My cat has had a sinus problem since earlier in the fall -- lots of sneezing, runny nose, etc. I took him to the vet who prescribed antibiotics. They were effective but about two weeks after completing the antibiotics, the sneezing started again. Since then we have gone throught two additional rounds of antibiotics, the latest was horrid pink stuff (Novolexin) that I could barely get down his throat! Each time the antibiotics appear to solve the problem, but after a couple of weeks the sinus problem flares up again. My cat is 15 years old and is also on thyroid meds (Methimazole), so his immune system isn't what it should be. However, he is very healthy in other respects, and when the antibiotics are working, we see his old, lively, playful self. Right now, when he's feeling poorly, he sleeps a lot and while he eats OK and seems reasonably normal, it's easy to see that he's under the weather. I'm wondering what to do next. I have heard that L-Lysine is very effective at boosting the immune system in cats with chronic upper respiratory infections, but my vet didn't endorse it. Suggestions please?