Teenagers who begin smoking at an early age are much more likely to start using marijuana by the time they're 17, researchers report.
Their analysis of data from a Finnish-American birth cohort study that began in 1994 found that by the time they were 17, about 15 percent of girls and 12 percent of boys had used marijuana or other illicit substances at least once.
Predictors of drug use included being female, binge drinking, father's binge drinking, peers who smoked, acquaintances with drug experience, and aggressive behavior among boys.
The researchers also found that starting smoking at an early age was an especially strong predictor of drug use. Participants who started smoking by age 12 or earlier were 26 times more likely to start using drugs by age 17 than those who never smoked.
"The findings support the gateway hypothesis, which asserts that illicit substances such as tobacco and alcohol are a stepping stone to harder, illicit drugs," project team member Tellervo Korhonen, of the University of Helsinki, said in an Academy of Finland news release.
The findings appeared recently in the Journal of the American Academy of Child and Adolescent Psychiatry.