One out of three of the most popular songs of 2005 portrayed substance use, according to new research which analyzed 279 of the year's most popular songs listed by Billboard magazine.

Rap music led the way with 77 percent of songs referring to substance use, followed by country at 37 percent and R&B/hip-hop at 20 percent. Rock and pop were on the lower end of the spectrum at 14 percent and 9 percent, respectively, according to a study at the University of Pittsburgh School of Medicine presented at the American Public Health Association's Annual Meeting & Exposition in Washington, D.C.

Alcohol and marijuana were the substances most frequently portrayed. Substance use was commonly associated with partying, sex, violence and/or humor, and was most often motivated by peer/social pressure, sex, and/or money (for instance, through trafficking).

Younger listeners may be more impressionable when it comes to such messages, said study author Dr. Brian Primack, an assistant professor of medicine and pediatrics at the University of Pittsburgh School of Medicine.

"This is significant because there are many impressions of substance abuse in popular music," he said. "Since young people are exposed to a lot of popular music, they also get exposed to lots of references to substance abuse."