Hello, I'm a 25yr old male with CF. I am actually on Zithromax as well. I take 500mg every other day. I've been on antibiotics for as long as I can remember because of the CF. Taking medication daily will possible be something that your child may need to get used to depending on the severity of his/her condition. You mentioned that your child is taking this because of a sinus infection, do he/she also have CF? I also need to take enzymes with every meal, as my pancrease has been effected from the CF as well. I currently take Ultrase MT20 tabs. 8-10 with snacks, 10-12 with meals. Checking the consistency of the bowel movements should give signs whether or not the antibiotics are affecting the digestive system. In some cases they cause diarreah. I actually find Zithromax to work quite well with myself. It is designed to work better with CF patience that have slow developeing Pseudomonas, which is my case. It is definitely best to ask as many questions as you can to your childs physician, then can work with you to maintaine the best possible quality of life for your child. If you ever have questions that you would prefer to ask someone that has been living with CF, feel free to e-mail me any time at "farbeyond1978@yahoo.com." Unfortunately I left work early today because of hemoptysis & I haven't been feeling well. I am actually waiting on a call back from my doctor and I may be admitted to the hospital for about a week. I will be sure to check to see how your doing once I'm back. I wish the best to you and your family,MikeP.S Don't worry about me mentioning the hospitalization, I will be fine... it's actually not as bad as it sounds.