Well, my son has never actually had a sinus infection. But he had horrendous (and worsening) allergies. His symptoms were mainly sneezing, sinus congestion, post-nasal drip, and (probably TMI) but copious amounts of ear wax.
He is 14 now, and has been receiving allergy shots for about 2 years. He is at the point where he goes every 2 weeks for injections. He is largely symptom free at this point. He still has times when it flares, of course... but before now, it was year round. Constant. No relief, no down time.
We just went through a terrible uptick in (in general, the area--- his doctor said he had tons of patients asking for additional treatments b/c theirs were flaring) and son's did flare a bit in January, but since then he's really been fine.
Before starting with the injection therapy, he did several prescription meds, tried the saline sprays, the neti-pot style rinses, OTC stuff... you name it. During most of his treatment, he was taking daily OTC meds also... he is pretty much weaned off of those now too. He may take a Zyrtec every few days... but I don't think he has taken one in the past two weeks.
I don't know if it would help you or not, but have you checked into it at all?
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He was put on singulair (oral tablets) when he was 7. I think it might have helped a tiny bit, but it caused stomach aches (about 45 minutes after he took it, every night) so we discontinued it. Then tried Nasonex (nasal spray) same thing. It was difficult to even tell if it was helping or not. But in general, his allergies seemed to be trending to worse, not better. The doc who prescribed both of those (and actually "diagnosed" allergic rhinitis for him) closed his practice here and moved elsewhere, where he could be closer to extended family and in a larger group practice so he could have days off, so when the prescription for Nasonex ran out, we didn't pursue refills with another doctor, b/c it just didn't seem to do much. So we began OTC. Alavert, Claritin (same thing --loratidine), etc. But he still had horrible drainage and used a half box of tissues a DAY. I don't know how he managed at school---I guess it was all he knew, so he just went on with life. In 6th grade, he got double pneumonia, and within a few months after that he had an asthma attack. Those scared him enough to consent to seeking allergy injection therapy. It's working for him.
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oh, and when I was in a health food store, the lady there suggested removing milk from his diet. (he LOVES milk). I did actually try that for 2 weeks. NO milk. No change, either. Right now, (after @ 2 yrs of allergy shots, still ongoing) he drinks milk every day. His symptoms are still almost gone, for the most part, even drinking milk every day. It's what he washes his breakfast down with every morning. I'm not saying it might not help you to avoid milk, but it didn't seem to have any effect on my son one way or the other.