"In 2006, more than one third (35.8 percent) of persons aged 12 to 20 who used alcohol in the past month also had used an illicit drug in the past month, and 16.0 percent of underage drinkers used an illicit drug within 2 hours of using alcohol on their last occasion of alcohol use.
"Marijuana was the illicit drug most used by underage drinkers, with nearly one third (30.0 percent) having used marijuana in the past month, and 15.0 percent having used marijuana within 2 hours of their last alcohol use."
Pemberton, M. R., Colliver, J. D., Robbins, T. M., & Gfroerer, J. C. (2008). Underage alcohol use: Findings from the 2002-2006 National Surveys on Drug Use and Health (DHHS Publication No. SMA 08-4333, Analytic Series A-30). Rockville, MD: Substance Abuse and Mental Health Services Administration, Office of Applied Studies, p. 4.