Lloyd Johnston, principal investigator for the Monitoring the Future study, told ABC News that if the government's ad campaign has not worked, there's something else out there driving down teen marijuana use. Johnston also said there is evidence that other government anti-drug messages have worked, adding that efforts to curb use of inhalants and ecstasy were followed by a drop in use of the drugs.
Johnston speculated that, assuming the anti-drug campaigns really were ineffectual, it may have been because the government "picked a hard target" by trying to demonize marijuana at the very time there was a competing push to legitimize medical marijuana.
Still, the recent report concluded the campaign was ineffective in changing behaviors.
Today, study author and professor at University of Pennsylvania's Annenberg School for Communication, Robert Hornik, told ABCNews.com that the reported decline in marijuana use "could be due to lots of influences, not just the campaign." He said he was expecting to conclude that the anti-drug campaign had positive effects, "but we couldn't find 'em."
"Despite extensive funding, governmental agency support, the employment of professional advertising and public relations firms, and consultation with subject-matter experts, the evidence from the evaluation suggests that the National Youth Anti-Drug Media Campaign had no favorable effects on youths' behavior and that it may even have had an unintended and undesirable effect on drug cognitions and use," the report said.
In other words, teens who specifically said they had a lot of exposure to the campaign messages were no less likely to stay away from marijuana than those who did not.
There is also a small amount of evidence that indicates the anti-drug campaign may have had the opposite effect for some teens. In one part of the analysis, teens who recalled seeing 12 or more anti-drug messages per month were actually more likely to start using marijuana than those who had seen fewer anti-drug messages per month.
The evaluation was conducted by the University of Pennsylvania's Annenberg School for Communication, after Congress called for the study. The study was based on four rounds of interviews conducted between 1999 and 2004, each involving about 5,000 to 8,000 youths between the ages of 9 and 18 years.
ABC News' Brian Hartman contributed to this report.