests are 'immunoassay' tests. These work by utilizing antibodies to react to specific drug compounds. When a specified drug is present in a test urine sample, an antibody binds to it, which produces a reaction. When a reaction occurs, it is recorded as a 'positive' result.

Unfortunately, immunoassays are subject to false positives. Sometimes an antibody can react with the wrong compound. It doesn't happen often, but when it does, it can create an unfortunate situation for the individual being tested.

If you believe a result you got on a drug test is incorrect, if possible, request that a more specific test be used to confirm or refute the results you received.

For example, gas or liquid chromatography with mass spectrometry can be used to identify the specific agent present in a sample as they don't rely on reactions with antibodies. While more expensive to use than your standard urinalysis test, they are far more accurate.


  • Mucinex DM contains two active ingredients, one of which (dextromethorphan) has been linked to causing false-positives on urinalysis drug screenings for opiates and PCP.

About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Office: 716-389-3076

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