<p>Estimates of first-time drug use, referred to as incidence or initiation, provide an important measure of the Nation's drug use problem. They suggest emerging patterns of use & identify periods of heightened risk for an immediate focus on the prevention of substance use, particularly among children & youths. Incidence data also suggest the future burden on substance abuse treatment systems. This report contains an analysis of the initiation of marijuana use. Marijuana is the most widely used illicit drug in the U.S. & is, in most cases, the first illicit drug used by persons who have used an illicit drug. The analysis is based on data from the 1999 & 2000 National Household Surveys on Drug Abuse. Charts & tables.</p>