Any number of elixirs meant to suppress the coughing reflex. There are a number of preparations meant to accomplish this through both narcotic and non-narcotic drugs. Most of them are considered to be foul tasting and generally unpleasant.
OTC cough syrups generally utilize DXM (dextromethorphan), an opiate isomer originally intended as an analgesic, but found lacking in this regard.
Prescription cough syrups, on the other hand, generally use narcotic cough suppressants, which are far more effective at achieving this end, but are generally tightly controlled by international agreements, and in the US by federal and state laws. The typical cough suppressants used in prescription cough syrups are codeine and, less frequently for very severe coughs, hydrocodone, the narcotic component of the very popular drug Vicodin.
Codeine is a naturally occuring alkoloid in opium and is transformed in the bloodstream into morphine. Hydrocodone is a semi-synthetic drug made by manipulating the codeine molecule. It is approximately six times more potent than codeine. Both drugs, in sharp contrast to morphine, are very effective analgesics and cough suppressants when administered orally.
Cough syrup is frequently used as a recreational drug, despite its foul taste. Codeine preparations have obvious potential in this regards as they produce euphoria and a very pleasurable inebreation. However these drugs have a high potential for addiction, and thus are tightly controlled.
A lesser known phenomenon is the recreational use of OTC cough syrups. DXM preparations have very powerful effects in some individuals often compared to the dissociative drugs PCP or Ketamine. Reported effects include, relaxation, feelings of well-being, hallucinations, enhanced comprehension and out of body experiences. However, OTC cough syrups are mixed with other drugs in order to curb this exact behavior, which can result in lethal overdoses. As such their recreational use should be highly discouraged without proper understanding of the drug and its various preparations. Additionally, psychotic episodes can occur at high enough dosages and chronic use can lead to psychological dependence or bromine poisoning.