The commercial name for the cough suppressant that contains the active drug dextromethorphan hydrobromide. When ingested in mass quantities (i.e. 2 4 ounce bottles), the drug suppresses a large portion of your inner brain, leading to hallucinations and euphoria.
Note: This WILL make you vomit, nothing you can do not to.

Be very careful to choose the right type of cough syrup when tussing. Choosing the wrong one can end you up in the emergency room or dead. The only listed active ingredient should be dextromethorphan hydrobromide. If you drink too much of the other types, say goodbye to mister liver. That said, it doesn't necessarily cause you to vomit -- it's just very common. That said, tussing is serious drug stuff...it's apparently a very heavy high, and not too fun for inexperienced users. Try something lighter for your first trip...

Robitussin Maximum Strength Cough and generic equivalents, which contain only dextromethorphan as the active ingredient, are the favored methods of getting high as a kite from DXM, as they are widely available in the U.S. and do not cause unpleasant side effects like vomiting and slow, excruciating death. (DXM will only make you throw up if you have a weak stomach or get motion sickness; guaifenesin will invariably make you puke your guts out.)

As there is no medical reason for putting DXM in icky syrup rather than tablets or gelcaps, it is suspected that this is done to discourage recreational use.