A study of 201 15-16 year-old students (from a sample of 2641) who reported using marijuana at least 40 times were the basis of this study. The authors found that three clusters of “heavy cannabis users emerged,” i.e., a small group of those where were distinguished by antisocial behavior, another group who had depression, low self-esteem and poor relationships with parents and friends, and a third group perceived to be “ordinary.” Those in the “ordinary” group were found to be less likely than those in the other two clusters to use other illicit drugs.

The authors reviewed numerous previous studies on the relationship between cannabis and the use of other drugs and concluded “In particular, cannabis use and dependence were highly associated with increased risks of other substance dependence.”