Good evening - I know the label cautions against long-term use, but when it was a prescription drug, many years ago, my ENT told me he has had patients use it for years without apparent issues. My current pulmonologist, primary and ENT all know I am taking it, and none has ever objected or suggested stopping.
The following statements are about pure guaifenesin - with no added decongestants, pain relievers or cough suppressants. There are no known effects on blood pressure, heart rate or blood sugar. There are no known medication interactions. According to my ophthalmologist, it is safe to use if you have glaucoma. I have taken it almost daily for over 35 years, without it my mucus is very sticky.

So, as always, each person is different, but if it is helpful for you and your doctor agrees, it is safe for long term use.
Have you asked the doctor yet about continuing to use it?
Sue