Actually, the missing component is Robitussin DM, which contains "dextromethorphan". It is common "recreational" drug. It is OTC, no prescription required!.
From a web search:
Dextromethorphan is a common ingredient in cough and cold medicines that acts as a cough suppressant. It manipulates the nerve signals in the part of the brain that controls your cough reflex. While it is not a prescription drug, it can have some dangerous side effects if not used correctly. It is also abused by some in a habit called “robo-tripping.” In high doses, it depresses the central nervous system and has a similar anesthetic effect to PCP, morphine or the animal tranquilizer ketamine, also known as “Special K.”
Read more: http://www.ehow.com/list_5796683_effects-dextromethorphan-abuse.html#ixzz2ZNrBCO89