A study of 6,000 California and Oregon adolescents found that those who use alcohol, tobacco or other drugs alone later experienced more health and behavioral problems than those who used these substances socially, Reuters reported Dec. 7.

The nine-year study, which began when subjects were in middle school and high school and featured follow-up interviews at age 23, found that the solitary drug users were less likely to graduate college, more likely to have drug problems as young adults, were in poorer physical health, and exhibited more delinquent behaviors.

The research was published in the December 2006 issue of the journal Psychology of Addictive Behaviors.