I figure that, plus the nerve that tickles your cough bone is dulled. I sing and have had too many surgeries so I have some experience with dilaudid. My vibrato is in my diaphragm and it isn't as responsive when I'm on dilaudid, but that could just be the drug slowing my responsiveness in general.
Alcohol is one of the most destructive things for your body. If you had the choice of taking morphine or Alcohol everyday for the rest of your life, you would live much longer on average with the morphine.
It's put in there for a few different uses. It's a very small amount and it's synergistic with the morphine and actually reduces respiratory depression. I have been also told it was put in there to prevent abuse, because if you take too much nausea starts happening.
It's funny, but what's also funny is that it's still true of benadryl. Relatively safe when used as directed but, well, check out the chubbyemu video on it for more details.