I have used Mucinex LA 600mg (the all guaifenesin version - no decongestant or cough suppressant) for 35 years without any bad effect. It thins the mucus and other secretions in my system, which tend to have a very sticky consistency. Before using it, I averaged 10 or more ear or sinus infections each year, as well as having a chronic cough. In 35 years, I have had fewer than 10 (total) ear or sinus infections. It also helps me be able to clear mucus from my lungs caused by a lung disease call bronchiectasis. This has helped keep mu lungs free of the infections that plague "Bronch" people - which means no long courses of antibiotics.
I am curious why your NP suggested you stop - were you taking a version that contained other medicines such as decongestants that are meant for only short-term use?
Sue