These drugs all have different mechanisms of action and specialties and should be used only as needed, i.e. if a symptom abates then you should stop taking whatever it is that treats it. The problem is that people are too used to combination formulations or, even worse, treating all of these drugs interchangeably. A chest cold has a different OTC treatment regimen than a sinus infection.
I used to take 24-hour allergy medicine with pseudoephedrine, and it took me years to realize it was the thing that was giving me insomnia during allergy season—for years I thought I just had periodic bouts of intense insomnia.
First 100% sleepless night in college. And the only sleepless night that wasn’t having too much fun or stressing over a final exam. Trash can.
If you pop OTC meds at the first sign of anything, your body gets used to it and it becomes a baseline; whereas before it would blow all the symptoms away, now you need it just not to get significantly worse.
That doesn't happen for every drug. But side-effects still exist for them all, so yeah, there are several reasons to limit how much you use them.
(It also gives me horrible insomnia if I take it at night so it wasn’t a huge hardship).
Guaifenesin thins the mucus instead, makes it more watery so it drains down the throat with no further complications like sore throat and coughing. And the extra volume helps flush bacteria out of the sinuses.