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Hi, this is Dr. Gabby. It could possibly. When did he start taking it? Why was it prescribed? When did he last eat?
The most common side effects of the minocycline are gastrointestinal related. That would include lack of appetite, vomiting, or diarrhea.
In rare cases the liver can be affected.
However, normally cats with an upper respiratory tract infection do not eat well.
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One reason they do not eat well is because they can not smell the food. Try warming smelly fish flavors up so they are more fragrant. I have good luck with "fancy feast ocean white fish" canned.
You can also try Human chicken flavored baby food. Just make sure it does not contains onions or garlic and warm it up.
Sometimes I have to put a taste in their mouths to get them eating.
It is recommended to give the Minocycline with food. Well that does not help now.
To help keep his blood sugar up you can rub Karo syrup, pancake syrup, or honey on his gums every few hours.
If his gums feel dry then he is probably dehydrated. In that case you can give him low sodium broth, children's pedialyte drink, or water with a syringe or eye dropper.
Keep a humidifier in the room.
If he is still not eating after all that then he needs to see the vet again tomorrow.
Does that help? Do you have any additional questions?