Ask your pharmacist when buying and then DOUBLE CHECK to make sure it doesn't say 'not suitable for diabetics'. Decongestants are usually not suitable for diabetics. Please see this link
I always find that it's quite easy to miss something that says 'not suitable for diabetics' when I'm standing in the shop reading it (I've gotten home so many times, re-read the packaging and then found that that it's NOT suitable!) that I always double check with the pharmacist.