Hey Clog.
Yeah I told them that I suffered from hayfever for the simple reason that if it ever flaired up badly and I needed to be taken out of work, at least they are aware that it is something I suffered from.
But I was honest and said that I dont take anything for it other than sudafed and that i was confident that it would not effect my sinus'.
As for the antihistamines, I really would not worry about it too much if you only have to take them once in a blue moon. I would be careful for your own sake however if you are flying on them and anything should happen requiring you to get an alco test or something. Not sure if it would show up in a urine sample but perhaps you could get in trouble. The best thing to do is seek out some advice about flying and using them for your own personal information. Im certainly no medical professional and I doubt there are many people on pprune who are. The last thing you want at 30000 feet, 1000miles from nowhere over an ocean or desert is a bad reaction to them. Iv seen it happen to passengers and I would hate to see it happen to a crew member.