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 on urine drug screenings.


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

Asked by Caddy On Jan 12, 2022

Answered by
Medical Content Reviewed By HelloPharmacist Staff

Published Jan 12, 2022
Last updated Mar 03, 2022

Key points

  • Mucinex DM contains two active ingredients, guaifenesin, and dextromethorphan.
  • Dextromethorphan has been linked to causing false-positives on urinalysis drug screenings for opiates and PCP.

Hello and thanks for reaching out! I can understand your nervousness!

While some over-the-counter medications have been reported to cause false-positive on urine drug tests, it's pretty uncommon.

Nevertheless, if you do get an unexpected result, it's good to know what could have contributed to it.

What Is In Mucinex DM?

Mucinex DM contains two active ingredients:

Now, guaifenesin has not been linked to causing false positives on urine drug tests but dextromethorphan has been.

However, dextromethorphan causing a false-positive is thought to be very uncommon and if it does occur, there are confirmatory tests available (although whether or not your future employer would allow further testing is something I don't know).

Can Mucinex DM Cause False-Positives?

Let's go over dextromethorphan, the cough suppressant contained in Mucinex DM. It 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.

It is chemically related to opioids, but does not exhibit typical opioid characteristics such as analgesia (i.e. pain relief) and respiratory depression.

Several studies, such as the one published by Mayo Clinic Proceedings, list 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.

One study on the matter concluded the following:

Although dextromethorphan is structurally similar to opioid drugs, the ingestion of a single normal (or even twice normal) dose of dextromethorphan is not likely to produce a falsely positive six-hour urine opioid EMIT screen.
Acad Emerg Med . 1995 Sep;2(9):791-4.

One thing to note here is that the amount of dextromethorphan you take seems to be correlated to your risk of showing a false positive. One study, in particular, noted that high doses (well over the recommended amount) are more strongly associated with false positives than lower doses.

Why False-Positives Occur

A quick word on how urine drug tests work.

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.

Final Words

Thanks again for contacting us and I hope everything works out!


  • Urine drug screening: practical guide for clinicians., PubMed
  • Commonly prescribed medications and potential false-positive urine drug screens, PubMed
  • High concentrations of dextromethorphan result in false-positive in opiate immunoassay test, PubMed

About the Pharmacist

Dr. Brian Staiger, PharmD

Dr. Brian has been practicing pharmacy for over 11 years and has wide-ranging experiences in many different areas of the profession. From retail, clinical and administrative responsibilities, he's your knowledgeable and go-to source for all your pharmacy and medication related questions!

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