Young people who start smoking at an early age have a much higher risk of starting to use cannabis by the age of 17, the results of a new study indicate.
Researchers looked at the data from a major US-Finnish study of twins, which began in 1994. The project gathered information on twins aged 12, 14 and 17, and also collected information from their parents and teachers.
By the time the twins were 17 years old, 15% of the girls and 12% of the boys had used cannabis or other illicit substances at least once.
The study identified a number of predictors of drug use. These included being female, binge drinking, having a father who binge drank, aggressive behaviour among boys and having acquaintances with drug experience.
However, early smoking was an especially powerful predictor. In fact, compared to those who had never smoked, those who had started smoking by the age of 12 or earlier were 26 times more likely to start using drugs by the age of 17.
"The findings support the gateway hypothesis, which asserts that licit substances such as tobacco and alcohol, are a stepping stone to harder, illicit drugs. The theory has come under much criticism in recent years.
"Many have argued that certain common factors may explain both smoking and drug use, factors such as problem behaviour and genetic influences. That's why we wanted to look into these issues more deeply in our studies," explained one of the researchers, Prof Tellervo Korhonen.
Details of these findings are published in the Journal of the American Academy of Child and Adolescent Psychiatry.