NAC for treating FIV and sinus issues in cats
My little rescue cat suffered from blocked nasal passages after having recurrent infections before he was rescued. We couldn't stop the nasal infections unfortunately but I found that he seemed to get headaches and would ask me to rub his face in an unusual way. So I tried sprinkling a tiny amount of NAC (N-acetylcysteine) on his food once a day and he stopped asking for face massages and seemed better.
I use NAC myself for sinus headaches. I found a scientific research paper showing a sprinkling was safe for cats. It is also used for treating respiratory inflammation and FIV in cats. If you look it up you will see vets use it. I used human-grade NAC with no additives.