Before we got our "official CF diagnosis" and didn't know why our daughter was getting lung infections, our PCP wrote a script for prescription Guaifenesin, which is now available over the counter and is the same drug they use in Mucinex. So, I'd have to say, yes you can give it a try -- it's really just an expectorant. We have used it from time to time, in addition to all the other things, it has helped some.