From the Vicks' Website:Overdose warning: Taking more than the recommended dose can cause serious health problems. In case of overdose, get medical help or contact a Poison Control Center right away. Quick medical attention is critical for adults as well as for children even if you do not notice any signs or symptoms.
What is DXM (dextromethorphan)?
The application for a patent on DXM was first filed in 1949, and it was approved for sale in the United States by the FDA in 1958. DXM is in a class of drugs known as dissociative anaesthetics. Many of these have powerful psychedelic effects at sufficiently high doses
I would suggest browsing http://www.dextromethorphan.ws/
Oh, and coincidentally - Nyquil does not contain Ephedrine any more (for the Crystal Meth) so it's not likely being used for that purpose any longer.