"Patterns in progression of drug use from adolescence to adulthood are strikingly regular. Because it is the most widely used illicit drug, marijuana is predictably the first illicit drug most people encounter. Not surprisingly, most users of other illicit drugs have used marijuana first. In fact, most drug users begin with alcohol and nicotine before marijuana, usually before they are of legal age."
Janet E. Joy, Stanley J. Watson, Jr., and John A Benson, Jr. Marijuana and Medicine: Assessing the Science Base. Division of Neuroscience and Behavioral Research, Institute of Medicine. Washington, DC: National Academy Press, 1999.