The National Youth Anti-Drug Campaign's initial television ads, designed to keep young children and teens away from marijuana, not only failed in their purpose but may have actually increased teens' risk of trying the drug, a new study concludes.

The Health Behavior News Service reported Oct. 16 that despite the $1 billion poured into the media campaign by Congress, researchers at the University of Pennsylvania found that exposure to advertisements aired between 1999 and 2004 had no effect on deterring kids from experimenting with marijuana.

The authors followed 8,117 children, ages nine to 18 (the campaign's target population) for five years beginning in 1999, surveying them at home four times. As exposure to the antidrug ads increased from four or fewer per month to a dozen per month, researchers saw a decrease from 82 percent to 78 percent in the proportion of children who "definitely" had no plans to smoke marijuana.

"Our basic hypothesis is that the more kids saw these ads, the more they came to believe that lots of other kids were using marijuana," said lead researcher Robert Hornik. "And the more they came to believe that other kids were using marijuana, the more they became more interested in using it themselves."

The Office of National Drug Control Policy (ONDCP), which administers the media campaign, termed the findings "old news."

"The campaign has made major changes to improve its effectiveness and, in fact, drug use among teens has dropped steadily in nearly every category since 2001," said ONDCP spokesperson Jennifer de Vallance.

The findings will be published in the December 2008 issue of the American Journal of Public Health.

This summary has been revised to reflect the following:

Correction, Oct. 20, 2008: We have updated the original version of this article to clarify that the research findings apply only to the media campaign ads aired between 1999 and 2004.