title: Opportunities to use drugs and stages of drug involvement outside the United States: Evidence from the Republic of Chile.
abstract: BACKGROUND
  Studying youthful drug involvement in the Republic of Chile, we sought to replicate North American research findings about the earliest stages of drug involvement (e.g., initial opportunities to use tobacco and alcohol, and transitions leading toward illegal drug use).
  
  
  METHODS
  A nationally representative multistage probability sample of middle and high school students was drawn; 30,490 youths completed surveys that assessed age at first drug exposure opportunities and first actual drug use. Cox discrete-time survival models accommodate the complex sample design and provide transition probability estimates.
  
  
  RESULTS
  An estimated 39% of the students had an opportunity to use cannabis, and 70% of these transitioned to actual cannabis use. The probability of cannabis use and the conditional probability of cannabis use (given opportunity) are greater for users of alcohol only, tobacco only, and alcohol plus tobacco, as compared to non-users of alcohol and tobacco. Male-female differences in cannabis use were traced back to male-female differences in drug exposure opportunities.
  
  
  CONCLUSIONS
  In Chile as in North America, when cannabis use follows alcohol and tobacco use, the mechanism may be understood in two parts: users of alcohol and tobacco are more likely to have opportunities to try cannabis, and once the opportunity occurs, they are more likely to use cannabis. Male-female differences do not seem to be operative within the mechanism that governs transition to use, once the chance to use cannabis has occurred.
authors.affiliation: University of Chile, School of Public Health, Santiago de Chile, Chile.
authors.forename: Luis
authors.initials: L
authors.lastname: Caris
authors.forename: Fernando A
authors.initials: FA
authors.lastname: Wagner
authors.forename: Carlos F
authors.initials: CF
authors.lastname: Ríos-Bedoya
authors.forename: James C
authors.initials: JC
authors.lastname: Anthony