FRIDAY, March 18 (HealthDay News) -- Anti-drug ads appear to be an effective way of delaying and reducing marijuana use by eighth-grade girls in the United States, a new study has found.

In 210 U.S. media markets that had advertised the "Above the Influence" anti-drug campaign, researchers analyzed the relationship between exposure to the campaign ads and use of marijuana and alcohol among students in grades 8, 10 and 12.

The campaign appeared to be most effective among 8th-grade girls, but did not seem to have much impact on similarly aged boys or teens of either gender in grades 10 and 12, the researchers found.

The study results were published online March 17 and in the May print issue of the American Journal of Public Health.

"By grade 10 and 12, most adolescents may have already made decisions about whether to initiate marijuana use, leaving less room for anti-drug advertising to have any meaningful effect," study author Christopher S. Carpenter, of Paul Merge School of Business at the University of California, Irvine, and colleagues wrote.

"Early adolescence is a period of major transition and vulnerability," they added. "Consequently, 8th-grade adolescent girls might be especially receptive to the Above the Influence campaign's anti-drug advertisements about achievement and living life above negative influences."

Above the Influence is a youth anti-drug advertising campaign created for the National Youth Anti-Drug Media Campaign (a program of the U.S. Office of National Drug Control Policy).

More information

The Nemours Foundation has more on what teens should know about drugs.