My son started it in May when he turned five. He is pretty healthy, but even still his PTF's increased. His doctor is really excited about Zithromax. After being on it for 3 months, Sean's PFTs of his smaller airways went from 70% to 100%. His doctor said that these airways are the first part of the lungs to get clogged and infected - so she was really excited to see this improvemnt. Basically Zithromax is used for it's anti-inflamatory properties. It helps reduce the swelling that is in all CF lungs - healthy or not-healthy. Because he isn't using it as an "anti-biotic," Sean won't built up a resistance to it. It is something that he can take his entire life.As a side note, Sean just went into the clinic for another check-up. He is fighting a cold right now and his PFTs are still 100%. I would recomend talking to your doctor about it. I am glad we did. (I know I sound like a commercial for Zithromax, but I really am thrilled with Sean's health recently.)--Katrina (mother of Sean (5 w/cf), Carolyn (4 w/o cf), and Allison (2 w/o cf)