Hello,
I have also had lots of sinus problems, not exactly like yours, but I have sinusitis, allergies, chronic rhinitis, and asthma, along with the fact that I had to get my adnoids out to be able to breath!!! what a relief that was!!!
Anyway, to get to the point. I have been prescribed doses of Prednizone many, many times throughout my life. Sometimes as frequent as you stated, and sometimes stronger doses. I wouldn't be concerned about the amount of Prednizone. The worst is that it will make you feel "antsy" and like you said, it will increase your appetite. But it should be fine, as long as this is your last dose for a couple months

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-Vers