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I'm sorry your girl isn't well. May I ask a few questions, please?
Has she been back to the vet since the initial antibiotic course?
Has she been on any other medications?
Has she had a chest x ray?
Thanks for the additional information about your boy. How often do you hear him cough now? Does he ever sneeze?
Thank you for that detail. There are a number of reasons cats can cough and I was trying to pin it down.
From your description, it sounds like your cat has allergies/asthma, compounded by nasal infection. My first and best advice is to bring him in for an exam and chest xray. The vet will probably want to put him on a course of antibiotics. The chest xray will show if he has feline asthma, which is an allergic/inflammatory condition which will get worse without treatment. The link below is to a video of an asthmatic cat coughing:
Regarding home cures-you can try the amino acid L-lysine 250 mg twice daily, as an anti-herpes virus treatment. It's safe, but I doubt it will do much for him. Another thing reasonable to try is Chlorpheniramine (Chlortrimeton) 1/2 of a 4 mg tablet twice daily. This may help a bit with his nasal signs. A vaporizer at night may ease his congestion a bit.
I really think a course of antibiotics is indicated. It won't hurt him to try the amino acid or Chlorpheniramine, but I am guessing it won't help much either. You can find plenty of people willing to sell you non-prescription homeopathic medications for anything your cat has. I advise saving your money for something that might work.
I hope this was helpful. Thank you for your patience, and please feel free to ask any other questions you have about your boy. I would be happy to talk further with you.