I've got prostatitis besides diabetes Type 2, and decongestants are a bad idea, as they affect the prostate and in effect the urinary tract.
However, newer antihistamines are fine, fexofenadine for example, like in the brand Telfast, great for bad colds. I smoke, so getting a cold is worse as it induces me to cough a lot as well. Telfast 180g works very well, does not affect my blood sugar and the prostatitis. Telfast-D has a decongestant pseudoephedrine, not good for me. I also take Paracetamol + Codeine sometimes, just to avert any fever.
My cough syrup also has no decongestants, just antihistamine that my prostate can still tolerate. Not sugar-free, but it's just a small amount anyway. All ok, no problem for my diabetes and all don't interact with my Metformin and Gliclazide meds.
Antibiotics when needed are fine for me too - Clarithromycin, Erythrocin, Augmentin, Flagyl, etc.