Dextromethorphan (sometimes called "DXM" or "robo") is a cough-suppressing ingredient in a variety of over-the-counter cold and cough medications.Drugs such as PCP (phencyclidine) and ketamine, which were initially developed as general anesthetics for surgery, distort perceptions of sight and sound and produce feelings of detachment - dissociation - from the environment and self. But these mind-altering effects are not hallucinations. PCP and ketamine are therefore more properly known as "dissociative anesthetics." Dextromethorphan, a widely available cough suppressant, when taken in high doses can produce effects similar to those of PCP and ketamine.

Like PCP and ketamine, dextromethorphan acts as an NMDA receptor antagonist. The most common source of abused dextromethorphan is "extra-strength" cough syrup, which typically contains 3 milligrams of the drug per milliliter of syrup. At the doses recommended for treating coughs (1/6 to 1/3 ounce of medication, containing 15 mg to 30 mg dextromethorphan), the drug is safe and effective. At much higher doses (4 or more ounces), dextromethorphan produces dissociative effects similar to those of PCP and ketamine.

The effects vary with dose, and dextromethorphan users describe a set of distinct dose-dependent "plateaus" ranging from a mild stimulant effect with distorted visual perceptions at low (approximately 2-ounce) doses to a sense of complete dissociation from one's body at doses of 10 ounces or more. The effects typically last for 6 hours. Over-the-counter medications that contain dextromethorphan often contain antihistamine and decongestant ingredients as well, and high doses of these mixtures can seriously increase risks of dextromethorphan abuse.

Dextromethorphan found in over-the-counter cough medicine is becoming an increasingly popular drug of abuse among teens, a new study shows.

Reports of dextromethorphan abuse to a California poison control hotline rose 10-fold from 1999 to 2004, and cases involving teens rose 15-fold, Dr. Jodi K. Bryner of the University of California at San Francisco and colleagues report.

"This increase in dextromethorphan abuse in adolescents is most likely due to the hallucinogenic effects of these easily accessible inexpensive over-the-counter products and the false perception that high-dose dextromethorphan is safe," Bryner and her team write.

While the use of illegal drugs like LSD and Ecstasy has fallen among adolescents in recent years, over-the-counter drug abuse has been on the rise since the late 1990s, the researchers report in the Archives of Pediatrics and Adolescent Medicine.

To examine the trend more closely, the investigators looked at records from the California Poison Control System, the American Association of Poison Control Centers and the Drug Abuse Warning Network from 1999 to 2004.

During that period, the number of dextromethorphan abuse cases reported to California Poison Control System, as well as the percentage of all drug abuse cases they represented, rose sharply. In 1999, just 48 cases of dextromethorphan abuse were reported to the state system; in 2004, there were 478. Out of every 1,000 cases reported in 1999, dextromethorphan was involved in 0.23, which rose to 2.15 in 2004.

Nearly three quarters of all dextromethorphan abuse cases reported to California poison control involved individuals aged 9 to 17, and the average age was 16 years old.

Data from the American Association of Poison Control Centers and Drug Abuse Warning Network mirrored those from the California Poison Control System, with most cases involving 15- and 16-year-olds.

Coricidin HBP Cough & Cold Tablets was the product most commonly reported to California Poison Control System, followed by Robitussin.

Explanations for the popularity of the tablet form of the drug, which rose during the course of the study period -- could include "widespread over-the-counter availability, high dextromethorphan content, palatability of the tablet formulation, and, most important, its frequent promotion as a product of abuse on many Internet Web sites," the researchers write.

"Preventive measures, such as placing dextromethorphan-containing products behind pharmacy counters, may be an effective action to limit this increasing trend of abuse in adolescents," they conclude.