At CVS pharmacies, you now have to be at least 18 to buy Coricidin Cough & Cold medicine. At Walgreens, there's a three-pack limit on an extra-strength variety of those pills. And at some independently owned drugstores, syrup bottles and blister packs of cough suppressants have vanished from shelves and reappeared behind the counter, near the cigarettes or the prescription drugs.

The nation's pharmacy giants are taking precautions in response to a trend that doctors and anti-drug abuse activists say could grow into an epidemic: teenagers and young adults using medicine to get high.

There are other, darker signs: One morning in May, on a lark, five ninth-graders in Loudoun County swallowed a "cocktail" of Coricidin and the motion-sickness drug Dramamine. Nauseated and loopy, they were rushed to Loudoun Hospital Center, where an emergency room physician explained that the drugs -- considered safe when used as intended -- can be fatal in very large doses.

From acid to ecstasy, patterns of substance abuse have evolved with the times, and in recent years, illicit use of prescription and over-the-counter drugs has soared among a certain demographic -- mostly suburban, mostly young and mostly middle class, according to researchers. They get the drugs through the Internet, at school and from their parents' medicine cabinets.

"We feel this is going to be the next big wave of substance abuse in the country," said Steve Dnistrian, executive vice president of the Partnership for a Drug-Free America. "It's limited to no one prescription drug or over-the-counter drug. It's a new and emerging category we've been watching over the last two years, and we've seen it's going to be a significant problem in the years to come if the data continue to head where they're heading."

The data, to some, are startling. Prescription drugs are now second only to marijuana as a category of illicit substance abused by teenagers, according to the 2003 National Survey on Drug Use and Health. The number of teenagers calling into poison control centers nationwide about cough medicine abuse has doubled in four years.

In a survey of more than 7,000 teenagers by the Partnership for a Drug-Free America, one in five reported taking a prescription painkiller without a doctor's prescription.

"Prescription drug use is all over the place," said Chrissy Trotta, a student at George Washington University and the founder of a campus group aimed, in part, at preventing that type of behavior. "Often painkillers, things like Vicodin, are mixed with other drugs. . . . It's a tremendous problem."

The motivation is often boredom and a sense of rebellion -- not unlike what motivated drug users of their parents' generation, according to interviews with more than a dozen Washington area high school students and an equal number of college students from across the country. Most spoke on the condition of anonymity for fear of repercussions from their parents and their schools.

Alex Kaplan, 17, a high school senior from Anne Arundel County, said that he has never used prescription or over-the-counter drugs to get high but that abuse of both is prevalent among some of his peers.

"When it comes to Robitussin," he said, "it's not like what you would drink if you had a cold, but kids, like, actually drinking half a bottle or 75 percent of the bottle. It just, like, makes you really out of it, I guess. When you're looking at them, it's just kind of creepy looking."

At a high school party on a Friday night, you won't always hear: " 'Want to smoke pot?' 'No, let's Robotrip, man,' but you will hear it sometimes," he said.

The mix of abused medicines has changed. Quaaludes, a type of sedative, are no longer widely available, but today's college students sometimes encounter punch bowls filled with drugs such as the painkiller Percodan at parties, said Andrea Barthwell, deputy director for demand reduction in the Office of National Drug Control Policy. And unlike their parents, young people can go to online pharmacies, where they sometimes can get those drugs.

"If you're going out in college, you're going to come across someone who's doing this," said a sophomore at the University of Pennsylvania, who spoke on the condition that he not be identified because of the illicit nature of the activity. "You'll go to frat houses. There'll be kids in a room. You'll just be drinking, and one kid will be like, 'Hey, I got some Xanax,' " a tranquilizer.

He added: "When somebody's drunk and they take Xanax, it's a horrible thing to watch. They're completely out of it."

Dnistrian compares the current surge in the abuse of illicitly obtained prescription medicines to the ecstasy boom of the 1990s or the increase in cocaine use in the 1980s. Some doctors and researchers attribute the trend more to fashion than to the drugs' effects.

"We're on the upswing with these prescription drugs, and usage is increasing more for social reasons than for medical ones," said Daniel Z. Lieberman, director of the Clinical Psychiatric Research Center at George Washington University Medical Center. "It's simply the thing to do among high school students who have an intense need to fit in."

In response, the Bush administration unveiled an anti-drug policy this year that focused on prescription drug abuse. The plan would dedicate nearly $150 million to augment prescription monitoring programs, to train physicians to combat abuse and to establish education programs on the dangers of taking such drugs recreationally.

"It's clearly a serious problem that we are working hard to correct," said Michael D. Maves, chief executive of the American Medical Association. "And it's a difficult problem, honestly. Some of this can go on and not be noticed by parents and peers because it doesn't have the same connotation of purchasing and using . . . drugs like heroin. It sometimes doesn't stand out like other things, in terms of truly illegal drugs, but it's no less serious."

The effect of many abused medications is psychological, Lieberman said, "a very relaxed and mellow high that can be accompanied by a powerful sense of well-being. It's not like cocaine, where you're feeling very energized, where you're able to stay up all night drinking and dancing."

But each category of medication has plenty of negative side effects. "Robotripping" -- one of the terms for abusing dextromethorphan, the active ingredient in cough medicines such as Robitussin -- can cause hallucinations, and it's almost always accompanied by the unpleasant symptoms of overdose, such as vomiting, said Rose Ann Soloway, clinical toxicologist at the National Capital Poison Center.

Linda Simoni-Wastila, a professor in the University of Maryland School of Pharmacy and an expert on youth prescription drug abuse, said perhaps the best way to combat the trend is through improvements in the drugs themselves. This summer, representatives of 20 pharmaceutical companies said they were working on ways to make drugs less addictive and less easily abused.

Still, many are unsure about the scope of the problem, let alone the best way to address it.

"I've been working in this area for a decade, and I still contend that prescription drug abuse is different from other abuse," Simoni-Wastila said.

"The truth is, we really don't know how to prevent it. We don't have specific guidelines on treatment. Right now, it's all very hush-hush."

"It's clearly a serious problem," said the AMA's Michael D. Maves.