Can Mucinex (Guaifenesin) Cause Diarrhea?

In our latest question and answer, the pharmacist discusses the side effects of Mucinex, and whether or not it can cause diarrhea.


I have a very sensitive stomach and am wondering if diarrhea is a common side effect of Mucinex.

Asked by Sal On Aug 10, 2022

Answered by
Medical Content Reviewed By HelloPharmacist Staff

Published Aug 10, 2022
Last updated Aug 10, 2022

Key points

  • Mucinex (guaifenesin) is generally very well tolerated, and side effects are infrequent and usually mild.
  • Diarrhea has been reported with Mucinex, but studies show that it is uncommon.
  • High doses of Mucinex are more likely to cause side effects, and if you are worried about them, you should start at a low dose and see how it affects you.


Hi Sal and thanks for reaching out to us!

Mucinex (guaifenesin), in general, is very well tolerated by most individuals and is only associated with mild side effects that happen infrequently.

Diarrhea Is An Infrequently Reported Side Effect

Clinical trials involving Mucinex (guaifenesin) have reported diarrhea as a possible side effect, but again, it appears to be relatively uncommon. In one clinical trial, involving 552 people, only 3 reported diarrhea as a side effect.

Guaifenesin Trial Diarrhea

Drug Healthc Patient Saf. 2019 -11: 87-94.

Most reported side effects of Mucinex don't happen at a frequency over what you would expect with taking a placebo.

It is certainly possible that an individual may have a sensitivity or allergy to an ingredient in Mucinex that may make them more likely to experience diarrhea, but again, it isn't commonly reported.

Does Dose Matter?

In studies that have reported side effects with Mucinex (which includes diarrhea), they tended to be more frequent with higher doses. High doses of Mucinex have been reported to cause headaches, nausea, vomiting, and abdominal pain although these side effects are still considered uncommon as they don't happen in many individuals.

Mucinex, at excessive doses, for long periods of time, has been reported to cause kidney stones, but this isn't definitive.

Final Words

Overall, most people have no side effects after taking Mucinex and if they do, they are generally mild.

If you have a sensitive stomach, it would be prudent to start at a low dose and see how you tolerate it.

Lastly, I do want to mention that there are a number of 'Mucinex' products on the market (e.g. Mucinex FastMax). This answer refers to the active ingredient in 'original' Mucinex only (guaifenesin). Other Mucinex products may contain additional ingredients that could be more likely to cause diarrhea.


  • Safety And Tolerability Of Extended-Release Guaifenesin In Patients With Cough, Thickened Mucus And Chest Congestion Associated With Upper Respiratory Tract Infection, PubMed
  • Role of guaifenesin in the management of chronic bronchitis and upper respiratory tract infections, PubMed
  • Guaifenesin Monograph, PubChem

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Dr. Brian Staiger, PharmD

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