It may depend on the deer in question, but last year I had a huge buck bed down about 23 yards away from me for 2 hours. Had to cough the whole time, to the point tears were rolling down my cheeks. When he finally got up, I was waiting for him to move out from behind some branches and I finally coughed. He was gone in a flash. Didn't stop for a second to check out what made the sound or anything. Just ran off immediately.
Thanks for the suggestions so far. I think I have a drug allergy that may prevent me from taking delsym. Any other suggestions?