My son's been on oral GSH for about three years. I think it helps him a lot, he still cultures PA (mucoid) which he cultivated before we started GSH.
He's only been having one course of antibiotics a year since we started GSH though. He has just hit a bit of a barrier and done four weeks of antibiotics still coughing though.
Just wanted to say though, we just had his annual review, and the doctor said that his chest xray doesnt look like someone with cf, and you wouldnt know he had it...so there you go! Even with culturing mucoid pseudo for about four years.
I do think the GSH provides protective factors against lung damage.