Tell them you have hay fever. They'll advise you get fexofenadine on prescription from your GP.
You can fly with that, I have done for years. (And actually I found it's much more effective than cetirizine/loratadine/acrivastine/chlorphenamine, all of which I tried in the past before I started flying)
If its something that you will need to take every day (like an antihistamine) then they'll put it on repeat prescription and if you ask for a big supply they'll often prescribe 3-6 months worth. That way you only have to pay the one prescription charge.