Well, there are days when I have to take something fast-acting above my regular antihistimines. I take two of them and still get away with a small dose of benadryl. You'll be tired and pretty dried out, but it won't hurt you. Just make sure you don't drive anywhere.
Oh, also, try Sudafed first. I am allergic to that, so Benadryl is my only other option. If you don't react to it though, it actually does better than Benadryl. I really wish I could take it.