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Contrary to the widespread use of cannabis in adolescence seems to really open the door to the use of hard drugs, suggest the results of a US-Dutch study published in the journal "Behavior Genetics."

One of the main arguments put forward to explain the association between early cannabis use and increased risk of other illicit drugs based on the assumption that the consumption of cannabis then facilitates access and exposure to other market drugs, thereby increasing their availability. This theory of the escalation of drug use in particular has motivated the establishment, in the 70s, the Dutch law on cannabis, which separates the market of this substance from that of "hard" drugs, remember Michael Lynskey, Department of Psychiatry, Washington University in St. Louis (Missouri) and colleagues.

U.S. and Dutch researchers wanted to clarify, in a study from the Dutch twin pairs discordant for early cannabis use, the influence of genetic and environmental risk factors shared by these sets of twins on the association between early use cannabis and excess risk of subsequent use of other illicit drugs.

Their analysis was based on 219 pairs of twins of the same sex monozygotic ("real") twins or dizygotic ("false") twins, only one member had used cannabis before the age of 18.

After adjustment (especially to take into account the influence of the early use of other substances such as alcohol or tobacco), they found that among participants who used cannabis before age 18, the probability of subsequent decision recreational drugs ("party drug" like ecstasy, for example) is multiplied by 7.4 and the consumption of hard drugs is multiplied by 16.5 compared to their sister had not used cannabis before majority. However, early cannabis use does not increase the risk of subsequent regular consumption of this substance, the authors note.

This observation conducted among twin pairs with only one early cannabis users suggests that the association between early cannabis use and subsequent use of illicit drugs is not due to family risk factors (genetic or environmental), conclude the authors.

A previous study conducted by Michael Lynskey and conducted with pairs of Australian twins, published in the "Journal of the American Medical Association" (JAMA), had led to comparable results.

Confirmation of these results in Dutch society, which does not endorse the use of cannabis, suggests that the association between early cannabis use and later other drugs taken is not only a matter of statutory factors and the surrounding context cannabis is used and obtained, say the authors.

For the authors, regardless of mechanism to explain the causal relationship of cannabis in adolescence does seem related to subsequent use of harder drugs and policies preventing the use and abuse of drugs and addiction, should therefore seek to prevent under 18 consume cannabis.