You don't need to be worried. If you took Zyrtec and Mucinex Fast-Max six hours apart, the worst you are likely to experience is added drowsiness and dry mouth. Using alcohol or other drugs with either or both of these medications will likely intensify the drowsiness considerably. This may greatly impair your ability to drive or use machinery, so please use caution and be safe.


Mucinex Fast-Max is a product line in which there are multiple versions intended to treat a variety of symptoms. Reviewing the ingredients of all the Fast-Max line of products, the nighttime varieties contain either diphenhydramine or doxylamine as an antihistamine.


A quick lesson on antihistamines. There are two main families of antihistamines, let’s call them old-school and new-school to make it easier to remember.

Old-school antihistamines include drugs like

  • Diphenhydramine (Benedryl)
  • Doxylamine (Unisom, NyQuil)
  • Chlorpheniramine (Chlor-Trimeton, Chlor-Tabs)

These drugs block histamine receptors in both the brain and the rest of the body; they also interact with several other receptor types. This makes them good at relieving allergy symptoms, but also gives them a few unpleasant side effects, particularly sedation/drowsiness, dry mouth, and dry eyes.


New-school antihistamines include drugs like:

  • Cetirizine (Zyrtec)
  • Fexofenadine (Allegra)
  • Loratadine (Claritin)


These drugs, unlike the old-school ones, are designed so that they can’t cross the barrier that protects the brain from harmful chemicals in your blood. The result is they can only block histamine receptors found in the rest of your body, and not in the brain, and so they produce allergy relief with much less drowsiness and other side effects.


That said, this doesn’t mean they can’t make you drowsy, and taking them with other antihistamines, alcohol, or drugs that affect the central nervous system (bupropion, benzodiazepines like alprazolam or clonazepam, and others) can produce substantial additive effects that could impair your ability to make decisions or operate a vehicle.


Again, mixing these substances should be avoided if possible, and if you must, use caution.Thanks for your question!