I don’t know alot about absorbtion rate through sinuses but the dosage is specific to the actual medication, not the amount of glucose in the product. I would assume that because you are not ingesting it that it doesn’t go through the same process and enter the bloodstream as if you were which would make the glucose less bioavailable.
I have had alergies and asthma my whole life and when im sick I worry about taking cough syrup mostly because of the impact on BG.
I’m assuming this is a steroid spray. I personally would just use it and stay on track with your diet as I doubt that the impact would be substantial enough to notice a real difference, especially if you are suffering from sinus problems. Antibiotics can cause their own set of issues that may also interfere, so avoiding them if you can is most likely what is currently in your best interest. By not taking this medication you could ultimately wind up on the antibiotics… so really it is up to you on what course you decide to take.
I wouldn’t be too concerned. But if you are really concerned you could test your BG. Test baseline prior to use/eating and then at 30 minutes after the spray and then again at 60 to see if there is actually an impact.