Drinking Robitussin is one of the easiest ways to get high on DXM, or Dextromethorphan, an expectorant (it makes you stop coughing) and moderatehallucinogen. It is often described as being very much like PCP and at the same time very much not. Drinking large amounts of cough medicine is also one of the best ways to get dead as most contain enough guaifenesin, acetaminophen, or pseudoephedrine to kill those people that try to down a whole bottle.
I've tried it twice. Your legs feel like they're attached to the floor and you get that very special feeling that you are not special, you have no destiny, and someday, just like everybody else, you are going to die.