Teenagers can get their hands on marijuana and prescription drugs more rapidly and easily this year than last, according to a new study. However, greater drug availability has not yet translated into greater drug abuse in the group – marijuana use among teens continues to decline.

While this paradox may be accounted for by a lag time between an uptick in drug availability and drug abuse, it also raises questions about what aside from drug supply determines substance abuse behavior.

The findings on drug availability, released Thursday morning by the National Center on Addiction and Substance Abuse (CASA) at Columbia University in New York, appear at a time when more teens are abusing prescription medication than any other drug except marijuana and inhalants, sparking concern nationwide.

In February, the Office of National Drug Control Policy kicked off a $14 million prescription-drug campaign with a Super Bowl ad profiling a drug dealer whose teenage customers had deserted him for the free prescription drugs in their parents' medicine cabinets.

According to the CASA study, 23 percent of 12- to 17-year-olds said they could buy marijuana in an hour or less and 42 percent reported they could do so in a day or less, jumps of 35 percent and 14 percent from 2007, respectively, after two years of decreases.

However, the most recent data shows that marijuana use among young people is falling steadily, with past-month use dropping 25 percent between 2001 and 2007. Data for this year is not yet available.

Teen usage of prescription medicine has not decreased as it has for most other illicit drugs, but it hasn't budged much either. Since 2005, the proportion of teens who say they've abused prescription drugs at least once has remained roughly 1 in 5.

This year also marked the first time in the CASA survey's 13-year history that more teens said prescription medication was easier to buy than beer. Among teens who know prescription drug abusers, 34 percent said abusers get the drugs from home, parents, or the medicine cabinet, while 31 percent said friends or classmates.

"A substantial number of American parents have become passive pushers," says CASA chairman Joseph Califano Jr. "A few decades ago, parents used to have a lock on the liquor cabinet. Maybe there should be a lock on the medicine cabinet."

The study cites parental negligence as key to the upswing in teen access to drugs.

There are different explanations for the divergence of trends in drug availability and use. Increased supply may yet convert into "upticks in usage" in the near future, says Mr. Califano. "Availability is the mother of use," he adds.

Drug price also plays an important role in usage, and it is not always affected by availability, notes Nora Volkow, director of the National Institute on Drug Abuse in Bethesda, Md. A 2007 NIDA study notes, for example, that changes in cocaine availability for 12th-graders over the last three decades has not proved a major determinant of use. What changed cocaine use was the introduction of low-cost crack cocaine in the early 1980s, which broke down social-class differences.

Besides, attitudes count most, says Steve Pasierb, president and CEO of the nonprofit Partnership for a Drug-Free America. "Use moves independent of supply to a degree."

For example, teen Ecstasy use increased dramatically at the beginning of this decade because young people viewed the pills as safe and saw few social consequences to taking "the hug drug." But news of Ecstasy-related deaths and a national campaign, launched in 2001, helped reduce usage by ratcheting up perception of risk and social disapproval, says Mr. Pasierb.

Teens and parents alike underestimate the risk of prescription drugs, he adds. "Grandma uses them or something like that, and that comes with a veil of safety."

It is not clear why marijuana and prescription drug availability rose this year, but contributing factors may include the influx of painkillers and attention-deficit drugs like Ritalin and the existence of websites where teens can learn how to grow marijuana, says Elizabeth Planet, who managed the survey.

But the single most important factor may be parental engagement, Ms. Planet adds, citing the fact that while nearly half the teens in the CASA survey said they typically leave the house on school nights to hang out with friends, only 14 percent of parents knew their children did so.

That disconnect between parents and children has serious consequences, she says. Fifty percent of teens who usually return home after 10 p.m. and 29 percent of those who come home between 8 p.m. and 10 p.m. reported that people drink alcohol, smoke marijuana or use other drugs at these nighttime get-togethers, usually at a friend's house.

Califano recommends that parents tackle drug issues at their children's schools and model good behavior. "Parents have got to send clear messages to kids," Califano says.

While it's critical that parents get involved in their children's lives, teens are not just abusing drugs because their parents are apathetic or poor role models, says Neil Bernstein, a clinical psychologist in Washington. "These aren't choirboys or choir girls," he says. "They are at risk: They often hang out with like-minded peers, take more risks, are thrill seekers, are less happy with their lives, do poorly in school, and lack adequate life skills."