I had a friend take acid and completely lose it. His girlfriend took him to the emergency room. They pretty much just talked him down until he could cope a bit better.

I used to go to a lot of Grateful Dead shows and they would always have an area set aside for dealing with people having bad trips. They just reassured people that everything would be ok. I think that just hearing that from someone who seems to be knowledgeable, like a doctor, etc, helps a lot.